Fifteenth Australian Weeds Conference Summary We

assessed the respective contributions of target-site and metabolism-based resistance to herbicides inhibiting acetyl-coenzyme A carboxylase (ACC) in 243 black-grass populations (Alopecurus myosuroides Huds.) collected across France using a herbicide sensitivity bioassay and ACC genotyping. Seedlings resistant to fenoxaprop were identified in 241 out of the 243 populations investigated. Among the five resistant ACC alleles known in black-grass, alleles containing an isoleucine-to-leucine substitu-tion at codon 1781 were predominant, occurring in 59.5 % of the plants containing resistant ACC alleles. Comparison of the respective results from herbicide sensitivity bioassays and from genotyping indicated that more than 75 % of black-grass plants resistant to ACC-inhibiting herbicides in France would be resist-ant due to increased herbicide metabolism. Analysis of herbicide application records suggested that, in 15.9 % of the populations studied, metabolism-based resistance to ACC-inhibiting herbicides was mostly selected for by herbicides with other modes of action. Our results revealed the importance of metabolism-based resistance in black-grass, an issue of practical importance when considering managing populations resistant to ACC-inhibiting herbicides by using her-bicides with alternative modes of action