Effect of a Pre-Irrigation Period on the Activity of Ethofumesate Applied to Dry Soil

Abstract

Applications of ethofumesate [(±)-2-ethoxy-2,3-dihydro-3,3-dimethyl-5-benzofuranyl methanesulfonate] at rates from 0.3 to 1.4 kg/ha were made in the field to soils of 2 and 30% (w/w) soil moisture. After 2 to 4 days, all soils were irrigated to establish stands of either sweet corn (Zea maysL. ‘Jubilee’) or winter wheat (Triticum aestivumL. ‘Stephens’) used as bioassay species for ethofumesate activity. Ethofumesate, at most rates, was significantly less effective on both corn and wheat when applied to dry soil than to wet soil. In greenhouse studies, ethofumesate activity was significantly greater when incorporated into soil of 12% than 2% (w/w) moisture 2 or 4 days prior to irrigation. An increase in herbicide activity was apparent as the length of time between herbicide application and wetting increased from 2 to 4 days. Analysis of wet (20%, w/w) and dry (2%, w/w) soils treated with equal levels of ethofumesate revealed no loss of herbicide applied to wet soil over a 12-day period, but in the dry soil, the amount extracted after 12 days was 10% of the amount extracted at 0 days. These data and other considerations suggest that chemical degradation of ethofumesate is the most likely mechanism for the activity loss in dry soil.