Mechanisms of resistance to acetyl-coenzyme A carboxylase-inhibiting herbicides in a Hordeum leporinum population

A failure of acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides t o control a population of Hordeum leporinum Link (barleygrass) occurred fol lowing eight applications of these herbicides in both crops and pastures. T his population was 7.6-fold resistant to fluazifop-P-butyl compared with st andard susceptible populations. The population was between 3.6- and 3.8-fol d resistant to other ACCase-inhibiting herbicides, except butroxydim to whi ch it was susceptible. ACCase extracted from resistant plants and assayed i n the presence of herbicides in vitro was susceptible to fluazifop acid and other aryloxyphenoxypropanoate herbicides, but was 4-fold less sensitive t o sethoxydim compared with ACCase from susceptible plants. Resistant plants metabolised fluazifop acid about 1.3-fold more rapidly compared with susce ptible plants; however, sethoxydim was metabolised equally in both populati ons. Resistance to fluazifop-P-butyl and other aryloxyphenoxypropanoate her bicides may be the result of increased herbicide detoxification, whereas re sistance to sethoxydim appears to be due to a modified target enzyme. Herbi cide resistance in this population is unusual in that different mechanisms appear to confer resistance to the aryloxyphenoxypropanoate and cyclohexane dione herbicides. (C) 2000 Society of Chemical Industry.