Managing herbicide-resistant blackgrass (Alopecurus myosuroides): Theory and practice

Blackgrass is the most important herbicide-resistant weed in Europe, occurring in 10 countries. Enhanced metabolism is the most common mechanism, conferring partial resistance to a wide range of herbicides, but acetyl-CoA carboxylase (ACCase) target-site resistance also occurs widely. Recently, acetolactate synthase (ALS) target-site resistance conferred by a Pro,97 mutation was identified in blackgrass in England and is of concern because of increasing use of sulfonylurea herbicides such as mesosulfuron + iodosulfuron. Resistance management strategies encourage (1) greater use of cultural control measures such as plowing, crop rotation, and delayed drilling; (2) reduced reliance on high-risk herbicides (ACCase, ALS); and (3) use of mixtures and sequences of herbicides with different modes of action. A key message is that, as weeds are relatively immobile, preventing and managing herbicide resistance is largely within the individual farmer's own control. In practice, financial and environmental pressures limit the scope for more cultural control, and the European Community Pesticide Review will result in fewer alternative herbicides being available. Consultants often feel unable to recommend lower risk but weaker herbicide strategies to farmers because the amount of blackgrass remaining might be unacceptable. This dilemma is exemplified by the recent introduction of a formulated mixture of mesosulfuron + iodosuffuron, which has given outstanding control of blackgrass. Farmers expect new herbicides to become available, but this optimistic view appears misplaced. A primary aim is to continue to encourage farmers to integrate cultural and chemical control in a long-term strategy.