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

Summary

The present project has a background in two earlier projects and in a standing debate about weed harrowing as part of organic farming. The first project partly used weed harrowing in a row crop while the second on effects of conversion to organic practice fully used weed harrowing. Results of both indicated that more than two weed harrowings might negatively influence wild flora more than economically necessary, and also damaged arthropods. These results, and the often postulated damage to nests of ground-dwelling birds, were turned into hypotheses.

In accordance with these hypotheses, the aim of the project was to investigate and quantify effects of weed harrowing on flora and fauna in the light of the potential of this weeding methodology as part of the approaches towards a reduced use of pesticides. A further aim was to calculate/model costs of particular practices directed towards improvement of flora and fauna in fields.

The project included six host farms on which we studied the effects of two or four times harrowing in spring sown wheat on weed cover and diversity, densities of selected spiders and beetles (agriculturally beneficial predators which are also bird food) and breeding success of Skylark, Lapwing and Oystercatcher. A large set of data from the Danish Agricultural Advisory Service made the modelling of yield effects possible, and the National Environmental Research Institute supplied additional data on breeding success of Lapwing and Oystercatcher in weed harrowed fields.

A marked reduction in weed occurrence was seen when the number of weed harrowings was increased from two to three or four. No significant change, however, in weed occurrence was found when harrowing frequency was increased from three to four weed harrowings. The weed biomass was reduced by 43% when the weed harrowing was increased from two to three times, and by 48% at an increase from two to four times harrowing. The crop biomass was reduced by 3% when the number of weed harrowings was increased from two to three, and by 6% at an increase from two to four times harrowing.

Weed harrowing had a significant negative effect on densities of the spiders Linyphiidae and the rove beetles Tachyporus spp., but not on the carabids Agonum spp. and Bembidion spp. Densities of the common arthropod predator complex were significantly higher (21-35%) in the plots subject to only two harrowings when compared with the adjacent plots subject to two additional harrowings.

A general positive relationship was found between weed biomass and predator densities, and the negative effect of weed harrowing could partly be explained as an indirect effect of lower weed biomass. 15-20 g weed biomass / m² (dry mass) is necessary to support a high density of beneficials in late May – early June.

Choice-experiments with two common carabid species revealed that Anchomenus dorsalis preferred un-harrowed soil whereas Bembidion lampros had no preference for harrowed or un-harrowed soil.

Weed harrowing had a strong, negative effect on the breeding success of Lapwings and Oystercatchers, while only a modest number of Skylark nests were damaged. The main reason for this was that breeding in Lapwing and Oystercatcher coincided with the weed harrowings, whereas most Skylark nests were established later.

Only 16% of the Skylark nests were exposed to weed harrowing, but 83% of these nests were destroyed. Predation was the main cause of nest failure, and predation risk was also affected by weed harrowing. In plots that were subject to two harrowings, 65% of the Skylark nests were successful, whereas this was the case in only 28% of the nests placed in plots where four harrowings occurred. There were two reasons for this difference: few Skylark nests were active during the first two weed harrowings and predation rates were significantly higher in plots with four times harrowing. It is concluded that weed harrowing performed no more than 37 days after sowing and no later than 20 May has only minor effects on Skylark breeding success.

For the Lapwing, weed harrowing was the most frequent cause of nest failure. On fields where weed harrowing was carried out, 40% of all Lapwing nests were completely destroyed and in a further 25% one or more eggs were lost. The mean number of hatchlings per nest was 0.63 in fields with weed harrowing, compared to 2.35 in similar fields where weed harrowing was not performed and 2.80 in perennial grassland. Because the breeding activities normally begin in April, even early weed harrowings (and other soil treatments) destroy a sizable number of Lapwing nests. Successful re-laying is only possible until shortly after crop emergence. It is concluded that weed harrowing (and rolling) performed after around 1 May, or later than a few days after crop emergence, has a strong, negative effect on Lapwing breeding success. Few Oystercatcher nests were found, but the available data suggest that weed harrowing is at least as damaging to this species.

Spring cereals are very competitive to weeds, and chemical as well as mechanical weed control often results in a negative net yield gain in these crops. To avoid harvest problems and long-run dynamic problems in the following crops the weed is however, controlled (>70% reduction in weed biomass) in spring cereals as well. Mechanical weed control appears to be an effective alternative to herbicide use, but the analyses indicate that the economically most efficient strategy is the use of a low (but still effective) dosage of herbicide. However, the weed control effect of an early, single, and intensive (>20% soil coverage of the crop) post-emergence harrowing is also very satisfactory, the only drawback being the 0.7 hkg/ha higher net cost than the low dosage herbicide treatment. For farmers with reduction of herbicide input as an aim it may be cost effective to apply a low dosage and, in case of insufficient control, follow up with harrowing. This has, however, to be done early to be efficient and to avoid damage to Skylark nests, and from a biodiversity point of view the mixed strategy cannot be recommended. Modelling has shown that the weed population needed to sustain a high density of arthropods may be allowed at a cost below 3€ per ha.

From the project results it is concluded that a certain weed cover is beneficial to insects and birds and such a weed cover may be a positive result of a harrowing which is not too intensive. By contrast, more than two times weed harrowing is damaging to plants, insects and birds and is not economically justified. The timing of harrowing is crucial to ground-nesting birds. If performed no later than 35-37 days after sowing, and never after 20 May, weed harrowing has only minor effects on Skylark breeding success. With this observed, there is no reason to accuse mechanical weeding of being damaging to Skylarks. Due to the very limited extra cost, weed harrowing might also be considered one of the tools for reducing pesticide use in other farm practices. The biodiversity effects of weed harrowing versus reduced herbicide dosages are unclear. The only clear exception to this is areas with high numbers of breeding Lapwing. This species is highly vulnerable to even early harrowings, and special protection measures for such areas should be considered. These measures should also include the omitting of herbicides and insecticides, to ensure sufficient amounts of food items and thus improve hatchling survival.