Pesticides Research No. 114 2007 – Effects of Mechanical Weed Control in Spring Cereals – Flora, Fauna and Economy

Effects of Mechanical Weed Control in Spring Cereals – Flora, Fauna and Economy

5 Conclusions and Perspectives

5.1 Conclusions
5.2 Perspectives
- 5.2.1 Perspectives for management
- 5.2.2 Perspectives for further research

5.1 Conclusions

Weed harrowing which is intensive in terms of number of treatments (more than two) is damaging to flora and fauna and has, with the exception of very rare situations, no economic justification.
More than two weed harrowings (one pre-emergence + one post-emergence) are in general non-economic.
Post-emergence harrowing is most efficient for weed control, but unfortunately also most harmful to arthropod predators and ground nesting birds.
More than two weed harrowing treatments cause a marked drop in biomass of wild plants.
More than two harrowing treatments cause a significant decrease of beneficial polyphagous predators that are also important birds’ food items.
Weeds in spring cereals are beneficial to both arthropods and bird life. Sufficient weed for supporting a high arthropod density (15 g weed dry mass / m²) can be allowed at a cost below 3 € per ha.
More than two harrowing treatments cause a strong reduction of the number of Skylark fledglings.
Timing of even few harrowing treatments is essential for the avoidance of damage to Skylark nesting success and can be done without affecting yield significantly: 35 days after sowing and 20^th May should be respected as the latest time for harrowing.
Lapwings are extremely vulnerable to harrowing treatments, which should ideally be avoided; only very early treatment(s) (no later than a few days after crop emergence) that allow successful replacement clutches can keep the damage to the production of offspring within acceptable limits.
For important Lapwing colony areas it should be considered how to avoid or restrict the use of harrowing as well as the use of herbicides and insecticides throughout the breeding period.
A combination of harrowing and herbicide treatment for weed control in the growing season may seem agriculturally attractive but is problematic for within-field biodiversity.
Harrowing alone with one or two treatments respecting the Skylark nesting period is a meaningful strategy that also economically is a fair alternative to herbicides.
Weed harrowing in organic farming is not a threat to Skylarks if the time rules expressed above are followed.
For early beneficial arthropods on the soil surface the use of low dosage herbicides may be a better alternative than weed harrowing.
The above conclusions open for a potential consideration of how to improve biodiversity in Danish agricultural fields.

5.2 Perspectives

5.2.1 Perspectives for management

The project has delivered some fairly clear answers to the issue of the fate of birds, weeds and arthropods exposed to weed harrowing. First of all, it is important to stress the finding that more than two times harrowing is not beneficial for the economic yield level, and therefore it can be assumed that a maximum of two times harrowing in spring cereals will be the realistic level of weed harrowing operations in the vast majority of cases.

On the other hand, it is clear that Skylark nests may be demolished and eggs destroyed by weed harrowing. This however can fairly easily be avoided by following some simple (and agronomical acceptable) rules of timing: if weed harrowings in spring cereals are carried out before 20 May and no later than 35 days after sowing, they will never cause much damage to young Skylarks, as has been argued. It must be stressed, though, that this holds true for the Skylark but not for the Lapwing, which has a nesting behaviour and phenology that makes it far more sensitive to weed harrowing. However, Lapwings tend to be more concentrated on specific areas for which special approaches may be considered, as mentioned later.

For the Skylark, one or two times harrowing in accordance with the above-mentioned recommendations on timing will also ensure an improved supply of arthropods and weed seeds partly due to a higher weed biomass. Weed harrowing no later than 20^th May will also secure that the population of early beneficial arthropods can partly recover before they are most needed to control insect pests. If this shall function as a benefit to the birds it is highly important not to add herbicide treatments and to avoid insecticides whenever possible. Should insecticide treatment become necessary, a reduction to 25% of the normal (label) dosage is recommendable. This will permit the survival of 25% more insect biomass than full dosage, and still a decent control of, e.g., aphids in cereals can be obtained (Esbjerg & Petersen 2002).

For the Lapwing, the results of the present project point at a quite delicate situation. For areas with only few Lapwings it might be acceptable that only very dedicated farmers limit their weed control efforts to one or two very early harrowings, which will not damage hatching success too much. However, Lapwings frequently nest in loose colonies and benefit from a common defence behaviour towards predators. Because of the normally high hatching success in these colonies they are potentially extremely valuable, and harrowing in such areas causes substantial population damage to this vulnerable species, which is already in focus in an EU context (Petersen submitted). On the other hand, herbicide and insecticide use is also damaging and removing the food items of the Lapwing chicks by indirect or direct means, and is thus incompatible with protective measures for the species. Therefore the results of this project, in a broader context, call for special attention to ensuring successful Lapwing breeding, maybe through Statutory Management Requirements (cross-compliance rules) or – using a voluntary approach – by the creation of particular agri-environmental measures available to farmers with important Lapwing colonies on their land.

It may at present seem most relevant to conclude that weed harrowing is a realistic alternative to the use of pesticides. In this context it should be noticed that the replacement of herbicide treatments with harrowing is possible at an average cost of only 0.5-1 hkg cereals per hectare – and the focus of the debate may be a different one than the economical. However, we do not know if an efficient use of low herbicide dosages is more Skylark and arthropod friendly than the most efficient weed harrowing alternative; that question needs further investigation.

5.2.2 Perspectives for further research

5.2.2.1 How does uncertainty about intensity, frequency and timing influence further conclusions?

It is at present not possible to safely separate weed harrowing effects into effects of intensity (soil covering), frequency or timing of harrowings. In case of arthropods, Skylarks and Lapwings it is without doubt that the effect of harrowing is a combined effect of timing, frequency and intensity. In the case of the weeding effect is has been shown, that the effect of additional treatments is an intensity effect. But in some cases the doubt about timing, frequency and intensity effects of the weed harrowing in spring cereals will make it difficult to change policy and make up the understanding and advices considering weed harrowing in spring cereals. There is no doubt that 1) to protect Lapwings their major nesting areas should not at all be harrowed, 2) to protect Skylarks late harrowings should be avoided, 3) densities of important ground-dwelling arthropods are reduced by harrowing through direct removal and indirectly trough reduced plant biomass. However, the question of arthropod effects and costs of one intensive harrowing (higher degree of soil covering of the crop) versus two less intensive harrowings remains unclear.

5.2.2.2 More research and field trials needed

In accordance with the remaining question more field trials that are testing the intensity, timing and frequency aspects in relation to weed control, crop damage, arthropods and weed flowering are needed to identify the most farmer and Skylark-friendly weed-harrowing strategies. Also the relevance of spring cereal species and varieties, seed density, sowing techniques, weed species, time of sowing, crop free plots (e.g. Skylark scrapes) and weed sowing should be integrated into these trials. Furthermore, different herbicide dosage levels should be included as references, to allow a comparison of the biodiversity effects of reduced dosages and “Skylark-friendly” harrowing. It should also be discussed how to measure the weed problem and the quality of the weed control, e.g. weed species density, biomass, seed production and germination, in relation to intensity, frequency and timing of the weed harrowing. All these parameters and measures can hardly be included in one, single, and traditional field trial design. Like in this project, a model framework could be used to integrate and analyse all these parameters and relations from less complex, but more varied and at the same time more specific field trials than used in the past.

Furthermore, it can at present not be argued to farmers that they will benefit (in terms of yields) from an increased beneficial effect of natural enemies such as the polyphagous predators included in the present project. The reason for this is not a lack of documentation about examples of increased abundance of beneficials. The documentation which is lacking is on more precise yield effects of particular levels of particular beneficial insects. Precise estimation of the reduction of insect pests is scientifically demanding, but it has been done in some cases, such as the recent documentation of reduction of yield loss to aphids caused by polyphagous predators (Östman et al. 2005). Secondly, there is a large annual variation in pest insect infestation levels. Accordingly, the assessment of annual benefits of predators and other beneficials is difficult, and basically the benefit will always be a mean benefit over years.

An important question in relation to breeding Skylarks also deserves further attention: how does weed harrowing in winter cereals affect breeding success? Skylarks often produce their first brood in winter cereals before moving to the spring-sown fields (e.g. Esbjerg & Petersen 2002), and weed harrowing carried out in winter cereals in mid or late April may potentially cause as much damage to Skylark breeding success as late harrowing in spring cereals.