Cross-resistance of a johnsongrass (Sorghum halepense) biotype to aryloxyphenoxypropionate and cyclohexanedione herbicides

Dose-response experiments were conducted in a greenhouse on a biotype of johnsongrass from Washington County, Mississippi, to determine the level of resistance to the aryloxyphenoxy-propionate (AOPP) herbicide fluazifop-P-butyl and the cyclohexanedione (CHD) herbicides clethodim and sethoxydim. Both seedling and rhizomatous plants were evaluated. Resistant/susceptible (R/S) ratios were calculated based on GR(50) values (the rate required to reduce shoot dry biomass, expressed as a percent of the control, 50%). The GR50 values for the resistant and susceptible seedling plants were I 10 and 10 g ai/ha for clethodim, 193 and 34 g ai/ha for fluazifop-P-butyl, and 265 and 48 g ai/ha for sethoxydim, resulting in R/S ratios of 11.0, 5.7, and 5.5, respectively. The GR(50) values for the resistant and susceptible rhizomatous plants were 609 and 39 g/ha for clethodim, 657 and 29 g/ha for fluazifop-P-butyl, and 668 and 30 g/ha for sethoxydim, resulting in R/S ratios of 15.6, 22.7, and 22.3, respectively.