Enhanced Biodegradation of Herbicides in Soil and Effects on Weed Control

Research conducted since 1979 in the north central United States and southern Canada demonstrated that after repeated annual applications of the same thiocarbamate herbicide to the same field, control of some difficult-to-control weed species was reduced. Laboratory studies of herbicide degradation in soils from these fields indicated that these performance failures were due to more rapid or “enhanced” biodegradation of the thiocarbamate herbicides after repeated use with a shorter period during which effective herbicide levels remained in the soils. Weeds such as wild proso millet [Panicum miliaceumL. spp.ruderale(Kitagawa) Tzevelev. #3PANMI] and shattercane [Sorghum bicolor(L.) Moench. # SORVU] which germinate over long time periods were most likely to escape these herbicides after repeated use. Adding dietholate (O,O-diethylO-phenyl phosphorothioate) to EPTC (S-ethyl dipropyl carbamothioate) reduced problems caused by enhanced EPTC biodegradation in soils treated previously with EPTC alone but not in soils previously treated with EPTC plus dietholate. While previous use of other thiocarbamate herbicides frequently enhanced biodegradation of EPTC or butylate [S-ethyl bis(2-methylpropyl)carbamothioate], previous use of other classes of herbicides or the insecticide carbofuran (2,3 -dihydro-2,2 -dimethyl-7-benzofuranyl methylcarbamate) did not. Enhanced biodegradation of herbicides other than the thiocarbamates was not observed.