Effect of a pre-irrigation period on the activity of ethofumesate applied to dry soil

Graduation date: 1981Applications of ethofumesate in Oregon have been observed from\ud previous research and from commercial use to be less effective when\ud applied to dry soils, even if subsequent precipitation occurred\ud within a few days. Two field studies were established in the\ud summer of 1979 using sweet corn (Zea mays L. 'Jubilee') and winter\ud wheat (Triticum aestivum L. 'Stephens') as bioassay species to\ud determine the effect of dry soil on ethofumesate efficacy. Applications\ud of various rates of the herbicide were made to soils of\ud approximately 2 and 30% w/w soil moisture. After 2 to 4 days, all\ud soils were irrigated for stand establishment and maintained at a\ud high moisture level. Ethofumesate, at most rates, was significantly\ud less effective on both corn and wheat when applied to dry soil than\ud to wet soil.\ud Two greenhouse studies were conducted using spring wheat\ud (Triticum aestivum L. 'Fielder') bioassays. One employed five soil\ud moisture levels, 2, 5, 9, 15, and 36%, and all soils were wetted to\ud field capacity 4 days after herbicide application. The other study used 2 and 12% moisture contents and soils were wetted 0, 2, and 4\ud days following herbicide application. The first study showed a\ud general decrease in herbicide activity with decreasing water content\ud with a greater than expected drop in activity at 15% moisture.\ud This drop in herbicide activity is believed to be the result of the\ud soil surface drying after application.\ud The second study showed substantially greater herbicide activity\ud at 12% moisture than at 2% moisture. The ethofumesate was incorporated\ud in this study, eliminating the effect of surface drying.\ud An increase in herbicidal activity was apparent as the length of\ud time between herbicide application and wetting increased from 2 to\ud 4 days for the 2% moisture soils. The reason for this effect is\ud not clear.\ud Soil samples (50 g, air-dried equivalent) at 2 and 20% soil\ud moisture were treated with 484 pg of ethofumesate. The herbicide\ud was extracted from the soils with hot methanol 0, 2, 4, 6, and 12\ud days after application. A gas chromatography analysis for ethofumesate\ud revealed no loss in the amount of herbicide applied to\ud wet soil over the 12-day period while in the dry soil, the amount\ud extracted after 12 days was 10% of the amount extracted at 0 days.\ud These data suggest chemical degradation of ethofumesate as the\ud most likely mechanism for the activity loss in dry soil