The Impact of Herbicides on Weed Abundance and Biodiversity PN0940
Total Page：16
File Type：pdf, Size：1020Kb
The impact of herbicides on weed abundance and biodiversity PN0940 IACR - Long Ashton Research Station & Marshall Agroecology Ltd Dr Jon Marshall Marshall Agroecology Limited, 2 Nut Tree Cottages, Barton Winscombe, North Somerset, BS25 1DU. Email: [email protected] CAER, University of Reading Professor Valerie Brown Centre for Agri-Environmental Research (CAER), Department of Agriculture, University of Reading, Earley Gate, P.O. Box 236, Reading, RG6 6AT. Email: [email protected] CSL, York Dr Nigel Boatman Central Science Laboratory, Sand Hutton, York, YO41 1LZ. Email: [email protected] IACR – Rothamsted Dr Peter Lutman IACR- Rothamsted, Harpenden, Hertfordshire, AL52JQ. Email: [email protected] Scottish Crops Research Institute Dr Geoff Squire Scottish Crop Research Institute, Invergowrie, Dundee, DD2 5DA. Email: [email protected] The impact of herbicides on weed abundance and biodiversity PN0940 Contents Page Acknowledgements EXECUTIVE SUMMARY AND CONCLUSIONS 1. INTRODUCTION 1 1.1. Policy Rationale 1 1.2. Scope of the Desk Study 1 1.3. Objectives 1 1.4. Target and Non-Target Plant (Weed) Species 2 2. ECOLOGY OF REPRESENTATIVE WEED SPECIES 7 3. UPDATING PN0923 - NON-TARGET EFFECTS OF HERBICIDES 13 3.1. Is Biodiversity Important? 13 3.2. Change in Weed Communities 13 3.3. Impacts of Farming 14 3.4. Interactions Between Weed Diversity and Biodiversity 15 3.5. Non-Target Effects Within the Crop 15 3.6. Non-Target Effects Beyond the Crop 15 3.7. Genetically Modified Herbicide Tolerant (GMHT) Crops 16 3.8. Spatial Distribution, Remote Sensing and Mapping of Weeds 16 3.9. Farming Systems 17 4. CHANGES IN ABOVE-GROUND WEED ABUNDANCE 18 5. NON-TARGET WEED SPECIES IN THE SEEDBANK 24 GR Squire, SCRI 5.1. Status Of The Non-Target Species 24 5.2. Abundance And Dynamics Of The Non-Target Species 27 5.3. Suppressive Management – Falling Seedbanks 27 5.4. Relaxing Management – Rising Seedbanks 29 5.5. Community Features And The Potential For Modelling 30 5.6. Dynamics Modelling Linking Trait to Community through 31 Management 5.7. Conclusions 33 6. CURRENT WEED CONTROL AND ITS IMPACT 34 PJW Lutman, IACR 6.1. Introduction 34 6.2. Arable Cropping Patterns 34 6.3. Herbicide Use in 1998 35 6.4. Principal Changes in Herbicide Use Between 1974 And 1998 37 6.5. Changes in Weed Susceptibility to Herbicides Between 1974 and 1998 42 6.6. Changes in Timing of Control of Weeds on Weed Species Diversity 46 PN0940 6.7. Effects Of Fertilisers On Weed Communities 47 6.8. Overall Conclusions 48 7. EFFECTS OF MANAGEMENT OTHER THAN HERBICIDES 62 ON FARMLAND BIODIVERSITY 7.1. Habitat Loss 62 7.2. Fertilisers And Nutrient Enrichment 62 7.3. Cropping Practices 63 8. THE IMPACT OF HERBICIDES ON INVERTEBRATES 64 VK Brown, CAER 8.1. Ecological Framework 65 8.1.1. Invertebrate Communities 65 8.1.2. Attributes Of Weed Communities Important To Invertebrates 65 8.2. Effects of herbicides on invertebrates 66 8.2.1. By Habitat Modification 68 8.2.2. By Prey Resources 70 8.2.3. By Plant Food Resources 71 8.3 New dimensions 79 8.3.1. Effects Of Herbicides On Soil Fauna 79 8.3.2. Sub-Lethal Effects Of Herbicides On Invertebrates 81 9. RELATIONSHIPS BETWEEN WEEDS, HERBICIDES AND BIRDS 90 ND Boatman, CSL 9.1. Introduction 90 9.2. The Diet Of Farmland Birds 91 9.3. Relationships Between Food Abundance And Bird Populations 95 9.4. Relationship Between Breeding Performance And Population Trend 98 9.5. Relationship Between Adult Mortality And Population Trend 99 9.6. Relationship Between Food Density And Foraging 100 9.7. Other Causes Of Bird Declines 101 9.8. Conclusion 102 10. RISK ASSESSMENT FOR NON-TARGET PLANTS 108 WITHIN CROPS 11. PRACTICAL WEED CONTROL, BIODIVERSITY 112 AND RISK AVOIDANCE 11.1. Weed Control And Biodiversity 112 11.2. Risk Management And Avoidance 114 12. KNOWLEDGE GAPS AND RESEARCH NEEDS 117 12.1. Weeds And Weed Management 117 12.2. Invertebrates 120 12.3. Birds 121 12.4. Priority Research Areas 122 EXECUTIVE SUMMARY AND CONCLUSIONS 123 REFERENCES 125 PN0940 Acknowledgements We wish to thank Dr Lena Ward and the Centre for Ecology and Hydrology for access to the Phytophagous Insect Database and for useful discussions. We also wish to thank staff of the Pesticides Safety Directorate for their useful input into the project, particularly Paul Ashby and Tim Godson. Ingrid Meakin and other staff of the Chief Scientists Group were also particularly helpful, including finding meeting accommodation under trying circumstances while moving offices. We also thank colleagues in our own institutions for their input, without whom this document would not have been as wide-ranging as it is. This report has been achieved in just over four months. While there is undoubtedly scope for further work and there are a number of gaps within the report, it provides a timely review of the impact of herbicides in the arable ecosystem. PN0940 EXECTIVE 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. PN0940 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.