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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.

BACKGROUND A 7 year experiment was set up in 2002 to evaluate the long-term effects of weed management strategies based on graminicidal sulfonylureas ( SUs) on the evolution of a Lolium rigidum population resistant to ACCase inhibitors in a continuous wheat cropping system. The strategies included the continued use of ALS inhibitors, the continued application of ACCase inhibitors and a simple resistance management strategy based on a biennial rotation of herbicide mode of action ( MoA). RESULTS The efficacy of the tested SUs in the field decreased significantly, starting from the fourth treatment in all control strategies. Regardless of control strategy, the few survivors of the ALS treatment in the third season produced a significant number of ACCase- and ALS-resistant (multiple-resistant) progeny. Continuous ALS and biennial rotation of herbicides reduced weed densities, but L. rigidum conserved its ACCase resistance trait. Enhanced metabolism was detected in ALS-resistant plants, whereas target site was primarily involved in the ACCase-resistant individuals. CONCLUSION At the end of the experiment, multiple-resistant individuals were found in all samples coming from the control strategies investigated. The biennial rotation between ALS and other MoA appeared to delay the development of resistance to SUs over continuous treatments, but additional measures will likely need to be taken in order to make this sustainable in the long term, whereas the field efficacy of SUs remained relatively high until the end of the experiment. Integrated weed management with more diversity should be introduced in oversimplified cropping systems in order to sustainably manage resistant L. rigidum populations. Copyright © 2012 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

The acetyl-coenzyme A carboxylase (ACCase)-inhibiting cyclohexanedione herbicide clethodim is used to control grass weeds infesting dicot crops. In Australia clethodim is widely used to control the weed Lolium rigidum. However, clethodim-resistant Lolium populations have appeared over the last 5 years and now are present in many populations across the western Australian wheat (Triticuin aestivum) belt. An aspartate-2078-glycine (Gly) mutation in the plastidic ACCase enzyme has been identified as the only known mutation endowing clethodim resistance. Here, with 14 clethodim-resistant Lolium populations we revealed diversity and complexity in the molecular basis of resistance to ACCase-inhibiting herbicides (clethodim in particular). Several known ACCase mutations (isoleucine-1781-leucine [Leu], tryptophan-2027-cysteine [Cys], isoleucine-2041-asparagine, and aspartate-2078-Gly) and in particular, a new mutation of Cys to arginine at position 2088, were identified in plants surviving the Australian clethodim field rate (60 g ha-1). Twelve combination patterns of mutant alleles were revealed in relation to clethodim resistance. Through a molecular, biochemical, and biological approach, we established that the mutation 2078-Gly or 2088-arginine endows sufficient level of resistance to clethodim at the field rate, and in addition, combinations of two mutant 1781-Leu alleles, or two different mutant alleles (i.e. 1781-Leu/2027-Cys, 1781-Leu/2041-asparagine), also confer clethodim resistance. Plants homozygous for the mutant 1781, 2078, or 2088 alleles were found to be clethodim resistant and cross resistant to a number of other ACCase inhibitor herbicides including clodinafop, diclofop, fluazifop, haloxyfop, butroxydim, sethoxydim, tralkoxydim, and pinoxaden. We established that the specific mutation, the homo/heterozygous status of a plant for a specific mutation, and combinations of different resistant alleles plus herbicide rates all are important in contributing to the overall level of herbicide resistance in genetically diverse, cross-pollinated Lolium species. [ABSTRACT FROM AUTHOR]