RESISTANCE TO ARYLOXYPHENOXYPROPIONATE AND CYCLOHEXANEDIONE HERBICIDES IN GREEN FOXTAIL (SETARIA-VIRIDIS)

Resistance to aryloxyphenoxypropionate and cyclo-hexanedione herbicide s (ACCase inhibitors) was characterized in five green foxtail populati ons from western Canada. Field histories indicated that these populati ons were exposed to up to seven applications of diclofop-methyl and se thoxydim over the previous ten years. Major differences occurred among the populations in their levels of resistance and patterns of cross-r esistance. With one exception, R/S ratios calculated from GR(50) value s derived from growth room experiments varied from 2 to 54 depending o n the population and herbicide. The exception was UM131 which was at l east 75 times more resistant to sethoxydim (R/S > 2981) than the other populations. UM59 was not resistant to sethoxydim. It was also the le ast resistant to the other herbicides with R/S ratios ranging from 2 f or fenoxaprop-p-ethyl to 8 for diclofop-methyl. In comparison, UM8 was resistant to diclofop-methyl, fenoxaprop-p-ethyl, sethoxydim, and tra lkoxydim with R/S ratios of 11, 5, > 39, and 27, respectively. In fiel d experiments, shoot dry weights of UM8 treated at two times the recom mended rates of these herbicides were reduced by 9, 45, 74, and 49%. I n contrast, the susceptible population, UM7, was completely controlled . UM7 and UM8 did not differ in their response to trifluralin, ethalfl uralin, quinclorac, propanil and TCA.