UPLAND SOYBEAN PRODUCTION - SURFACE AND SHALLOW GROUNDWATER QUALITY AS AFFECTED BY TILLAGE AND HERBICIDE USE

Information on the effects of tillage practices on pesticide transport in runoff and shallow groundwater is lacking for much of Mississippi, particularly the uplands of northern Mississippi. Therefore, herbicid e transport in surface runoff and shallow groundwater was determined i n no-till (NT) and conventional-till (CT) soybean [Glycine max (L.) Me rr.] watersheds (about 2 ha each) in these loessial uplands (fragipan soils). For the 1991 crop year, runoff losses of the preemerge herbici des metribuzin utyl-4,5-dihydro-3-methylthio-1,2,4-triazin-5-one, Lexo ne(TM)) and metolachlor ethyl-N-(2-methoxy-1-methylethyl)acet-o-toluid ide, Dual(TM)] were about 20 and 9%, respectively, of that applied to the NT watershed and about 23 and 11%, respectively, of that applied t o the CT watershed. Greater than 90% of these losses occurred as a res ult of the first runoff event. Although sediment concentrations from t he NT watershed were reduced by Mo orders of magnitude compared with t he CT watershed, herbicide losses in runoff as well as runoff volumes, were about equal. Herbicide losses in runoff in crop years 1992 and 1 993 were lower for both watersheds due to the longer time interval bet ween herbicide application and first runoff. Herbicide movement into t he soil profile was greater in the NT watershed.