The Impact of Herbicides on Weed Abundance & Biodiversity
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Institute of Arable Crop Research ( IACR) & Marshall Agroecology Ltd;
Centre for Agri-Environment Research ( CAER), University of Reading;
The Food and Environment Research Agency (FERA);
Scottish Crops Research Institute ( SCRI) - 2001.
Executive Summary and Conclusions
In considering non-target plants within arable fields, the majority of plant species that are found are of only minor concern to farmers, unless present at high population density. Under horticultural conditions, it can be argued that all weeds are targets, providing some difficulty for formal risk assessment. In arable, there are a number of key weed species that are typically controlled irrespective of density. In contrast, rare arable weeds may require specific conservation protection; these species may be non-targets under almost all conditions. The majority of species usually present can be both targets and non-targets and are most likely to be of greatest significance for biological diversity within fields, as they occur frequently and with moderate abundance.
Data on farmland birds and invertebrates indicate that there have been significant reductions in populations and ranges over the past thirty years. In the case of the grey partridge, there is good evidence that herbicides have played a significant role in their decline. Whilst habitat loss and fragmentation may play a role in bird declines, the evidence indicates that habitat degradation is of greater importance. Changes in farming practice in general are the cause of most population declines of farmland birds. Whilst the exact causal links are not known for most species, herbicides are implicated.
This review has shown that there have been changes in weed assemblages over the past century, with some species becoming less common, other increasing in frequency and others remaining static. Studies of weed seed banks indicate little change in weed seed abundance or a slight trend for reduced densities. Where weed control has been relaxed, either as set-aside or where herbicide use has been halved, weed seed banks can increase rapidly. However, the commonest and most competitive weed species tend to become the most abundant, under these conditions. Rare species may not recover.
Analysing changes in cropping and herbicide use, the move from spring to winter cropping since the 1970s has been a dramatic change in cropping practice. Co-incident with the change to winter cropping, there have been major changes in the pattern of herbicide use. In the 1970s, herbicides were used primarily for broad-leaved weed control and on only about 50% of fields. Today, herbicides are used on most fields and are targeted on grass weeds as well as dicotyledonous species. An examination of the weed spectra controlled by the herbicides in use over the past 25 years indicates that on average today’s herbicides control more weeds. Broader spectrum products were introduced in the early 1980s. Factors other than herbicides may play an important role in changing weed assemblages, particularly fertilisers and cropping pattern.
Data collected from the literature and from the Phytophagous Insect Database demonstrate close links between invertebrates and a range of representative weed species. Different weed species support differing numbers of insect herbivores, with some species hosting numbers of rare species, as well as pest species. The data indicate that a number of weed species that are particularly important for insect biodiversity in the arable habitat can be selected.
Data on the use of weed species by birds has also been examined. Whilst, as with the invertebrate data, there is some lack of quantitative information on preferences, it is clear that bird species of conservation importance utilise particular genera of weeds. Thus it is possible to identify genera that are of greater importance for farmland birds.
The data indicate that herbicides, by controlling weeds and modifying abundance and species assemblages, have impacted on wildlife in arable land. These non-target effects need to be considered for regulatory reasons, particularly with the requirements under EU Regulation 91/414. With such dramatic changes in biodiversity, there are also calls for more sustainable production methods. The challenge will be to grow crops and maintain an appropriate population of weed species to support farmland wildlife. Under horticultural conditions, this may be difficult, in terms of crop quality protection. Nevertheless, under arable and horticultural production, there may be opportunities to develop sacrifice areas, such as conservation headlands, or to develop much greater selectivity of herbicide action, either through selective chemistry or application or a combination of these.
In terms of regulatory needs, the approach of selecting representative weeds and assessing their importance for biodiversity has been successful. A shortlist of species has been identified. The approach can now be applied to other weed species, to check the most important species have been identified. Regulatory approaches reviewed in PN0923 can be applied as non-target protocols, with adjustment of acceptable risk to achieve control where required.
There are a number of areas where knowledge is lacking. These are briefly reviewed and a priority list for research and development is given below:
1. Classification of the competitive ability of a wider range of weed species under different cropping conditions.
2. Confirmation of the trends shown from data derived from the Phytophagous Insect Database linking plants to insect herbivores by ecological field study.
3. Assessment of the biodiversity importance of common weeds not included in this study.
4. Surveys of the status of weed and invertebrate populations.
5. Quantification of the importance of particular weeds for invertebrates and birds, including preferences and resource values.
6. Investigation of the interactions between weeds, invertebrate fauna and birds, including those that are insectivorous at the chick stage.
7. Modelling the functioning of the agricultural ecosystem to identify clearer causal links between population change and agronomic practice.
8. Investigation of the nature and effect of selection pressures within agroecosystems at genetic, individual, population and community levels.
9. Development of weed management systems that allow biodiversity to be maintained in the crop.
10. Tests of spatial methods of herbicide risk avoidance at appropriate spatial scales.
PN0940: The Impact of Herbicides on Weed Abundance and Biodiversity (pdf, 147 pages)
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