Herbicide losses in runoff from conservation-tilled watersheds in a corn-soybean rotation

Soil and nutrient losses are tolerable when corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] are grown in rotation on steep slopes in northern Appalachia if conservation tillage is used and a rye (Secale cereale L.) cover crop follows soybean harvest. Herbicide losses and concentrations in runoff resulting from this rotation, however, may be unacceptable. Therefore, runoff from two chiseled and two no-till watersheds was monitored for 4 yr to determine the effect of this cropping sequence on losses of four commonly used herbicides: alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl) acetanilide], atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea], and metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin- 5(4H) -one]. The spectrum of weeds controlled by linuron is similar to that of atrazine and their losses were compared to determine if substitution of linuron for atrazine would decrease herbicide loss in runoff. As a percentage of applied chemical, average losses were small with atrazine (0.31%) linuron (0.20%) metribuzin (0.14%) alachlor (0.05%). At times, however, herbicide concentrations may be a concern. Atrazine concentrations consistently exceeded the lifetime Health Advisory Level-Maximum Contaminant Level (HAL-MCL) of 3 micrograms L-1 in the first few runoff events after application. In addition, atrazine was detectable in runoff during soybean years, at times above the HAL-MCL. Average linuron loss was significantly less (P less than or equal to 0.05) than that of atrazine and linuron was rarely detected in runoff following corn harvest or during soybean years The 2 micrograms L-1 MCL for alachlor was only exceeded during the first few events after application, whereas metribuzin concentrations never exceeded the HAL of 200 micrograms L-1