Report from the Sub-committee on Agriculture

8. Alternative methods of controlling and preventing pests

8.1 Prevention and control of fungal diseases
8.1.1 Possibilities with resistance and breeding
8.1.2 Possibilities offered by cultural practices
8.1.3 Perspectives
8.1.4 Conclusions
8.2 Seed-borne diseases
8.2.1 Present situation
8.2.2 Possibilities of prevention and control without pesticides
8.2.3 Perspectives
8.2.4 Conclusions
8.3 Control of pests
8.3.1 Pest-resistant varieties
8.3.2 Alternative methods
8.3.3 Perspectives
8.3.4 Conclusion
8.4 Prevention and mechanical control of weeds
8.4.1 Present knowledge
8.4.2 Effect on the seed pool
8.4.3 Weed problems in vegetable production
8.4.4 Control of Couch grass without pesticides
8.4.5 Perspectives
8.4.6 Conclusion
8.5 Alternative methods for growth regulation
8.5.1 Growth regulation in cereals
8.5.2 Growth regulation in seed grass
8.5.3 Growth regulation in pot plants
8.5.4. Conclusion
8.6 Biological control
8.6.1 Present use and potential in greenhouses
8.6.2 Future perspectives in greenhouses
8.6.3 Biological and microbiological prevention and control in fields
8.6.4 Perspectives for field use
8.6.5 Possibility of using plant extracts and similar
8.6.6 Conclusion
8.7 Effect of alternative methods on the need for nutrients
8.8 Use of damage thresholds
8.8.1 Weeds
8.8.2 Diseases and pests
8.8.3 Potential for use of damage thresholds
8.8.4 Perspectives
8.8.5 Site-specific plant protection
8.8.6 Conclusion
8.9 Use of genetically modified organisms (GMOs)
8.9.1 Present situation
8.9.2 Future perspectives
8.9.3 Conclusion
8.10 Possibility of new spraying techniques reducing pesticide consumption and undesirable environmental techniques
8.10.1 Use of new spraying technology to reduce pesticide consumption
8.10.2 Possibility of reducing drift by the choice of spraying method
8.10.3 Possibility of reducing point-source contamination with pesticides
8.10.4 Conclusion
8.11 New pesticides
8.11.1 Herbicides
8.11.2 Fungicides
8.11.3 Insecticides
8.11.4 Conclusion

The sub-committee was required to examine alternative, non-chemical methods of preventing and controlling diseases, pests and weeds as a basis for clarifying the consequences of totally or partially phasing out pesticides within a 10-year period. In this chapter we review existing knowledge in this area.

The methods examined are:

	prevention and control of fungal diseases
	control of seed-borne diseases
	prevention and control of pests
	prevention and mechanical control of weeds
	alternative methods for growth regulation
	biological control

The sub-committee has also considered measures that could help towards reduction in a scenario with partial phasing out of pesticides. The potential of the following measures has been examined:

	use of damage thresholds
	use of alternative spraying techniques to reduce pesticide consumption and environmental impacts
	use of genetically modified organisms
	the pesticides of the future

We can start with some general considerations concerning prevention in connection with choice of crop and crop rotation.

General considerations

Prevention of pests through choice of crops

Crop rotation is very important to the overall level of pests and their significance to crops. Thus, it is well-known that there is far less need for pesticides in crop rotations at dairy farms with a big production of coarse feed than at specialised arable farms with such productions as sugar beet and potatoes. There is generally a great need for research in order to assess the long-term perspectives of different crop rotations in which pests are to be controlled by alternative methods, as would be the case, for example, in a scenario without pesticides. Many technical changes could be made in the existing cultivation systems, such as adjustment of sowing time, fertilisation, quantity sown, etc., which would improve the possibilities of minimising problems with pests. These measures are discussed in chapter 8.1., 8.3 and 8.4.

We do not believe that there is any realistic possibility of changing the choice of crop to production of fodder and vegetables. We do, however, consider that there are certain viable alternatives in the use of wholecrop for sows and biomass production for non-food purposes. Both these types of production are estimated to have only a small need for pesticides (Olesen et al., 1998). Intercropping, in the form of mixed seed, is not considered to offer any great crop-rotation potential in arable farming, although its potential could be greater in organic production, where the inclusion of nitrogen-fixing species affects the yield.

8.1 Prevention and control of fungal diseases

Resistance in agricultural crops

Breeding for resistance has been one of the main ways of reducing diseases in our cultivated species. Action in this field has to be long-term and continuous because resistance is not necessarily static. When species are cultivated on large areas of land, the fungal population’s ability to attack the plants can change, whereby the resistance of the species becomes ineffective after some years’ cultivation.

Cereal breeding is carried out in Denmark, but most of the species cultivated at the present time come from other countries. In the last few years, the acreage used for Danish varieties of spring barley and winter wheat has been between 5 and 10%. However, all the varieties of rye and winter barley grown have been of foreign origin.

2 kinds of resistance-race-specific and non-race-specific

When evaluating resistance in the present varieties it is important to differentiate between the different types of resistance, their genetic basis and the response of the pathogens to the resistance. Resistance is often classified as either race-specific or non-race-specific (partial) resistance. The two categories are defined on the basis of the effect of the resistance to specific pathogenic isolates. Race-specific resistance is characterised by a high effect on certain pathogenic isolates (those with avirulence against the resistance in question) and no effect on isolates with the corresponding virulence. The frequency of isolates with virulence/avirulence in the pathogen population thus determines the effect of race-specific resistance in field conditions. Race-specific resistance is normally inherited through one gene, whereas partial resistance depends on many genes.

Loss of resistance

The proliferation of virulent isolates in a pathogen population is expressed in practice by the resistance in question losing its effectiveness. The speed at which such proliferation occurs depends on the pathogen’s biology and spreading pattern, but also the size of the area on which the variety in question is cultivated, whether as part of a mixed crop or a monoculture, whether the crop is grown in both spring and winter form and, last but not least, the level of partial resistance of the available varieties. Partial resistance has approximately the same effect on all pathogenic isolates. However, selection also takes place in the pathogen population to adapt to partial resistance, but this selection is less intense and partial resistance is therefore regarded as more "durable".

Good resistance in spring barley

Spring barley is the species of cereal that has been subjected to the most intensive breeding in the last 30-40 years and on which the most breeding-related research has been done in Denmark. It is also the species of cereal with the highest resistance to fungal diseases. The species cultivated already have effective resistance to mildew, brown rust and Rhynchosporium and moderately effective resistance to net blotch. Some of the most widely cultivated species combine good resistance to all four diseases. The relatively successful breeding for resistance in spring barley must be seen in the light of extensive use of ml-o resistance to mildew. There has thus been a distinct move towards cultivating more mildew-resistant species in the last 10 years. Ml-o resistance has thus given researchers time to incorporate good resistance to the other main fungal diseases.

Resistant species often have love yield losses

In spring barley, the level of yield of the most resistant varieties is equal to that of the highest yield varieties and, generally speaking, it does not pay to take measures to prevent and control fungal diseases in either the highest yielding varieties or the most resistant varieties (table 8.1). This is reflected in the present choice of varieties, especially within feeding barley. It can pay to use fungicides in wholly or partially sensitive varieties in localities and years with favourable conditions for epidemic development of fungal diseases.

*Resistant varieties: Meltan, Bartok, Heron, Tofta, Wren
**Highest yielding varieties: Henni, Bartok, Sultane, Cork, Paloma

Varieties with good mildew resistance generally give a smaller additional yield when fungicides are used than more sensitive varieties. This can be seen from table 8.2, which shows figures from 3 years in which disease attacks varied considerably. Variety mixtures generally have a low level of mildew and less need for disease control. Other diseases, such as brown rust, net blotch and Rhynchosporium cause a considerable loss of yield in some years if fungicides are not used. This was the case, for example, in 1998.

Table 8.2
Gross additional yields in hkg/ha for control with fungicides in different varieties of spring barley, with a breakdown by resistance and year. The mildew attack is given in brackets for each of the 3 years. All the figures are based on at least 15 trials. (Annual Reports of Field Trials, Supplementary Trials).

Feeding barley and malt barley

In Denmark, varieties of feeding barley are grown on about half the acreage, and varieties of malt barley on the other half. Within feeding barley, great attention is paid to resistance in the choice of variety and cultural practices, with minimum use of fungicides. In the case of malt barley, farmers do not have the same possibility of choosing varieties with high resistance to disease because the malt industry in Europe only accepts very specific varieties that are of good quality, well known and well tried. To maintain Denmark’s big export of malt (the additional value of malt barley compared with feeding barley in a normal season is often as much as DKK 10-30/hkg), resistance to disease must often be given lower priority than quality.

Low resistance to disease in winter barley

Winter barley has relatively low resistance to leaf diseases. The average loss in the five highest yielding varieties as a consequence of not using fungicides is about 2 hkg/ha after deduction of costs. Winter barley is attacked by the same diseases as spring barley, but in winter barley it is Rhynchosporium, net blotch and brown rust that cause most losses. The varieties exhibit considerable difference in the level of resistance, but only a single variety has had fairly high resistance to all diseases. Within the last few years, individual, new varieties have shown clearly improved resistance in trials, but none of them has shown a sufficiently high yield and resistance to winter conditions in Denmark. The varieties that best combine yield, winter resistance, stem characteristics and grain quality are normally grown on most of the acreage. As the variation in resistance among the varieties of winter barley is limited and no varieties combine sufficient resistance to all the main diseases, there is no possibility in the short term of further reducing the dependence on fungicides through choice of varieties (Hovmøller et al., 1998).

Winter wheat’s four most serious leaf diseases are mildew, yellow rust, leaf spot of wheat and glume blotch of wheat. It suffers severe attacks of these diseases, resulting in heavy loss of both yield and quality. There are varieties with moderate to good resistance, particularly to mildew and yellow rust, but no varieties on the market combine effective resistance to all the main leaf diseases (table 8.3).

Table 8.3 Look here!
Frequency of damaging attacks of fungal diseases in winter wheat in Denmark, level of resistance of available varieties, and genetic basis and future potential for resistance (modified on the basis of Hovmøller et al., 1998).

A simple breakdown of the varieties into sensitive and resistant, depending on the pressure of disease in the individual year shows that the loss of yields due to diseases varies between 6 and 21 hkg/ha (table 8.4). In the case of mildew, the varieties widely grown today are not fully resistant, although some of the very latest varieties appear to have effective resistance. For many years, the resistance of wheat to mildew has proved relatively short-term. Many of the varieties grown possess effective resistance to yellow rust, but here, too, the resistance lasts too short a time (Hovmøller et al. 1998). Leaf spot of wheat and glume blotch of wheat are diseases that could also cause serious problems in the future because the varieties worth cultivating today are insufficiently resistant. The strength of epidemic attacks depends greatly on the weather, and severe attacks can cause high yield losses.

Table 8.4
Gross additional yields in hkg/ha when fungicides are used in different varieties of wheat, with a breakdown by resistance and year. All the figures are based on at least 16 trials. (Annual Reports of Field Trials, Supplementary Trials).

Resistant contra high-yielding varieties

As for spring barley, we have selected the five ‘most resistant’ and the five ‘highest yielding’ varieties of winter wheat in the period 1995-97 on the basis of the results from the underlying trials. In this connection it should be noted that the highest yielding varieties in one year are seldom highest yielding in the following year. A later analysis of the same material year by year shows that only one of the five highest yielding varieties in one year is to be found in the five highest yielding varieties in the previous year.

On average, in the period 1995-97, the use of fungicides resulted in an additional increase in yields in both the most resistant varieties (which were only partially resistant to leaf spot of wheat) and the highest yielding varieties. The most resistant varieties yielded 3.5 – 4.5 hkg/ha less than the highest yielding varieties, both with and without treatment with fungicides (table 8.5).

Subsequent trials in 1998, when most varieties suffered severe attacks of leaf spot of wheat and new fungicides were used, show that the net additional yields were of the order of 9-12 hkg/ha in the variety trials, with the highest additional yields in the group of highest yielding varieties (Munk et al., 1999). It was also found that when the grouping of ‘most resistant’ and ‘highest yielding’ varieties for 1998 was based on results from 1997-98 there was no difference in yield between the two groups of varieties when treated with fungicides, while the most resistant varieties gave the highest yield without treatment.

Resistance does not cost yield from nature´s hand

The above results underline the fact that it is not the property ‘resistance’ that is the reason for a possibly lower yield than in varieties that are highest yielding for a period. We believe that with different breeding priorities the level of resistance can be raised while maintaining a high level of yield. However, since Danish plant breeders participate in international cooperation, the magnitude of the improvement in yield depends on the extent to which the same selection criteria are used and on how much material can advantageously be exchanged between the breeding companies. If Denmark concentrates unilaterally on resistance, it may mean that the improvement in yield of Danish varieties will have difficulty in keeping up with the expected improvement for foreign varieties. Conversely, increased focus on resistance in other countries as well could give Danish breeders a competitive advantage.

Table 8.5
Yield relationships in the most resistant and the highest yielding varieties of wheat with and without use of fungicides in 1995-97. On average, both groups produced additional yields of the same order of magnitude with treatment with fungicides. The classification into ‘most resistant’ and ‘highest yielding’ varieties was carried out after the results from the underlying trials had been calculated. (Munk et al., 1998).

Big breeding potential

Within the next 10 years the potential for breeding and use of varieties with effective resistance to mildew and to some extent to yellow rust is deemed to be relatively good because there are new effective resistance genes in breeding material and a comparatively high level of partial resistance in a number of well adapted varieties. However, if the population dynamic of the yellow rust fungus continues at the same rate in the years ahead, there may be a shortage of good resistance to yellow rust within 10 years. In the breeding material there is not at present fully effective, race-specific resistance to either leaf spot of wheat or glume blotch of wheat. There is a considerable difference in the level of partial resistance between new lines, but partial resistance is polygenetically determined and is therefore difficult to work with in breeding. The level of partial resistance can probably be increased in new varieties within 10 years, but it does not seem likely that a high enough level can be achieved to avoid epidemic attacks in weather conditions that are favourable to the two fungal diseases.

Many breeding aims

It must be stressed that wheat does not appear to have resistance comparable to the very durable Mlo resistance of spring barley. It will always be necessary to introduce new resistance to replace the specific genes that have lost their effectiveness. That means working to introduce new resistance to all four leaf diseases at the same time. New varieties must also have good winter hardiness, i.e. a high level of tolerance to frost and cold, combined with resistance to out-wintering diseases. We are therefore unlikely to see, within a 10-year horizon, the development of varieties that combine resistance to all four leaf diseases, let alone varieties with effective resistance to all the main diseases combined with good winter hardiness.

There is a considerable potential for reducing losses from leaf diseases genetically. In this connection it must be stressed that even though, in cereals for example, there is a possibility of remedying the problems within each of the areas leaf diseases, seed-borne diseases and weed competition, it is not given that all the problems can be solved at the same time by breeding within a short space of years. It is therefore necessary to carry out a combined assessment of unsolved tasks for each individual crop, while at the same time considering the level of yield and quality.

Resistance in rape, peas and seed grass

Only limited use is made of fungicides in rape, peas and seed grass. Too little is known about the significance of pathogens in rape, peas, seed grass and beets and about resistance genetics and the population dynamics of pathogens. There is generally a genetic basis for raising the level of resistance to diseases in rape, peas, seed grass and beets, but that calls for a major research and breeding effort. In the case of rape, peas and seed grass, breeding is primarily carried out for the export market, where one cannot expect any direct increase in demand for resistant varieties.

Potato blight

Potato blight is the principal disease in potatoes, and it occurs every year. The effect of attacks depends mainly on how early they occur and how severe they are, and on the receptivity of the potatoes cultivated, cf. figure 8.1. Losses from blight of 30-40% are common (Hansen & Holm, 1996). Good resistance to blight could considerably reduce the losses in potatoes.

Varieties with specific resistance genes to potato blight have been widely used for more than 40 years, but up to the present time all known, specific resistance genes lost their efficiency. Researchers have been working since the 1960s on non-specific resistance genes that act against different races of blight. Such varieties are attacked more weakly and the epidemic develops more slowly. However, in several varieties with non-specific resistance, the resistance has become partially or completely ineffective after many years’ cultivation (Holm et al., 1998).

Present resistant varieties

Although there are some varieties with fairly good resistance to potato blight, it has not yet proved possible to transfer this resistance to all the varieties preferred by the consumers and the starch industry. Danva is an example of a late starch potato with good resistance. At the breeding stations there are several new, late starch and ware varieties with non-specific resistance on the way. However, in neither Danish nor foreign material are there any ware varieties with good resistance on the way. High resistance seems only to occur in late varieties (Holm et al., 1999). A common feature of resistant varieties is that nobody knows how long it takes for resistance to break down because of a change in the virulence of the fungus. Types that break down resistance develop faster in widely grown varieties.

None of the known resistances are strong enough to keep attacks of fungal diseases at bay and ensure against loss of yield. All varieties with non-specific resistance genes are strongly attacked towards the end of the growing season if the climatic conditions are favourable to blight. However, these varieties can manage with reduced spraying – especially early in the season (Holm et al., 1999).

Heavy consumption of fungicides in potatoes

Today, 40-50% of all fungicides, measured in kg active ingredient, is used to prevent this disease. The varieties’ resistance to potato blight is not fully utilised because the risk of attack in the different varieties cannot be sufficiently differentiated at the present time. For that reason, varieties with some resistance are generally sprayed as though they have no resistance to attack.

(Antal behandlinger = Number of applications
Mest modtagelige = Low resistance
Moderat modtagelige = Moderate resistance
Mest resistente = Higt resistance)

Figure 8.1
Need-determined treatment frequency index against potato blight in 3 localities in Denmark up to 15 August, with a breakdown into 3 resistance levels. The figures are the average of the years 1988-95. With routine treatment, six applications are taken as the normal treatment level (Source: Hansen & Holm, 1996).

Strategic use of varieties, incl. variety mixtures

Besides direct use of resistant varieties, disease attacks can be reduced by strategic use of resistance, including use of mixed species and varieties and distribution of resistance over time, e.g. in winter and spring forms. Experience with the use of variety mixtures is good in spring barley, while there is only limited experience from other crops. The possibility of regulating and stabilising resistance by this means has often been advanced but has been of only limited practical importance owing to problems of regulation.

Resistance in market-garden cultures

The market gardening sector covers many cultures. Only a few of the most important will be mentioned. Most varieties within this sector come from other countries (particularly the Netherlands and the UK). In Denmark, very little breeding work is done in this area.

Problems in apples and strawberries

The biggest problem in pomes is fruit scab. The main varieties grown in Denmark today are not all the healthiest varieties. If pesticide consumption is to be reduced, the choice of varieties will have to be changed. That takes time (10-15 years) because the present production has to be written off before a new production can be established. There are varieties of apples with resistance to scab, but the resistance has become less effective in the same way as resistance in other crops. Choosing resistant varieties will reduce the problem of diseases but will not solve it (Lindhard et al., 1998).

There are strawberry varieties with relatively good resistance to grey mould and mildew. Unfortunately, however, they do not meet the retail sector’s requirement concerning a long shelf life. When planting strawberries it is important to ensure that the material is free of red rot, wilt and stem rot.

Vegetables cover many cultures and are grown both outdoors and, in many cases, indoors. One of the biggest problems in greenhouse vegetables is pythium, which attacks the plants’ root system. There are at present no varieties that are resistant to this disease in, for example, cucumbers, although resistant wild types have been identified that could improve the level of resistance in the longer term (Thinggaard, 1998). In lettuce, downy mildew is a serious problem. Even though breeding is going on towards resistant varieties, the resistance is seldom particularly lasting.

Onions are a field culture that suffers severe attack from onion mildew. This fungal disease reduces both quality, yield and shelf-life. Screening of the varieties of onion cultivated has shown very little difference in their receptivity (Kjeldsen, Bjørn & Thinggaard, 1998).

The supply of ornamental plants is very varied, and as more than 400 different species are produced it is difficult to incorporate broad resistance in all cultures. Owing to rapid development in the choice of varieties it is thus difficult, with the present form of production, to ensure the cultivation of disease-resistant varieties (Ottosen et al., 1998).

8.1.2 Possibilities offered by cultural practices

Agricultural crops

The crop rotation is of great importance to the total pressure of diseases and the reduction in yield caused by diseases. Table 8.6 shows some examples of crop-rotation diseases and nematodes and the number of years it normally takes before the crop is grown again on the land in question.

Generally, however, there is considerably less need for fungicides in dairy farm crop rotations with a big production of coarse fodder than in specialised arable farm crop rotations with, for example, cereals and potatoes. This is not due to attacks by specifically crop-rotation diseases but to the fact that cereals and potatoes are generally the crops that are most severely attacked by leaf diseases.

Not all these diseases can be prevented or controlled chemically. In Denmark, the general rule is a crop rotation that prevents proliferation of most of the crop-rotation diseases. Among other things, there is no tradition for controlling nematodes, which has been widely practised in Dutch agriculture, for example, in order to have a large proportion of potatoes and beets in the crop rotation.

There is some possibility of reducing the impact of diseases in winter cereals by postponing the sowing time. However, there can be a considerable reduction in yield with late sowing (5-7 hkg/ha), and late sowing also reduces the crop’s N-absorption in the wintertime and increases the risk of leaching (Annual Report of Field Trials, 1997).

Table 8.6  Look here!
Table showing crop-rotation diseases/nematodes and the number of years that normally have to elapse before the crop is again grown on the land in question (Source: Godt Landmandskab år 2000 (Good Agricultural PracticeYear 2000)).

Effect of nitrogen

Increased quantities of seed and increasing quantities of nitrogen normally increase the risk of mildew in cereal crops (table 8.7), but the interaction between the cultural practices and the diseases, and thus the need for pesticides, is normally significantly less than the main effect (Jørgensen et al., 1997).

Table 8.7
Effect of quantity of seed on percentage of plants with mildew in wheat in the first disease evaluation and the need-determined quantity of fungicide calculated by means of PC-Plant Protection (Jørgensen et al., 1997).

Attacks by diseases can therefore not be prevented simply by adjusting the cultural practices. The widespread practice of split application of the nitrogenous fertiliser helps to reduce the risk of severe mildew epidemics, compared with earlier strategies in which all the fertiliser was applied in one go.

Experience from organic potato production

In organic potato production, some cultural practices are used to minimise attacks of potato blight and their effect. The earlier the potatoes finish growing, the higher the yield that can be achieved. Growers not only select a variety with considerable resistance but also preheat and pre-germinate the seed potatoes and use spring ploughing, which results in warmer soil and faster germination (Holm et al., 1999).

Second crops are deemed to be neutral with respect to attacks by diseases and pests provided unfortunate combinations of main and second crops are avoided.

Market-garden crops

In tree fruit, different methods are available for reducing attacks by diseases. However, none of them keeps the culture completely free of problems. For example, good orchard hygiene, including removal of old, mummified fruit, reduces the risk of monilia, while removal or good decomposition of old foliage, together with pruning that results in open trees, helps to reduce attacks of fruit scab in apples. Rapid decomposition of foliage also helps to reduce leaf spot in cherry trees and rust and leaf spot in blackcurrants. Good drainage and cutting out of wounds reduces the risk of fruit-tree canker, and clipping off of shoot tips infected with mildew, especially in young plantations, can reduce attacks (Lindhard et al., 1998).

Grey mould is reckoned to be one of the diseases causing most losses in strawberries. Moist conditions during flowering encourage this fungus, which attacks through the flower. The risk of attack can be reduced by ensuring rapid drying of the plants – for example by reducing the supply of nitrogen – which results in smaller plant mass, cultivation of isolated plants and harrowing to remove old, died-down plant material (Lindhard et al., 1998).

8.1.3 Perspectives

Cultural practices

Many cultural practices can easily be incorporated in the present cultivation systems, such as adjustment of the sowing time, fertilisation, quantity sown, etc., whereby most problems can be reduced. However, several of these measures reduce the level of yield slightly or require extra work, especially in market-garden production.

Breeding

To utilise the potential for resistance, far more needs to be done in the way of breeding for resistance, and research and development are needed to support the primary breeding work. Besides this, more research and development are needed within resistance genetics and the epidemiology and population dynamics of pathogenic fungi, and testing of varieties and advisory services should be expanded. That applies to all crops and all pathogens (diseases) that are potentially harmful in the crop in question. Utilisation of the potential for resistance also requires close contact between the aforementioned players and activities and integration of the cooperation with agricultural advisers and seed producers so as to ensure the possibility of rapid adjustment of the varieties grown.

Strategic use of resistance

Strategic use of resistance should also be considered and more closely analysed. This would mean cultivating varieties with different resistance genes between each other (like a patchwork blanket) to reduce the risk of breakdown of resistance. Variety mixtures are another good strategy that reduces the risk of losses from diseases. Extensive use is already made of variety mixtures in spring barley (10%). Several years’ trials in wheat have shown that there may also be a potential in this crop.

8.1.4 Conclusions

Breeding and cultural practices offer several options for preventing and controlling fungal diseases. However, the methods in question cannot at present make up for the losses that would result if fungicides were no longer used.

In most crops significant losses occur at intervals due to attacks by diseases. In agricultural crops the losses from diseases are potentially largest in potatoes and wheat. This is also clear from the fact that fungicide consumption is highest in precisely these two crops. Within market gardening, too, there is a risk of heavy losses in many crops – particularly pomes and strawberries. In the event of a total or partial phase-out of pesticides, greater use would have to be made of resistant varieties in order to reduce the loss of yield from attacks by fungal diseases.

There is a great potential for reducing the expected losses from leaf diseases genetically within the next 10 years. However, we do not think it possible for all the breeding objectives to be achieved simultaneously within a 10-year period. Besides resistance to leaf diseases in cereals, work is also needed on resistance of seed-borne diseases and better weed competition. It will therefore be necessary to prioritise the breeding activities. There is a great need for a major intensification of work on breeding and of research on breeding if we are to see any noticeable change in the range of resistant varieties, compared with the range that is available today.

Foreign plant breeding is of great general significance to the Danish range of varieties and production; there is also close collaboration between Danish and foreign breeders. The feasibility of changing Danish breeding priorities in favour of breeding for resistance would thus depend on the interests of foreign breeders.

There is a significant potential for reducing losses from fungal diseases through strategic use of resistance (including variety mixtures).

Many cultural practices can be incorporated in the present cultivation systems, such as adjustment of the sowing time, fertilisation, amount of seed sown, etc., to improve the possibility of minimising pest problems. However, diseases can neither be prevented nor minimised only by adjusting cultural practices. Several of the changes in cultural practices would reduce the level of yield.

Within market gardening there are also different methods that can reduce attacks by diseases. However, none of them can keep cultures free from all problems. For example, good orchard hygiene, which includes removal or good decomposition of old foliage, together with pruning that results in open trees, helps to reduce attacks. Several of these methods are associated with an increased need for manpower.

8.2 Seed-borne diseases

Current practice

Thanks to effective and systematic seed-dressing of most Danish seed for drilling, serious, loss-producing, seed-borne diseases have been kept at a very low level. Today, a large proportion of all seed for drilling (85-90%) is dressed, while seed-dressing of other crops varies somewhat, depending on the actual occurrence of diseases. It is absolutely essential to treat infected consignments. Other consignments can be left untreated, but with our present-day resources and technology, it is not always possible to determine which consignments need dressing. The total consumption of seed-dressing agents corresponds to around 3% of total pesticide consumption. Several seed-borne diseases can proliferate and spread relatively rapidly, so continued action is needed to keep these diseases down. In the worst event, lack of control and prevention would result in serious losses of yield and very serious impairment of the quality of seed for drilling and cereals in general produced in Denmark (Nielsen et al., 1998). The principal diseases/pests, damage thresholds and loss levels that could occur in connection with the establishment of the main agricultural crops are shown in table 8.8. The quantities in the different generations of seed are given in table 8.9.

Table 8.8  Look here!
Table showing the principal agricultural crops and their need for treatment with seed-dressing agents against diseases and pests. Indication of damage thresholds and expected losses from severe attacks (Nielsen et al., 1998).

If seed-dressing agents were not used, we would expect the biggest problems to be stinking bunt in winter wheat, leaf stripe and naked smut in barley and, possibly, stripe smut in rye. These fungal diseases multiply unusually rapidly, and even a small amount of inoculum on the grains can result in heavy attacks in the same growth season. Insect attacks and soil-borne diseases can cause very serious losses in beets and moderate losses in peas and rape.

Table 8.9
Quantity of certified seed for drilling in tonnes. Breakdown by seed and categories, together with estimate of the farmer´s use of own seed in 1996/97. Cert. 1. Generation and Cert. 2 Generation.: Certified seed for drilling of 1^st and 2^nd generation, respectively. The certified breeding acreage is also shown. Source: The Danish Plant Directorate

8.2.2  Possibilities of prevention and control without pesticides

Alternative methods not fully developed

The possibility of regulating seed-borne diseases without seed-dressing agents is very limited in the short term because there are no alternative methods that would have sufficient effect. The risk, particularly in cereals, of directly phasing out seed-dressing agents is so serious that we suggest that treatment with seed-dressing agents be retained where alternatives have not been developed and faster methods of analysis in winter cereals have not been implemented. We know too little today to be able to say whether these methods could entirely replace chemical seed-dressing agents within a 10-year period. Alternatively, we would expect this breeding work to take place abroad, where use of seed-dressing agents would still be permitted.

The only possibility in a 0-scenario would be disease analysis of consignments of seed for drilling and subsequent rejection of infected consignments. This would depend entirely on fast, effective and representative analysis in the period from harvest to sowing. Since the quantities of seed for drilling are very large, a very big bottleneck would occur in winter cereals. The time needed to analyse a sample is between 2 and 10 days, depending on what the sample is being analysed for. The present systems of analysis would not be able to cope with the tasks in the short term. If each sample were to cover 50 tonnes, that would correspond to having to build up capacity for handling about 30,000 samples. The analysis capacity would thus have to be increased considerably from the present level of about 1,000 samples (Bent Nielsen, Pers. com.). We would expect large quantities of seed for drilling to be rejected, which would necessitate a substantial increase in the breeding acreage. There would be a real risk of uncontrolled proliferation of the diseases and large yield losses. The problems would be greatest in winter wheat, spring barley and rye, while they are difficult to predict in spring wheat. In triticale, the varieties investigated up to the present time have had few problems with seed-borne diseases. Triticale varieties with a large "proportion of wheat" would, however, be at risk of attack by stinking bunt.

Problems in rape, peas and beet

Today, there are no alternatives to treatment with seed-dressing agents for rape, peas and beets, and there would be very large losses in the case of beets and moderate losses in the case of rape and peas. The risk – particularly in the case of cereals – of directly phasing out seed-dressing agents is so great that we suggest retaining seed-dressing of the first generations of seed up to and including C1. With 1.5 hkg seed/ha, the C1 generation of 26,833 tonnes, could be sown on 175,000 ha, but the C2 acreage is only 58,157 ha (table 8.8), which means that only about one third of the C1 quantity could be sown. Since a larger breeding acreage than actually used would be needed, it might be necessary to dress a larger area in order to ensure sufficient supplies of varieties.

Intermediate

In the intermediate scenarios it is assumed that the breeding generations up to and including certified seed for drilling C1 are treated with very effective seed-dressing agents, while the large commonly used generation (C2, 80% of the seed for drilling) are analysed for seed-borne fungal diseases and only dressed if the damage thresholds are exceeded. Owing to serious capacity problems in analysing winter cereals, it would be necessary to determine whether a sufficiently extensive need analysis could be carried out. On the face of it, the model appears usable in spring-sown cereals, where the need for analysis would be of manageable size if the resources were increased. In Denmark, seed for drilling is produced at a relatively small number of very large facilities. If the consignments of seed were to be separated according to the need for seed-dressing, considerable restructuring (of silos, containers, etc.) would be required at the grain merchants.

8.2.3  Perspectives

Research in alternative methods

Chemical seed-dressing, which is the only method used to prevent and control seed-borne diseases in seed for drilling, has been practised since the beginning of this century. There are other possibilities, however, but only limited research and development have been done in this area up to the present time. Prevention of seed-borne diseases by means of resistant varieties is possible, particularly in the case of stinking bunt and leaf stripe, for which effective genes are available. However, that calls for a continuous and systematic breeding programme and monitoring to check whether the resistance lasts. The last few years have brought some development in biological control and prevention of plant diseases, and several products are also suitable for dressing seed. However, the products in question have not been fully developed and must also be tested for effect and usefulness in practice. Other alternative methods of prevention and control include the use of hot water, hot air, swabbing (in the case of diseases like stinking bunt, which sit on the surface of the seeds). However, these alternative methods will rarely have sufficient effect against high-level attacks. The alternative methods are far from usable in practice. The technology and methods need testing, and the investment requirement and capacity must be investigated. It is thus necessary to review control and prevention strategies and to look at possibilities of combining different measures.

10-year perspective

As described in the foregoing, there are some prevention and control measures with a considerable potential against seed-borne diseases. They include, particularly, resistance and biological prevention and control. However, too little is known today to be able to say whether these methods could replace chemical seed-dressing agents altogether within a 10-year period. Analyses of the incidence of diseases are of vital importance in this scenario as well, and work is going on to determine whether a system can be established that can handle the very large quantities of winter cereals in question within a very short space of time. Larger breeding acreages will be needed than today because more areas/consignments must be expected to be rejected than at present. Furthermore, a requirement concerning resistance to seed-borne diseases would mean a reduced choice of varieties, so it would not always be possible to use the varieties with the highest yield potential.

8.2.4  Conclusions

Problems in a 0-scenario

Today, 85-90% of all seed for drilling and the seed of a large proportion of other crops in Denmark are dressed. If dressing were generally omitted, we would expect rapid proliferation of several of the seed-borne diseases that cause losses. The risk of phasing out seed-dressing agents is so great, especially in the case of cereals, that we propose that dressing be retained where alternatives have not been developed and faster methods of analysis in winter cereals have not been implemented. Too little is known today to be able to say whether these methods could entirely replace chemical seed-dressing agents within a 10-year period. Alternatively, one would have to expect this part of breeding to take place in countries where such agents could still be used. Continued dressing of the first generations of cereals, followed by an assessment of the need of subsequent consignments of seed, would be one way of reducing fungicide consumption - a way that should be examined more closely and tested. The need assessment would demand fast, reliable analytical methods, the separation of seed consignments and probably the discarding of significant quantities of grain for multiplication. In beets, too, there could be substantial losses due to uncertain establishment if seed-dressing agents were banned.

Alternative methods

Today, work is in progress on several alternative methods of combating seed-borne diseases, including resistant varieties, the use of biological agents and technical methods involving the use of hot water/air or brushes. None of these methods has been fully developed, and much research and development work needs to be done before it can be judged whether these methods can directly replace chemical methods. For this reason, we propose that, in the event of a phase-out of pesticides, dispensation be granted for prevention of seed-borne diseases in the early generations of seed, since the consequences of an uncontrollable proliferation of seed-borne diseases are incalculable.

In spring barley, we believe that dressing of the first generations, combined need assessment of the C2 generation, could be practised, whereas for winter cereals a more detailed evaluation would be needed to determine whether need assessment of C2 was practicable owing to the short time elapsing between harvest and sowing.

8.3 Control of pests

Pest-resistant varieties - unknown

Insect resistance in plants is seldom an "on/off" phenomenon. The resistance is usually partial. Insect resistance is reflected in different ways: for behavioural reasons, the pests in question choose a different species or variety because the plant is impossible to colonise, lay eggs in, is repellent, etc., or for physiological reasons, the pests do not ‘thrive’ well on the plant and therefore do not do much damage to it. In extreme cases, the insects die (Hansen, 1998).

However, partial resistance is of great importance because even small differences in varieties can have major effects with respect to population dynamics. Another form of partial insect resistance is tolerance. Here, the plant has developed a ‘system’ that enables it to continue growing and produce a largely normal yield despite an insect attack.

As far as concerns resistance in existing varieties, it can be concluded that there are no pest-resistant varieties of cereals in Denmark with the exception of some varieties of barley and oats, which are resistant to the cereal cyst nematode. Similarly, there are no pest-resistant varieties of grass, beets, rape and peas, whereas many varieties of potato are resistant to both kinds of potato cyst nematode.

8.3.2  Alternative methods

Agricultural crops

Many cultural practices affect the quantity of insects in fields. That applies to both harmful insects and useful animals. Some of the difference is due to natural variation and influence from adjoining habitats, while some depends on the cultural practices. The choice of crop, crop rotation, sowing time, application of fertiliser, soil preparation and use of pesticides all have an influence (Hald & Reddersen, 1990, Goldschmidt, 1995, Petersen et al., 1996). There are at present no clear directions on how to promote the population of useful animals as an element of the strategy to keep pests down in the field. A study from 1994 and 1995 showed that harrowing had no distinctly negative effect on the number of ground beetles, which play an important role as aphid predators (Petersen et al., 1996).

Effect of nitrogen

Aphid attack in cereals is affected by nitrogenous fertiliser. Increasing amounts of nitrogen and, especially, late applications promote the proliferation of aphids. That is because the crop stays green and thus attractive to aphids for a longer period and the aphids are looking for protein. The current levels of nitrogen are deemed to have only a limited effect on the aphid population. The effect of the level of nitrogen on the need to spray against aphids and on the additional yield from insecticide spraying is unclear (Nielsen & Jensen, 1998).

Pest attacks are also affected by the sowing time. For example, oats sown late are attacked most by frit flies, and winter cereals sown early are attacked more by barley yellow virus because the aphids have more time in the autumn to transfer the virus (Nielsen & Jensen, 1998).

In recent years, field slugs have been a growing problem in many autumn-sown fields. The attack depends not only on the climate and the type of soil but also very much on the method of cultivation. Ploughing reduces the number of slugs, while direct sowing promotes attacks because the slugs are not disturbed. Black soil that is harrowed repeatedly for as long as possible before sowing of winter cereals/winter rape impedes the slugs because they are disturbed and starved (Nielsen & Jensen, 1998). Neither frit flies nor slugs are of any great significance for the present level of pesticide consumption.

Market-garden crops

Within fruit growing, work is going on abroad on alternative methods of controlling moths based on the development of confusion techniques using sex hormones. With this method, a large number of dispensers containing pheromones are set out per ha. In IPM production, predatory mites are set out to control spider mites and rust mites. Within blackcurrants, bud gall mites are the main pest. Attacks can be prevented to some extent by using healthy plant material in new plantations, keeping distance from infected bushes and removing infected buds by hand. Usable varieties with resistance to bud gall mites are expected on the market within 10 years (Lindhard et al., 1998).

Attacks by several kinds of pest on outdoor vegetables can be reduced by practising good crop rotation. This applies, for example, to carrot flies and cabbage flies. Attacks can also be reduced by adjusting the sowing in relation to the lifecycles of the pests. Attacks by cabbage flies, carrot flies and thrips can also be partially kept down with netting, while cutworm attacks can be reduced by watering (Friis et al., 1998).

Within the greenhouse sector, great efforts are being made to ensure that biological prevention and control is the main alternative method. It is that already in the production of greenhouse vegetables. Another method that is now undergoing pilot trials is gassing with mixtures of environment-friendly gases. In the case of pot plants, this could, for example, be done just before sale to rid the plants of pests and useful animals (Ottosen et al., 1998).

8.3.3  Perspectives

Pest problems are limited in Denmark

Research in insect-resistant plants has primarily taken place in countries with economically serious pest problems. There, some use is made today of insect-resistant varieties. The reason for the low input in this area in our part of the world is that pest attacks are rarely all-destructive, and in severe attacks farmers have been able to spray with insecticides, which is a very cheap and effective measure. Genetically modified plants with resistance to pests in maize, among other crops, are becoming widely used in the USA and other countries. The possibilities offered by these methods are discussed in section 8.9.

Screening of the varieties used in Denmark would probably reveal differences between varieties with respect to host-plant properties in relation to their respective pests. Preliminary screening in this area could create the basis for incorporation of factors concerning insect resistance in breeding work. The results of such breeding activities have a long time horizon (10 years or more).

Biological prevention and control and the

With respect to the use of biological control of pests in the field, there are deemed to be certain possibilities, which are described in detail in section 8.6. The sub-committee believes that there is a potential for utilising the natural fauna to regulate pests in agricultural crops, but it cannot be utilised immediately.

8.3.4  Conclusion

Pest resistance

Little is known about the resistance of Danish varieties to pests. Simple screening for receptivity to pests could prove this to be an unused resource. The use of genetically modified plants with resistance to, for example, pests in maize is rapidly gaining ground in the USA and some other countries. The same thing could happen in other crops. However, the sub-committee does not think that serious progress, and thus use in Denmark in this area, could be achieved within a 10-year period.

Only very limited use is made of biological control of pests in fields at present, and further development and trials are needed before biological control can become a realistic alternative to chemical control.

It is a well known fact that natural field fauna, e.g. ground beetles and spiders, affect the pest population. In some years, they make a significant contribution to controlling, e.g., aphids, whereas they are insufficient in other years owing to high proliferation rates. We lack specific knowledge of the effects in this area.

Major attacks at interval of years

The development of pest attacks depends greatly on the climate. Severe, loss-producing attacks that cannot be prevented occur at regular intervals, usually in seasons with hot weather, when the proliferation rate is high.

Cultural practices, such as sowing time, fertilisation and soil preparation, affect the population of some pests. Methods involving such factors should be used where they can reduce losses from pests.

Within market gardening there are several alternative methods that can be used to control certain pests. They include placing crops in satisfactory crop rotations, adjusting sowing times, use of netting and watering.

8.4  Prevention and mechanical control of weeds

Experience from mechanical control

In all current crops, trials have shown that weeds can be controlled with non-chemical methods, but in most cases the effect is not quite as good as with chemical agents. Therefore, more weeds must be expected in the crops if one only looks at the effect of the direct methods. To achieve an acceptable effect, both preventive and direct control methods would have to be used in a planned combination. Relevant methods are shown in table 8.10 for a wide range of crops, while the general principles are illustrated in figure 8.2, which shows that it would be necessary to use both crop rotation, various preventive methods and direct control. In localities with a high pressure of weeds or particularly aggressive species of weeds, weeds that could not be controlled sufficiently effectively with non-chemical methods would cause yield losses and harvesting problems.

Table 8.10  Look here!
Table showing relevant non-chemical methods of controlling weeds in important crops. The table expresses typical processes in mechanical weed control and the number of applications (Tersbøl et al., 1998).

Methods of prevention

Postponing the sowing time in winter cereals offers good possibilities of reducing the importance of weeds. However, late sowing can result in a considerable loss of yield, reduce the crop’s absorption of nitrogen during the winter and increase the risk of leaching.

A number of methods of prevention, including cultural practices, have a regulating effect on the relationship between crop and weeds. In addition, they can act in interaction with the direct, mechanical methods of control. Preventive methods would have to be used in a situation without access to herbicides. Only limited use is made of these methods today.

Figure 8.2
Principles for weed control without pesticides, where the overall result depends on the crop rotation, the prevention measures used and the direct control measures used. (based on Michael Tersbøl).

Increasing the quantity of seed and the amount of nitrogen applied normally increases the weed competitiveness of cereal crops. Applying fertiliser seems to increase the competitiveness. The interaction between the cultural practices and weeds is normally significantly less than the main effect. Adjusting cultural practices would not make up for the losses that would occur if herbicides were no longer used.

The effect of second crops on seed-propagated weed is not known, but serious problems must be expected with root weeds such as couch grass. The success of the direct mechanical methods depends very much on the timing of the action and adjustment of the treatment intensity to the sensitivity of the crop. It also depends on the quality of the seed bed and the weather conditions around the time the control measures are carried out. In current cultivation practice, extensive and successful use is made of mechanical control in rape, where the methods are able to compete with chemical methods.

Experience from rape, maize and peas

Table 8.11 shows figures for mechanical control in rape and maize sown in rows. As will be seen, in rape, a result similar to that with chemical control is achieved. In peas, harrowing produced rather good results in 1998. The number of plants and the additional yield have been smaller with harrowing, but harrowing two or three times has produced competitive net yields compared with chemical agents, which are relatively costly in peas (Annual Report of Field Trials, 1998). The long-term effect and the consequences of the lower effect for the subsequent crops are not known.

Harrowing can make it difficult to carry out measures aimed at preventing wind erosion, such as applying liquid manure, which is done on light soils.

Table 8.11
Comparison of weed control and additional yields in different crops with chemical and mechanical control methods (Annual Report of Field Trials, 1997).

* 50 cm row spacing + reduced quantity of seed

Table 8.12
Comparison of weed control and additional yields in peas with different quantities of weed and chemical and mechanical weed control (Annual Report of Field Trials, 1998). The result of the control measures depends greatly on the amount of weed.

In potatoes, too, provided the control measures are carried out at the right time and in the right way, there is no appreciable difference between chemical and mechanical control. Without chemical control, root and grass weed can cause serious problems in certain circumstances (Holm et al., 1999).

Capacity problems

In some situations, the capacity of the mechanical methods is deemed to be generally lower than that of chemical methods. In potatoes, for example, it is estimated that mechanical control would take 2-3 times as long as chemical control. That might cause problems for the approximately 300 large potato producers, who account for 40% of the total production. Without the use of chemical agents it is doubtful whether the present structure – with potato production concentrated in large and specialised farms – could be retained (Holm et al., 1999).

In rainy periods it can be extremely difficult to carry out mechanical control, let alone achieve a good effect. Some fields and crops might therefore have to be abandoned in some years.

In certain periods the capacity of the mechanical methods is greater than that of spraying because, at some stages (blind harrowing and some late harrowing), harrowing takes less time than spraying and time is not spent on filling, washing and cleaning. Neither spraying nor harrowing can be done in rainy weather or just after it. Unlike spraying, harrowing can be carried out in windy weather. For a good result, harrowing must be carried out at the right time because the weather can be a limiting factor. Harrowing is much less effective than pesticides and the effect that can be achieved depends greatly on the quality of the seed bed.

8.4.2  Effect on the seed pool

The effect on the seed pool is not known

There are no trials that throw any light on weeds’ production of seeds with mechanical control measures. Trials with reduced dosages of herbicides indicate that the seed pool will not increase when the weeds’ production of biomass is significantly reduced. The goal for all non-chemical control of weeds must therefore be to keep the quantity of weeds at such a low level that the weeds do not cause yield losses, do not increase the weed-seed population or develop vegetative reproductive organs, do not impair the quality of the crop and do not cause unnecessary harvesting problems. We have no reliable basis today for predicting the development of the seed pool on the basis of crop rotation and control methods (Tersbøl et al., 1998).

The possibilities and limitations of non-chemical control measures against weeds have been evaluated and described within different types of crop rotation, in which the prevention and control methods are integrated. The types represent a selection of typical agricultural productions on relevant types of soil. With the reservations mentioned, concerning especially root weeds and special situations with respect to pressure of weeds and aggressive species of weeds, it is believed that the systems described could regulate weeds satisfactorily in the main crops. However, this evaluation is based on a very slender body of research. In the event of heavy pressure of weeds or aggressive species of weeds, the crop rotation/choice of crops would have to be adjusted still further. It might be necessary to grow some crops (cereals and rape) in rows to make room for row cultivation and, on certain soils and in some localities, it would probably be necessary to switch to grass.

8.4.3  Weed problems in vegetable production

Poisonous species of weed

With the present cultivation conditions, the occurrence of poisonous plants in Danish farm produce presents no health problems for humans. There are occasional cases of poisoning of livestock, but there are no data indicating the magnitude of the problem. There are reports of fatal poisoning of livestock, but most cases are less serious. In Denmark, spring groundsel and deadly nightshade are considered to be the two most significant poisonous species.

Spring groundsel

In the period 1994-98, spring groundsel spread to a rapidly growing number of set-aside fields and fields under grass, especially in Jutland. Species of groundsel contain toxic alkaloids. Horses, cattle, sheep and pigs can be poisoned. The toxicity of the species of groundsel in question is not destroyed by ensiling and drying. Few cases of poisoning of animals or humans by spring groundsel have been described. This is due partly to the fact that there is no usable Danish survey of cases of plant-poisoning of animals and humans. Spring groundsel can be controlled by mowing set-aside fields, the banks of ditches and other natural ecosystems when flowering commences to prevent the plants from setting seed and by establishing the best possible grass cover on set-aside fields and newly established grass meadows. Preventive measures alone cannot prevent the threshold of 35-40 spring groundsel plants per 100 m2 from being exceeded where there is a lot of air-borne seed (Jensen et al., 1998a). The problem has been aggravated by the ban that has been introduced on the use of chemical agents on set-aside land.

Deadly nightshade

Deadly nightshade is widespread in Danish fields in which crop rotation is practised. The fruit of deadly nightshade has the same colour, size and specific gravity as green peas and can therefore not be discarded during harvesting and processing of tinned peas. The crop is therefore rejected if deadly nightshade is found during a field inspection. To avoid the problem of deadly nightshade, it is necessary to prevent its fruit from reaching a size at which it can be confused with peas by sowing early or late in the growing season. Cultivation without pesticides would therefore imply a considerable reduction of the production period, which would call for a larger production capacity if the same production were to be maintained (Jensen et al., 1998a).

In the event of restructuring for pesticide-free farming, a proliferation of the above-mentioned poisonous plants could therefore not be excluded. In Denmark, there would hardly be any question of an increased risk of poisoning of humans, but there could be more cases of poisoning of livestock, which would cause some loss of production in the form of a reduced milk yield, reduced growth rates and similar.

Control of wild oat

As a consequence of the Act on Wild Oat, seed-bearing wild oat must not occur during the growing season. When producing cereals without pesticides, it would be necessary to replace chemical control of wild oat with manual weeding. This is a realistic method of controlling relatively small populations of wild oat, but it is not realistic for large populations. Here, the crop rotation would have to be changed towards production of coarse fodder to reduce the population (Jensen et al., 1998a).

Grass and clover seed, as well as vegetable and flower seed, cover a broad range of cultures. Some of the cultures could be cultivated without serious consequences for the gross seed yield. Common to the cultures is the fact that the production is used as seed, that the main settling criterion is high purity and germination capacity, and that the seed is free of or has a very low content of seeds of other cultures and weeds. The seeds of many cultures are the same size as those of common species of weed, which in some cases makes seed-cleaning impossible or very costly. Therefore, to meet the high quality requirements, the raw material the farmer grows in the field must largely fulfil the criteria. That means high requirements concerning cleaning of the crops. With our existing expertise and the alternative methods available today, it would be difficult, if not impossible, to maintain most of the production without herbicides. We estimate that pesticide-free production of grass seed would make the export-oriented part of seed production in Denmark unprofitable (Jensen et al., 1998a). 92% of production from the 61,000 ha cultivated is exported. Strenuous efforts are being made to develop a production of organic seed, but experience from this is limited. Denmark also has a substantial production of vegetable and flower seed. This production covers a wide range of cultures, most of which are exported. Here, we would expect serious problems in meeting the purity requirements and with profitability.

Organic soil

On organic soil, which often has special species of weeds that are difficult to control, problems could be expected with mechanical control methods. Denmark has 660,000 ha of organic soil. Estimated on the basis of individual municipalities, about 75% of this land is used for cereals (Hjort Caspersen, Pers. com.). Weighing this up in relation to the fact that not all organic soil is cultivated, we estimate the probable total acreage on which rotation is practised to be at least 150,000 ha.

8.4.4  Control of Couch grass without pesticides

Mechanical control of couch grass

Comparative analyses of the need to control couch grass with mechanical stubble-harrowing or spraying with glyphosate have been carried out by (Melander, 1993, Permin, 1990, Høstmark, 1990). According to Permin, stubble-harrowing would be needed every year to avoid proliferation. This is close to the figure indicated by Høstmark (1.2-year interval), whereas, according to Melander, stubble-harrowing each year would not, on its own, suffice in certain crop rotations. Høstmark gives a treatment interval of 2 – 3.2 years with chemical control of couch grass in a cereal crop rotation in order to maintain the status quo. The 2 years are for a control effect of 85%, while with a control effect of 95%, the interval can be increased to more than 3 years. In 1994, sales of glyphosate products corresponded to treatment of 480,000 ha with normal dosage, and the average for 1995 and 1996 was just under 600,000 ha. This consumption accords well with the need for control of couch grass according to the above sources if the weed is primarily controlled with chemical agents.

Control of couch grass at organic farms

At organic farms couch grass occurs in 15% of the observations, judged a short time after earing (Kristensen, 1998). On average, 13 couch-grass shoots/m2 were counted. It should be noted that the time of observation was not ideal for evaluation of a couch-grass population because a small stand of couch grass can easily be overlooked beside small cereal shoots. Thistles occurred in 5% of the observations, with an average of 2 thistles/m2. For comparison, thistles were not found at conventional dairy farms, while couch grass also occurred in 10-15% of the observations (953 nos.). On average, there were only 6 couch-grass shoots/m2 at these farms. It is thus possible to control both couch grass and thistles without herbicides. On the other hand, the variation from field to field would become greater because it can take several years to achieve effective control of a large population of root weeds.

Negotiations are in progress on a voluntary agreement between the milling trade and farmers on discontinuation of "pre-harvest treatment" with glyphosate in the production of bread cereals.

8.4.5  Perspectives

New machines for mechanical control

The existing mechanical methods could be made more efficient through development for more targeted use, and the development of autonomic cultivators could provide the necessary greater capacity. The need for manual weed control could be reduced using existing technology, and hand-hoeing could possibly be dispensed with in planted-out crops grown in rows, although this would at the same time mean higher costs for establishment. Similarly, a number of new technologies could in time be made operational, thereby obviating the need for hand-hoeing in sown crops grown in rows. The greatest potential lies in the use of sensors to identify crops and weeds with automotive machines. Effective detection, together with a low speed, would enable the use of a number of non-chemical technologies that have not yet been used. Whether the technologies will be developed for practical use depends on the demand for them and on support for product development.

Success requires restructuring

For successful non-chemical weed control, the whole of the way in which weed is controlled today would have to be changed. That would involve costs in connection with a changed choice of crops, higher direct energy consumption, loss of yield and quality in special situations in which the weed is not effectively controlled, investments in new tools, development and research in farm-oriented weed control, and retraining and supplementary training of farmers and agricultural advisers.

8.4.6  Conclusion

Prevention and mechanical methods

If pesticides were totally or partially phased out it would be necessary to combine preventive methods, cultural practices and mechanical methods to achieve sufficient control of weeds. Research results have shown that mechanical control is feasible in almost all crops. However, owing to a generally lower effect, it is not yet clear how the transition to mechanical weed control would affect the soil's seed pool. In crops like rape and potatoes, mechanical methods are already equal to the chemical methods used.

Problem situations

In certain situations with, for example, special types of soil, unstable weather and poor establishment of crops, mechanical control could be problematical. Crop damage after harrowing and a generally lower level of weed control would mean increased losses and adjusting crop choice and cultural practices in order to control weeds would involve increased costs. The capacity of mechanical methods is generally less than that of chemical methods, which could be problematical in conjunction with unstable weather. It is believed that there is plenty of potential for improving the existing mechanical methods to replace hand-hoeing. Switching to non-chemical methods would require considerable retraining and supplementary training, and most farms would have to invest in new machines.

Poisonous plants

Under present cultivation conditions, the occurrence of poisonous plants in Danish farm produce presents no health problems for humans. There are occasional fatalities among livestock. In Denmark, spring groundsel and deadly nightshade are considered to be the two most significant poisonous species. Restructuring for pesticide-free farming might therefore result in proliferation of the poisonous plants in question. It is hardly likely that there would be any increased poisoning risk for humans, but there could be more cases of poisoning of livestock, which would cause some production loss in the form of lower milk yields, reduced growth rates and suchlike.

Wild oat

Pursuant to the Act on Wild oat, seed-bearing wild oat must not occur during the growing season. If cereals were to be produced without pesticides, chemical control of wild oat would have to replace manual weeding. This is a realistic method in the case of relatively small populations of wild oat, but not in the case of large ones. Here, it would be necessary to change the crop rotation towards production of coarse fodder in order to reduce the population.

Problems in seed production

Grass and clover seed, as well as vegetable and flower seeds, cover a wide range of cultures. Over 90% of our production is exported. All the cultures are grown for seed. The main price criteria are high purity and germination capacity, in addition, the seed must be free of or have only a very low content of other cultures and weed seed. These criteria set very high requirements concerning the purity of the crops - requirements that, for the greater part of production, would be difficult to meet without the use of herbicides, given our present level of expertise.

Control of couch

Couch grass can be controlled without herbicides on most soils. In several studies of the crop rotations practised at arable farms, the need for mechanical harrowing after harvesting has been compared with the need for glyphosate treatment. Mechanical stubble-harrowing is required every year to avoid proliferation of couch grass in certain crop rotations, as replacement for glyphosate treatment approximately every four years. We have reasonably good experience of controlling couch grass in organic dairy farming, but the crop rotations practised in that form of farming are very different from those practised at the various types of arable farm. Experience from organic farming shows that thistles can be a major problem. Variations in the quantities of root weeds from field to field would become greater without access to pesticides, as it can take several years to attain effective control of large stands of such weeds.

8.5  Alternative methods for growth regulation

Growth regulation prevent lodging

Some use is made of growth regulation in cereals to prevent lodging. The cereals are typically treated during elongation and the agents used have a shortening effect on the internodal length and strengthen the stem itself. Experience shows that the spraying is done to safeguard against lodging. The use of growth regulators has decreased in recent years in step with the cultivation of stiff-stemmed varieties and use of less nitrogen. The results of trials on varieties with strong stems have shown that it seldom pays to spray. Growth regulators are used almost exclusively in winter cereals. The treatment frequency index was reduced from 0.6 in the period 1981-85 to 0.1 in 1997. (Environmental Protection Agency statistics on pesticide consumption). The highest level of consumption is in rye, where the varieties cultivated have relatively poor straw strength, and where lodging can greatly reduce the baking quality of the rye. There is also a small consumption in certain species of seed grass, including cock's foot. On average, 11% of the seed acreage was treated in 1997.

8.5.1 Growth regulation in cereals

Alternatives in cereals

As mentioned, there are good strong-stemmed varieties of wheat that have only a slight tendency towards lodging. In addition, the risk can be reduced by sowing fewer grains per m2 – 200-250 instead of 280-350 and using less nitrogen (10-30 kg/ha).

The disadvantage with some of the strong-stemmed varieties is that they generally have difficulty competing with weeds. Terra is a good example of a tall, relatively soft-stemmed variety of wheat with good resistance to disease and good competitiveness with weeds. To minimise problems with lodging in such a crop it is necessary to reduce the amount of nitrogen applied.

In 26 trials in 3 varieties representing different types, carried out by the Danish Agricultural Advisory Centre in 1995-97, reliable additional yields for growth regulation were not achieved (Petersen et al., 1997). The varieties in question included Terra. The lack of yield increase was due in part to the fact that there was no serious, long-term lodging of the crop in the trials.

Problems in rye

In rye there is a generally greater risk of lodging. Rye from fields affected by lodging is usually of a poorer baking quality than rye from standing crops owing to moister conditions during ripening close to the soil. There are also practical problems in the form of increased harvesting difficulties when the ears of rye have to be combined from lodging. Crops affected by lodging have a greater risk of a lower cereal yield, a higher water content, a lower falling number, which impairs the baking quality of the grain, a lower grain weight, more stones in the product harvested, a reduced harvesting capacity, higher drying costs and a risk of total rejection as bread rye at falling numbers lower than 100 (Anon., 1998). Generally speaking, there are more trials in rye than in wheat that show positive additional yields for growth regulation (table 8.13). Trials with different, current varieties of rye have shown some difference in the varieties' response to growth regulation - something that is to some extent reflected in the assessment carried out of the lodging of varieties (table 8.14). However, positive additional yields have been achieved in several varieties with growth regulation. There are thus no varieties that can definitely be recommended if one wants to avoid lodging.

Due to consumer demand for bread from cereals that have not been growth-regulated, increasing attention is being paid to ways of cultivating rye without growth regulators. One of the problems focused on here is how to avoid lodging via cultural practices and what costs this involves.

Alternatives in rye

In rye, trials are in progress to determine the effect of alternative methods on growth regulation. Elements that have an effect and that are being studied are postponed sowing, reduced quantity of seed and reduced application of nitrogen. Too large a reduction in the quantity of seed (<150 plants/m2) increases the risk of ergot. In an open stand many side shoots form, which flower unevenly, increasing the chance of ergot infecting the flower formations. On good soil, sowing would have to be postponed until the beginning of October and the quantity of nitrogen would have to be reduced to 80 kg/ha to reduce the risk of lodging. It is estimated that that would reduce the net yield by 6-7 hkg/ha, necessitating an additional price of DKK 7-8/hkg to compete with growth regulated production (Anon., 1998).

Table 8.13
Additional yield in different years with growth regulation of rye (2 applications: Cycocel at gs 30-31 and Terpal at gs 37-39),with a breakdown into trials with and without lodging (Annual Reports of Field Trials).

Table 8.14
Additional yield in different varieties of rye treated with growth regulators (1.5 l Cycocel + 0.5 Cerone). Annual Report of Field Trials, 1996-97)

2-year trials with a reduced quantity of seed sown and reduced application of nitrogen have shown that it is possible to avoid the use of growth regulators in rye (Petersen et al., 1998). However, multi-year trials are needed to get a clear idea of what that implies for stability of cultivation and yield.

On very light soil without irrigation, where rye has traditionally been grown, there is less risk of lodging. The risk is thus greatest on better soils. Only 20% of the present rye production of approx. 500,000 tonnes is used for bread. The remainder is exported or used in mixed fodder. The demand for rye for bread can be covered with about 22,000 ha.

Voluntary agreements

Efforts are being made to get a voluntary agreement established between the milling trade and farmers on discontinuation of the use of growth regulators in the production of bread cereals.

8.5.2  Growth regulation in seed grass

Growth regulation is used in certain species of seed grass to minimise the risk of lodging and straw breakage, thereby reducing the risk of harvesting losses and problems. Trials with cock's foot grass and red fescue show that a better yield can be achieved with one or two applications of the growth regulator Cycocel (table 8.15).

Table 8.15
Growth regulation of seed grass with two applications of Cycocel during stem elongation. Trials from Annual Reports of Field Trials (1986 and 1990).

In a trial series in red fescue, the interaction between the amount of fertiliser applied (40 or 60 kg/ha) and growth regulation has been investigated. The results show no certain interaction or greater need for growth regulation at the highest amount of nitrogen.

8.5.3  Growth regulation in pot plants

Growth regulation is widely used in pot plant cultures to create compact plants with greater flower density. There are seven approved agents for growth regulation in greenhouse cultures. The most widely used is Cycocel. In many cultures, a combination of chemical and alternative methods is used today. One extensively used method is "negative dif", which is based on regulation by means of light and temperature.

Research in alternative methods

Current research focused on reducing growth by various methods, including applying less phosphorus, indicates that there are methods that could in time be used to reduce the use of chemical growth regulators (Hansen & Nielsen, 1999). At the present time, a combination of "negative dif" at the beginning of the growing period and chemical regulation is used in many cultures, but it has not been determined whether the agents can be omitted altogether or whether the use can be reduced dramatically (Ottosen et al., 1998).

8.5.4  Conclusion

Winter cereals

Growth regulators are used in about 10% of winter cereals – particularly in rye. A little is also used in seed grass and ornamental plants. There are good prospects for the use of alternative methods in winter wheat, to minimise the risk of lodging. The risk is thus small when cultivating varieties of good stem strength and reduced plant counts. If less strong-stemmed varieties are grown, it might be necessary to reduce the amount of nitrogen applied by 10-30 kg/ha. There is a considerable risk of lodging in rye on good soils, but less risk on sandy soil. There are no varieties of rye that can completely eliminate the risk of lodging, although some varieties can contribute to reducing this risk. The risk could also be reduced by postponing sowing until the beginning of October and reducing the amount of seed sown and the amount of nitrogen applied. Together, these measures would reduce the net yield by 6-7 hkg/ha.

Seed grass

The use of alternative methods of growth regulation in seed grass has only been clarified to a limited extent. We can expect a reduction of stability of cultivation in certain soils until alternative methods of growth regulation have been developed.

Pot plants

In pot-plant production, growth regulators are used mainly to promote the especially richly-blossoming and compact plants that have the best sales value. No methods of replacing chemical growth regulators are immediately available for pot plants. Considerable research would be needed to determine whether there are alternative methods for the many different pot plant cultures.

8.6  Biological control

Here, biological prevention and control in agriculture is defined as placing living organisms in the crop. The methods include use of both "macrobiological" animals and microorganisms.

8.6.1  Present use and potential in greenhouses

Biological control of pests in greenhouses

Biological and microbiological control of pests is widely used in greenhouses today. At present there are products of 50-60 species of useful organisms for use against the main greenhouse pests in Denmark (table 8.16). In greenhouse vegetables, biological pest control is routinely practised on largely the entire acreage, while it is used on about 30-35% of the acreage used for greenhouse-grown ornamental plants.

Potential in greenhouses

The present scope of biological pest control in greenhouse vegetables is largely equal to the potential, while in greenhouse-grown ornamental plants, it is below the potential, given our present level of knowledge and experience. That is due to special factors concerning ornamental plants – including the fact that ornamental plants include many species that are attacked by a wide variety of pests and that the damage threshold is low. Some other special factors are also involved – for example, that biological control can be costly, that it is new, so growers are unaccustomed to it, and that it may be associated with bad experience. In addition, the requirement made by countries importing pot plants concerning complete freedom from pests plays a role, and there is a lack of essential knowledge and experience in a number of areas (Enkegaard et al., 1998).

Microbiological prevention/control and diseases

Microbiological prevention and control of plant diseases in greenhouses: Internationally, there are now about 30 microbiological organisms (MBOs) for prevention and control of plant diseases in agriculture. Of these, around 20 have been developed for use in greenhouses. Only a few agents are marketed in Denmark today. The use of MBOs in Denmark is at present restricted (table 8.17).

Work is in progress on the establishment of an approval procedure for MBOs in Denmark and the EU, but no organisms are at present officially approved. Dispensation has been granted for the time being for sale of the organisms that are marketed.

Table 8.16  Look here!
Pests in Danish greenhouse crops and the useful organisms that can at present be used to control them – depending on the species of pest, the species of plant and specific cultural practices. It should be noted that each category of pest and useful animal (e.g. aphids and ichneumon) comprise up to several species. In all, there are 46 different useful organisms.

Table 8.17
Diseases in Danish greenhouse crops that are at present controlled with microorganisms and the acreage on which the organisms are used. A detailed breakdown by disease is not possible because MBOs are often used prophylactically by adding them to the growth medium on the basis of an assumption that they have an effect on one or more of the diseases mentioned.

8.6.2  Future perspectives in greenhouses

Considerable potential in greenhouses

The future potential for biological pest control in greenhouse vegetables is thought to be limited, whereas the potential in greenhouse-grown ornamental plants is considerable. It is difficult to say how much of this potential can be realised within a 10-year period.

In greenhouses, we expect to see specially developed MBOs for controlling some selected, serious root pathogens and some few leaf pathogens, such as mildew and grey mould. MBOs are unlikely to be developed specifically for control of pathogens of limited economic importance in small cultures or for control of so-called 0-tolerance pests.

Realisation of the potential for biological prevention and control of diseases and pests in Danish agricultural and market-garden crops within a 10-year horizon will depend on several factors, including the price and availability of the biological organisms, the availability of pesticides, the situation with respect to pests, political measures and the scope of research in Denmark and elsewhere.

8.6.3  Biological and microbiological prevention and control in fields

Very limited use in fields

Very little use is made of biological control in fields. Bacillus thuringiensis is used against butterfly larvae, nematodes are used against weevils, virus is used against cutworm, and predatory mites are used against mites. The acreages on which these methods are used are very small (table 8.18).

 In fields, MBOs are at present being tested against seed-borne diseases in cereals. We expect it to be possible for microbiological control measures to be used in fields against these types of diseases in both vegetables, flowers and agricultural crops within the next few years.

Table 8.18
Pests in Danish field crops that are at present controlled with useful organisms, and the acreage on which these are used. Readers are referred to table 1 for the full names of species.

8.6.4  Perspectives for field use

Uncertain potential in fields

In the short term, the sub-committee expects some expansion of biological control of pests in fields, using agents that are at present marketed abroad and under development in Denmark. In particular, there will be products against aphids, butterfly larvae, mites and soil-borne insects. This expansion of biological control will mainly be directed towards high-value crops: specialised crops, ornamental greenery and nurseries. In the long term, more possibilities could be realised. Potentially, all pests can be controlled biologically – outdoors as well as indoors. Biological control of a single pest affects many of the main crops, e.g. aphids in cereals, or different forms of biological control against all pests in a single crop could probably be achieved with an intensive R&D effort in Denmark and abroad, combined with other political measures.

The sub-committee estimates that development of MBOs for outdoor prevention and control of other diseases than the seed-borne diseases and fungal diseases that affect germination would, except in a very few cases, require a longer time horizon than 10 years. The use of MBOs will depend on continued intensive research and close cooperation between researchers, producers of MBOs, agricultural advisers and farmers.

8.6.5  Possibility of using plant extracts and similar

Limited experience and knowledge

Plant extracts and non-synthesised natural substances are attracting renewed interest as alternatives to chemical control agents. However, very little is known about their effect spectrum and use, so detailed studies under relevant conditions are needed before their use can be recommended. Extensive research and trials are necessary in order to be able to evaluate continuously products introduced in this field (e.g. pyrethum, garlic extracts, stinging-nettle extracts, neem, etc.).

There are at present 10 approved active ingredients that occur naturally, including five extracted from plants (soaps, pyrethrin, soya oil, rotenon and citronella oil). Four of these are used as plant protection agents, although outside the actual agricultural sector. Two ingredients are extracted from the mineral kingdom (sulphur and paraffin oil) and two from the animal kingdom (gelatine and blood meal), while one is microbiological (toxin of Bacillus thuriengensis). With the exception of blood meal and citronella oil, these substances are used for controlling insects (Memorandum from the Danish Environmental Protection Agency, 17 December 1998).

8.6.6  Conclusion

Biological pest-control methods, which include both useful animals and microbiological agents, have great potential within greenhouse production. They are already used extensively in vegetable production, while there is still an unexploited potential within greenhouse production of ornamental plants. Effective methods of preventing and controlling diseases in greenhouses are still limited. Outdoors, biological pest-control methods are deemed to have some potential within specialised crops, whereas biological prevention and control of diseases within a short time horizon is only deemed to have a potential in the case of seed-borne diseases and fungi that affect germination.

There is a need for a major research effort in this area, aimed at developing methods for outdoor use. Besides that, there is a need to improve the use of biological control of diseases and pests in ornamental plants cultivated in greenhouses and to spread the use of this method of control.

8.7  Effect of alternative methods on the need for nutrients

Lower yield means reduced need for nitrogen

With a reduced yield as a consequence of weeds, diseases and pests, changes in optimal nitrogen of 1-1.25 kg N/hkg cereal (Olesen et al., 1998) must be expected. In the event of deviations from normal yields, the sub-committee recommends use of the Plant Directorate’s fertiliser guidelines, where a 1 hkg change in cereal yield in winter cereals gives a 1.3 kg change in the need for nitrogen, and in spring cereals, a 1 kg change in the need for nitrogen. If a lower yield is largely due to weeds and pests, the need for nitrogen might actually not fall as indicated because both weeds and pests absorb nitrogen and can contribute to reduced nitrogen utilisation.

Effect of nitrogen on pests

Reducing and splitting-up of the amount of nitrogen applied could to some extent reduce the risk of both pests and diseases. The fertiliser should also be used strategically to strengthen the crop’s competitiveness to weeds. This is an area in which the sub-committee believes that there is a clear advantage over organic production.

Mechanical control of couch grass in the autumn is thought to have a negative side-effect in the form of increased leaching of nitrogen in winter owing to increased nitrogen-mineralisation, while mechanical weed control in spring is also known to increase nitrogen metabolism. This is often seen as a positive effect on the crops, which have good possibilities of utilising the quantities of nitrogen released during their growing season.

An increase in the acreage with second crops to replace winter cereal, such as winter-green fields, can help to reduce leaching of nitrogen in the winter-time.

8.8  Use of damage thresholds

Growing attention has been paid to damage thresholds up through the 80-90s because such thresholds are essential for need-adjusted use of pesticides.

8.8.1  Weeds

Use of reduced dosages

Preliminary trials have also shown that use of a crop’s competitiveness and other preventive measures can reduce the use of herbicides still further. However, only limited data and experience are available concerning this. That also applies to the combination of preventive measures and need-based, non-chemical weed control.

PC-Plant Protection

In Denmark, the most widely used decision-support system for weed control is PC-Plant Protection/weeds. This is a robust system that is targeted on recommendation of herbicides and dosages. The program is generally based on economic optimisation. Despite relatively widespread use of PC-Plant Protection/weeds, the treatment frequency index has not fallen in the last few years due in part to growing acreages with winter cereals and consequently increased proliferation of annual grass weed, which has often meant a need for two herbicides – one for monocotyledonous weed and one for dicotyledonous weed. In systems under development, greater importance is being attached to preventive measures and need analyses. We do not have a decision-support system or guidance system for non-chemical weed control at present; nor do we have damage thresholds or a decision-support system for weed control in outdoor vegetables or forestry.

8.8.2  Diseases and pests

Warning models

Over the years, economic damage thresholds have been developed and used as the basis for need analyses of several important diseases in the main agricultural crops. In a cooperation between the Danish Agricultural Advisory Centre and the Danish Institute of Agricultural Sciences, agricultural advisers pass on results from a number of weather-based warning models and from monitoring of certain diseases and pests in important crops. Table 8.19 lists monitoring and warning systems in Denmark. Most of the systems are also communicated by means of the Internet-based system Planteinfo.

Registration system for diseases and pests

Since 1992, the Danish Agricultural Advisory Centre has operated a registration system (Nielsen & Juhl, 1996). Here, the occurrence of the principal diseases and pests is monitored in the part of the growing season in which treatment may become necessary. Occurrence is monitored in untreated plots throughout the growing period and in a broad selection of varieties.

In cereal crops, the different damage thresholds are combined in PC-Plant Protection/diseases and pests. Results from four years’ trials with PC-Plant Protection are shown in table 8.20.

PC-Plant Protection

As will be seen, there are equal net yields after all treatments. In accordance with PC-Plant Protection, the principles of plant protection in surrounding fields and the recommendations from the farmers’ groups, a considerably lower treatment frequency index was used than according to the standard treatment and also reduced dosages of fungicides but full dose of insecticide. The very similar results from the last three treatments are due to the fact that principles from PC-Plant Protection are often also used by the farmers’ groups and in the general principles on which many farmers base their treatments.

In the disease area, PC-Plant Protection/diseases in cereals has contributed towards a more targeted use of fungicides. Among other things, it has supported the use of reduced dosages (Secher et al., 1995). For example, the treatment frequency index for cereal fungicides has been reduced by 50% compared with the reference period in the first pesticide action plan.

Table 8.20
Treatment frequency index and yield with PC-Plant Protection for diseases and pests. 26 trials in wheat from 1995-1998 with comparison of PC-Plant Protection with standard treatment (2 x 0.5 l Tilt top+sumi alpha in 1995-97 and 0.33 l Tilt top/0.33 l Amistar in 1998), the treatment in surrounding fields and the recommendation from a local Farmers’ group (Source: Annual Reports of Field Trials, 1997-98).

Treatment	TFI	Yield and additional yield hkg/ha	Net yield hkg/ha
Untreated Standard treatment	- 1.6	71.6 9.3	2.6
PC-Plant Protection Treatment in surrounding fields Farmers’ groups	0.8 0.9 0.7	8.1 7.9 7.9	3.5 3.2 3.8
LSD95		2.5

Warning against potato blight

PC-based decision-support systems (Negfry) are being developed within potato blight, where there is a large consumption of fungicides. Negfry’s calculations of the need for treatment with fungicides are based on knowledge about the fungus’s biology; temperature and moisture also play a role (Hansen & Holm, 1996). Negfry has shown promising results with possibilities of reducing the number of applications by 2-3. In a few seasons, delayed application has been recommended in some localities, which reduces the confidence in a high-value crop like potatoes, where an early attack of potato blight results in substantial yield losses and reduced quality. Negfry is still under development.

In the case of pests there are several operational warning systems and damage threshold models. In many crops these tools have been very targeted and have led to a marked change in the use of chemical control. There are warning systems for a number of pests in the main agricultural crops with the exception of rape.

8.8.3  Potential for use of damage thresholds

15% of farmers use PC-Plant Protection today

Increasing use has been made of decision-support systems, warning systems and damage thresholds for plant protection in recent years, primarily because of greater awareness of and interest in more environment-friendly farming, but also because the systems offer farmers the possibility of saving on spending on chemicals. Today, about 15% of Danish farmers use PC-Plant Protection directly or indirectly (Svendsen et al., 1997).

Wider use of decision-support systems depends on several factors. The main ones are:

	the systems’ technical foundation and further development
	the farmer’s professional interest and training in the use of decision-support systems and the time he has available for using them
	feasible and reliable detection and monitoring methods and the cost of using them
	the value of the additional yield from control and the costs involved.

Aversion to risk

Another and perhaps equally important factor in the spread and use of decision-support systems is the farmer’s willingness to take risks. For many years it has been "good farming practice" to keep fields clear of weeds, diseases and pests. This ensures a stable and high yield that is not necessarily economically optimal because farmers do not like admitting to an over-consumption of pesticides. Under-consumption, on the other hand, is visible and directly measurable in the yield. In most years less treatment is often sufficient, but in unfortunate circumstances things can go wrong. Seen over a span of years, need-based control will be economically optimal.

8.8.4  Perspectives

Major future potential

The sub-committee believes that there are good possibilities of reducing herbicide consumption still further, partly through greater use of mixed herbicides that are adjusted to the composition of weed species and the total pressure of weeds. In the case of diseases and pests, we think that further knowledge concerning the relationship between cultural practices and climate could help towards further reduction of consumption. Assumptions for reduced consumption fall into the following categories (Christensen et al., 1998):

A. widespread dissemination and use of existing damage thresholds and warning and decision-support systems

B. further development of existing damage thresholds and warning and decision-support systems and development of new systems that take account of the following factors:

	development of robust, simple and effective monitoring methods
	strengthening of the data and experience base for need-based planning and decisions
	adjustment of consumption to falling prices for plant products as a consequence of the EU’s dismantling of subsidy schemes and increased taxes on pesticides
	use of site-specific plant protection, with control measures limited to those areas of a field in which there is a need for prevention and control of pests
	integration of preventive methods, including choice of crop rotation, soil preparation, competitiveness and resistance properties, in damage threshold models and warning and decision-support systems
	development of robust warning systems that combine use of chemical and non-chemical/biological control.

Considerable potential for reduction

In the opinion of the experts, if several or all of the above-mentioned assumptions can be fulfilled, pesticide consumption in many crops, including cereals, could be reduced by 20-25% in relation to what is possible with the present computer-based decision-support systems (Christensen et al., 1998).

8.8.5  Site-specific plant protection

Possibilities of patchwise treatment

Research is now in progress on site-specific plant production and plant protection, with control measures limited to those areas of a field in which there is a need for prevention or control of pests. The sub-committee believes that the development of methods for handling such a system would help to reduce consumption considerably. Trials and research have shown that purposeful use of fertiliser, pesticides and other factors related to the phase-out can contribute to satisfying environmental requirements and simultaneously optimise production economically. Also under development is advanced spraying equipment that operates on the basis of a treatment map and in which one or more herbicides are only mixed with water when this is pumped out into the pipe system. Spraying systems of this kind, in which the water and the pesticide(s) are kept apart, could eliminate problems with spray residue. The new technology also offers the possibility of automatically storing data on location-specific conditions and treatments that can be used as an experience/database in subsequent years and as documentation for the treatment carried out. A prerequisite for wide use of site-specific plant protection is automation of pest registration. Several technologies that could be used for that purpose are expected to become available in the next few years.

8.8.6  Conclusion

In recent years, decision-support systems have been developed for several of the main agricultural crops as support for the farmer when evaluating the need for prevention and control measures. Such decision-support systems have contributed significantly to reduction and adjustment of dosages, not only through direct use of the programs, but also through advisory services and newsletters from the Danish Agricultural Advisory Centre. Although the use of damage thresholds and decision-support systems has spread, it has not been possible to reach all farmers.

Damage threshold systems still need to be developed for a large number of crops and there is plenty of room for improvement of several of the existing systems.

In a number of crops it is believed that a 20-50% reduction could be achieved by combining decision-support systems and chemical and non-chemical control methods.

8.9  Use of genetically modified organisms (GMOs)

Reduction potential in beets

In connection with the desire for greater use of resistant varieties, it is interesting to determine whether new breeding techniques based on molecular-biological methods can be expected to produce more and faster progress. The question is whether these methods can decisively reduce the losses from diseases and thus change the need for pesticides. It is also of interest to determine whether plants with built-in herbicide resistance can significantly change the present consumption.

8.9.1  Present situation

Only a few genetically modified plants are ready for immediate introduction on the Danish market. They include herbicide-resistant varieties of beet and rape and insect-resistant maize. Only limited official studies of the effect they will have on pesticide consumption have been carried out. However, it is believed that:

	Cultivation of glyphosate-resistant and glyphosinate-resistant sugar beet and mangolds could lead to a significant reduction in herbicide consumption compared with present-day consumption in the cultivation of these crops. As a cautious guess, there could be a saving of about 2 kg active ingredients per ha with glyphosate-resistant varieties of beet. Trials have shown that it is possible to achieve a reduction from the present level of 2800 g traditional beet herbicides to 540 g glyphosate/ha (Elbæk-Jensen, 1998).
	Cultivation of herbicide-resistant rape is not expected to result in a significant reduction in herbicide consumption. Today, only a few herbicides are available for this crop and there is a trend toward increasing use of mechanical weed control. However, if this trend spreads, there might be a risk of increased herbicide consumption if and when the herbicide-resistant varieties are widely used.
	Cultivation of glyphosate-resistant or glyphosinate-resistant maize in Denmark would mean that the agents used today could be replaced with glyphosate or glyphosinate. This would probably not result in any significant change in total consumption.

8.9.2   Future perspectives

Limited effect within 10 years

In Denmark, the general view is that genetically modified plants with good resistance to disease could have a significant effect on the present need for pesticides. Genetic modification to achieve improved resistance to fungal diseases still seems to be at a preliminary stage. A great deal of knowledge has been built up concerning the basic mechanisms of attacks by fungal diseases and fungus-plant interactions, but relatively few field trials have yet commenced. The first varieties with effective resistance to fungal diseases will definitely not appear on the market for another 10 years.

Widespread use of GMO in the USA

In North America and elsewhere, vast areas of land (several million ha) are used for cultivation of genetically modified soya beans, maize and rape that are resistant to herbicides or that have had pest-resistant genes built into them. This has reduced the consumption of herbicides and insecticides.

Possible disadvantages in connection with the cultivation of genetically modified plants are evaluated in the Sub-committee on Environment and Health’s report.

8.9.3  Conclusion

In the GMO field in Denmark, most progress has been made in the development of herbicide-resistant plants that have a possibility of being marketed within a few years. The introduction of genetically modified, herbicide-tolerant varieties of beet is expected to result in a significant reduction in herbicide consumption - about 1 kg active ingredient per ha. Herbicide-tolerant rape and maize are not expected to result in any major reduction. With our present knowledge, it is not possible to predict how much genetically modified plants will affect the consumption of pesticides in Danish agriculture in a coming 10-year period. Intensive research is going on in this field all over the world and will undoubtedly in time greatly change our cultivated plants. Particularly if methods were developed for rapid breeding of genetically modified, disease-resistant plants, we would expect the basis to be created for reducing losses from disease.

8.10  Possibility of new spraying techniques reducing pesticide consumption and undesirable environmental techniques

8.10.1  Use of new spraying technology to reduce pesticide consumption

Sprayers and reduced dosages

There are different types of sprayer on the market, including hydraulic sprayers with and without air assistance and air-assisted sprayers. Over the years a number of trials have been carried out to determine whether a better biological effect that makes it possible to use lower dosages can be achieved by using different types of nozzle, quantities of water and pressure. These trials have generally shown that, in relation to the most widely used spraying system (hydraulic sprayers with flat sprinkler nozzle, about 100-200 l water and a pressure of 3-5 bar), there are only limited possibilities in Denmark of further reducing present-day dosages.

A technology that combines site-specific application of pesticides via GPS (Global Positioning System) is being developed that will open the way for more varied application on a field basis on the basis of a need analysis (Christensen et al., 1998).

8.10.2  Possibility of reducing drift by the choice of spraying method

A spraying technique that combines good effect with minimum drift can be used is some situations but not in others.

Effect of drift

Various factors affect the extent of drift. In calm weather, most of the liquid sprayed reaches the crop/the weed. In windy weather or periods with atmospheric instability, some of the liquid is transported out of the areas treated. The amount that drifts depends on:

	the wind velocity and relative humidity
	the mean drop size and drop-size distribution, which depend on the type and size of nozzle, the hydraulic pressure and the surface tension and viscosity of the liquid sprayed
	the distance between the mouth of the nozzle and the spraying target (the height of the boom)
	the spraying equipment used (conventional, air-assisted, screening, size and design of the spraying equipment, electric charging of the drops, etc.).

The climatic conditions – particularly the wind velocity – are critical for the extent of drift. The practitioner cannot affect this. He can, on the other hand, ensure that there is sufficient spraying capacity on the property – also in peak-load periods. Spraying in unfavourable conditions can thereby be avoided or limited in years with significantly fewer favourable spraying hours than normal. In Denmark, strong wind is the most limiting factor for the kind of spraying that enables use of the lowest dosages (Jensen et al., 1998b).

Morning spraying causes least drift

Spraying conditions are normally best in the hours of morning, when the relative humidity is at its highest and wind velocity is at its lowest. Climatic conditions that are favourable with respect to effect also reduce drift. Air assistance on the basis of Hardy’s Twin Principle is another example of the fact that drift during field spraying can be reduced without reducing the biological effect.

New low-drift nozzles minimize drift

On the other hand, the choice of nozzle, and thus the drop size, is a point where there will be a conflict in some cases between the two objectives: effect and drift. That is because small drops settle and cover crops best during spraying with leaf agents, but small drops also involve the highest risk of drift. Trials carried out so far with small, low-drift nozzles have shown that it is possible to maintain the biological effect and at the same time reduce the risk of drift compared with the traditional flat-sprinkler nozzles that are otherwise used. It has not yet been documented whether low-drift nozzles and coarse-atomising air-injection nozzles can replace the traditional flat-sprinkler nozzles in all situations without serious loss of effect.

8.10.3  Possibility of reducing point-source contamination with pesticides

Problems during filling and cleaning

During filling and cleaning of sprayers there is a risk of contaminating the surroundings – the groundwater, the farmer’s own wells, other wells and watercourses – via drainage systems from the farm. There is not much documentation showing that washing water or pesticide spills cause serious groundwater pollution, but a few sources suggest that farm wells and other wells can be seriously contaminated with pesticides (Anon., 1995). Although there is very little documentation showing that washing yards and pesticide spills cause serious groundwater pollution, there are a number of factors that mean that the said areas may be particularly critical because the pesticides reach the surroundings in very concentrated form. In addition:

	The area load is high because the same washing/filling yard is often used regularly for many years.
	Washing/filling yards are often surfaced with gravel and stone without any topsoil, which considerably increases the risk of pollution. With topsoil, microorganisms that can cause biological degradation are removed and the potential for bonding and retention is reduced. That means that the rate of transportation of water and pesticides is relatively high.
	For the same reason, weed control on farmyards and lanes involves a risk to well water and groundwater.
	Remnants of pesticide are led to the soil with a relatively large quantity of water increases the risk of percolation.
	Some yards are connected with drainage pipes or waste pipes, which means that watercourses and drain water can be affected.
	Handling often takes place close to wells and drilled holes.

Need for information campaign

All in all, we expect increased focus on the use and handling of pesticides, together with information and guidance, to help to minimise point-source contamination of groundwater, farmers’ own wells, other wells and watercourses. This will result in improved conditions (Jensen et al., 1998b).

The following points should be impressed on farmers:

	Filling of concentrated pesticide and washing of spraying equipment should be done on an area with soil covered with vegetation, a bio-bed or a concrete-surfaced yard with drainage to a separate tank – possibly a liquid-manure tank. A grass-covered area is very suitable. The grass prevents run-off and the formation of flow channels in the soil.
	Residual spraying liquid should never be disposed of by emptying it out on topsoil or a surfaced area or into a liquid-manure tank. The right way to dispose of it is to dilute it and spray it on the crop. The heavily diluted washing liquid should be sprayed over as large an area of the field as possible. The diluted liquid can also be discharged into a liquid-manure tank.
	Washing and filling should never be done on areas surfaced with gravel or concrete, where the washing water and waste can percolate to sewers, drains or watercourses. In addition, washing and filling should never be done near wells.
	Packaging from pesticides must be disposed of via the municipal waste collection scheme.

It is important for users of pesticides to be aware that even very small, accidental spills can cause pollution of groundwater and water courses.

EU-standards for sprayers

At European level, common standards are being prepared for field sprayers. Work is in progress on:

	obligatory rules for mounting of a clean-water tank on sprayers so that the sprayer can be washed through before it leaves the field.
	In addition to a clean-water tank, there must be an extra water tank for washing hands.
	common codes for filling equipment and equipment for cleaning chemical packaging.

Compliance with a standard is normally voluntary, but in some countries it will form the basis for approval of spraying equipment. In Denmark there is already a trend towards voluntary installation of clean-water tanks on sprayers. Clean-water tanks can be added to most sprayers without serious technical problems (Jensen et al., 1998b).

8.10.4  Conclusion

Compared with the spraying technology used at present, the introduction of alternative, known spraying technologies offers only a limited possibility of reducing the quantities of pesticides used. There is, however, an exception to this in the form of site-specific application, which will in time enable varied application at field level by means of GPS technology.

Possibilites of reducing drift

The risk of drift can be reduced by using new nozzles that minimise the proportion of drops that are particularly exposed to drift. Some of the new types of nozzle increase the capacity in relation to earlier sprayers, which at the same time improves the possibility of getting spraying done in calm weather. Within fruit growing, we also think that new screened equipment, which collects spray residue, offers good possibilities of reducing the impact on the surrounding environment.

Information on the risk of point-source contamination

We consider that, with intensified information and guidance for farmers on filling and cleaning sprayers, there are good prospects for minimising point-source contamination of groundwater, farmers’ own wells, drilled holes and water courses in connection with the filling, washing and cleaning of sprayers. These possibilities require only a limited investment.

8.11  New pesticides

Development of low-dose agents

New pesticides are constantly being developed for use in agriculture. It is difficult to judge whether new pesticides will in the future change significantly the present consumption of pesticides. There has been a general trend towards agents containing a smaller quantity of the active ingredient. For example, the average content in agents developed in the 1940s corresponded to 1.5 kg/ha, while the average content of those used today is just over 100 g/ha. The cost of developing new agents has risen very greatly in the last 20 years, in step with the increasingly stringent requirements concerning the agents’ environmental profile. This has at the same time reduced the success rate for finding new agents.

Today, there are some pest/crop combinations for which there are no pesticides. Traditionally, there has, for example, been no means of preventing or controlling take-all disease in cereals. Recently, however, seed-dressing agents have been introduced that can reduce the risk of take-all disease (Beale et al., 1998, Löchel et al., 1998).

As resistance to many products is constantly increasing, constant development of products that act through other mechanisms is vital if we are to ensure continued effective pest control.

Some trends can, however, be described.

8.11.1  Herbicides

In the last few years some very effective agents have been developed that are active in very small dosages (<100g active ingredients per ha). In the same period there has been some focus on the development of herbicides from naturally occurring chemical substances, although success has so far been limited. Up to the present time, only one of these herbicides has been marketed: Bialafos in Japan. Glyphosinate (Basta) is a synthetic copy of this agent. In addition, there has been a development towards herbicides that affect processes that are specific to plants, which has resulted in falling toxicity to humans and animals. As a result of these developments, there are now fewer different modes of operation than there used to be, which increases the risk of weeds developing resistance.

8.11.2  Fungicides

Strobilurines

In connection with the development of fungicides, attention has similarly been focused on utilising naturally occurring active substances from fungi and bacteria. Of the new types, strobilurines, in particular, have proved to be very effective, and at the same time broadly acting, against some of the principal pathogenic fungi that attack cultivated crops. Compared with fungicides used earlier, the strobilurines have proved to increase additional yields significantly – for example, by 4-10 hkg/ha in wheat. The reason for this is that, unlike earlier agents, the strobilurines already attack the fungi at the spore stage and thus do not trigger the plants’ own defence system. The new agents have improved the economy of control of fungal diseases and mean that it will remain financially advantageous to combat fungal diseases in cereals despite lower cereal prices.

Other new fungicides act by mobilising the crop’s own defence mechanism. The agents do not in themselves have any toxic effect on fungi but have proved able to impede certain fungi after the plant’s defence system has been activated. An example of this group is benzothiazol (Bion), and a substance extracted from a brown alga (Joubert et al., 1998). Bion is marketed in Germany and some other countries as a "plant activator".

8.11.3  Insecticides

More seed-dressing agents

New types of insecticide are characterised by the fact that they are also active in very small quantities (5-20 g/ha) and that agents are being developed with action mechanisms that differ from those previously used. Examples are triazamat (Aztek) and imidaclopride (Confidor). These make it possibly to control species of pest that have developed resistance to the agents used previously. Within pest control in beets, for example, increasing use is being made of seed-dressing agents that are able to replace 1-2 applications of, e.g. pyrethroids. Seed-dressing agents are generally thought to have a minor environmental impact because less than 1% of the acreage is directly exposed to the pesticide.

Another innovation within insecticides is the development and use of attractants (including pheromones), which have the effect of stopping proliferation of the insects or means that they are caught in a kind of trap with pesticides, where they are killed. These methods are believed to be considerably more environment-friendly because the agents are not applied broadly in the culture (Jones & Langley, 1998).

8.11.4  Conclusion

New pesticides are being constantly developed to replace old agents, and agents are being developed that offer new means of controlling diseases, including take-all disease. The agents are generally used in smaller quantities than previously and there is a trend towards increased use of certain insecticides as seed-dressing agents. The search is being intensified for active ingredients derived from nature's own substances, although significant modification is often needed to achieve stable and suitable pesticides. The success rate for finding new agents has decreased owing to more stringent environmental and health requirements concerning the agents.

As resistance to many products is continuously increasing, constant development of products that act through other mechanisms is vital if we are to ensure continued effective pest control.

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