Integrated management of herbicide resistance

Abstract

Research was conducted to improve our understanding of herbicide resistance in the grass-weed, black-grass (Alopecurus myosuroides). Research aims were to: Quantify the effectiveness of resistance mitigation strategies; Develop robust tests for resistance to ALS (acetolactate synthase) inhibitors; Investigate sampling strategies to improve resistance detection; Quantify the impact of population dynamics; Develop more sustainable resistance management strategies for individual fields.

Target site resistance (TSR) to ALS herbicides (e.g. mesosulfuron+iodosulfuron, ‘Atlantis’) can build up rapidly following repeated annual use of this herbicide. Use of other modes of action in combination with ALS herbicides improved weed control, but did not reduce selection for ALS TSR. There was also evidence for development of enhanced metabolic resistance to mesosulfuron+iodosulfuron, as well as ALS TSR.

Robust and reliable resistance tests were developed. Glasshouse pot assays are more robust than Petri-dish assays, but take longer. Testing showed that resistance to ALS inhibiting herbicides occurs in at least 21 counties in England. Improved advice for farmers/agronomists on collecting representative seed samples for resistance testing was obtained; sampling from several patches improves the assessment for the whole field, but sampling from a single field can give a highly misleading representation in terms of the resistance status of the whole farm.

Resistance to ALS herbicides was shown to increase faster in minimum tillage systems compared with ploughing. ALS TSR did not decline when ALS herbicides were not used for 3 years, so there was no loss of resistance in the absence of herbicides. Modelling studies showed that: Pre-emergence herbicides can compensate, to some degree, for the declining performance of post-emergence herbicides; Modifiers in the form of alternative herbicides or non-chemical methods slowed, but did not prevent, the buildup of resistance; Non-chemical control methods are increasingly important in combating resistance by reducing the reliance on post-emergence herbicides.

Key aspects of more sustainable resistance management strategies are: greater use of non-chemical control methods; less reliance on high resistance risk post-emergence herbicides; greater use of pre-emergence herbicides; more critical monitoring of herbicide performance in individual fields; regular testing for resistance.