Metabolic resistance to pre-emergence herbicides in grasses

Black-grass (Alopecurus myosuroides Huds.) and rye-grass (Lolium spp) are very competitive grass weeds. During the last decades, they have evolved resistance to post-emergence herbicides, in particular ACCase- and ALS- inhibitors. This requires more complex weed management strategies. To ensure good control of both weeds, pre-emergence treatments become increasingly important. Particularly flufenacet has become a key herbicide for the control of multiple-resistant black-grass and rye-grass. Yet, in some of those populations, reduced flufenacet efficacy start to be observed. In a screening with black-grass and rye-grass populations of worldwide origins, most populations could be controlled with the registered field rate of flufenacet, however differences in the level of flufenacet efficacy were observed and were in correlation with enhanced flufenacet metabolism. This was particularly the case for rye-grass populations. The use of flufenacet in mixtures with diflufenican, particularly in combination with flurtamone or metribuzin or aclonifen, improved the efficacy significantly. In decreased flufenacet sensitive populations, the efficacy of other pre-emergence herbicides like pendimethalin, prosulfocarb, S-metolachlor, dimethenamid-P and pethoxamid, was also significantly decreased whereas other herbicides like pyroxasulfone or diflufenican remained highly active. This decreased efficacy of flufenacet as well as S-metolachlor was associated with higher metabolism involving glutathione-s-transferases. Although differences between populations were sometimes relatively small, best weed management practices (e.g. application of full dose rates and mixtures) should be applied to reduce selection pressure and prevent the development of resistance. This is particularly important as flufenacet is one of the few still active herbicides suitable for the control of multiple-resistant grass weeds. Use of mixtures associated with agronomic solutions in an Integrated Weed Management approach has to be the main approach.