Resistance to aryloxyphenoxypropionate and cyclohexanedione herbicides in green foxtail (Setaria viridis)

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