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 aryloxyphenoxypropionate (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 GR₅₀ 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 110 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 GR₅₀ 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.

Nomenclature: Clethodim; fluazifop-P-butyl; sethoxydim; johnsongrass, Sorghum halepense (L.) Pers. #³ SORHA.

Additional index words: ACCase inhibitors, herbicide resistance, graminicides.

Abbreviations: ACCase, acetyl coenzyme-A carboxylase; WAT, weeks after treatment.