Clomazone is effective as a chemical fallow herbicide; however, its soil residual properties in the Northern Great Plains are unknown. Clomazone was applied to soil at 0.6, 1.1, and 2.2 kg ai/ha at two locations in Montana in the spring of 1986. Soil samples were taken at each location at monthly intervals for 6 months. Residual levels of the herbicide were estimated by measuring the percent chlorosis by height in oat leaves. Clomazone at 2.2 kg ai/ha applied to a loam soil dissipated to levels below 0.1 mg/kg in 3 months and applied to a silty clay loam soil dissipated to 0.2 mg/kg 6 months after application. Half-lives (t½), determined from first-order rate plots, were 33 and 37 days in the Willow Creek loam and Bozeman silty clay loam, respectively. Thus, clomazone residue from labeled-use rates should not inhibit wheat in a wheat-fallow-wheat cropping system in Montana.
