Response of Alopecurus myosuroides Huds. to varying intensities of acetolactate synthase-inhibiting herbicides in a crop rotation including imidazolinone-tolerant oilseed rape

Abstract

Herbicide-tolerant winter oilseed rape (OSR) varieties offer the opportunity of using imazamox for weed control, an active ingredient belonging to the chemical group of acetolactate synthase (ALS-) inhibitors. However, ALS inhibitors are used in many different crops and are the most resistance-prone herbicide mode of action. Their frequent application in a crop rotation increases the selection pressure for ALS herbicide resistance in weeds, which has to be considered when designing resistance management strategies. Alopecurus myosuroides Huds. is a frequent and economically important grass weed in Northwestern Europe, which has evolved resistance to several herbicide modes of action. For the sustainable use of herbicide-tolerant OSR varieties, studies on the effects of the different intensities of ALS inhibitor use on A. myosuroides population dynamics and resistance development are required. Two field trials were conducted including susceptible and multiple resistant A. myosuroides individuals and four weed control strategies varying in their intensity of ALS inhibitor use over a three-year trial period. A. myosuroides head numbers, the presence of target-site mutations in surviving plants, and crop yields were assessed annually, and the amount of A. myosuroides seeds in the soil seed bank was determined at the end of the trial period. The results show that the intensity of ALS inhibitor use significantly influenced the density of A. myosuroides and the development of resistance. Under weed control strategy IV (no ALS inhibitors), an increase in A. myosuroides head number was observed due to multiple resistance in the A. myosuroides population employed in the field trial. None of the four weed control strategies was able to control A. myosuroides infestation to an acceptable level. The results on A. myosuroides densities in the soil seed bank were highly variable and inconsistent. Molecular analysis of surviving plants showed a selection of ALS-resistant biotypes depending on the ALS inhibitor selection pressure. This study did not reveal any specific deterioration of A. myosuroides infestation associated with the use of imidazolinone-tolerant OSR in a short-term crop rotation. However, this OSR production system should not be employed if ALS-resistant A. myosuroides plants are present in the field.