title: Comparative losses of glyphosate and selected residual herbicides in surface runoff from conservation-tilled watersheds planted with corn or soybean.
abstract: Residual herbicides regularly used in conjunction with conservation tillage to produce corn ( L.) and soybean [ (L.) Merr] are often detected in surface water at concentrations that exceed their U.S. maximum contaminant levels (MCL) and ecological standards. These risks might be reduced by planting glyphosate-tolerant varieties of these crops and totally or partially replacing the residual herbicides alachlor, atrazine, linuron, and metribuzin with glyphosate, a contact herbicide that has a short half-life and is strongly sorbed to soil. Therefore, we applied both herbicide types at typical rates and times to two chisel-plowed and two no-till watersheds in a 2-yr corn/soybean rotation and at half rates to three disked watersheds in a 3-yr corn/soybean/wheat-red clover ( L.- L.) rotation and monitored herbicide losses in surface runoff for three crop years. Average dissolved glyphosate loss for all tillage practices, as a percentage of the amount applied, was significantly less ( ≤ 0.05) than the losses of atrazine (21.4x), alachlor (3.5x), and linuron (8.7x) in corn-crop years. Annual, flow-weighted, concentration of atrazine was as high as 41.3 μg L, much greater than its 3 μg L MCL. Likewise, annual, flow-weighted alachlor concentration (MCL = 2 μg L) was as high as 11.2 and 4.9 μg L in corn- and soybean-crop years, respectively. In only one runoff event during the 18 watershed-years it was applied did glyphosate concentration exceed its 700 μg L MCL and the highest, annual, flow-weighted concentration was 3.9 μg L. Planting glyphosate-tolerant corn and soybean and using glyphosate in lieu of some residual herbicides should reduce the impact of the production of these crops on surface water quality.
authors.affiliation: USDA-ARS, North Appalachian Experimental Watershed, OH, USA. martin.shipitalo@ars.usda.gov
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Shipitalo
authors.forename: Lloyd B
authors.initials: LB
authors.lastname: Owens
mesh_terms: D000383:Agriculture; D004784:Environmental Monitoring; D005998:Glycine / Q000031:analogs & derivatives* / Q000032:analysis; D006540:Herbicides / Q000032:analysis*; D029902:Medicago; D012989:Soil Pollutants / Q000032:analysis; D013025:Glycine max; D014908:Triticum; D014872:Water Movements; D014877:Water Pollution, Chemical / Q000032:analysis* / Q000517:prevention & control; D014881:Water Supply / Q000032:analysis; D003313:Zea mays; D000097797:Glyphosate
chemical_list: D005998:Glycine; D006540:Herbicides; D012989:Soil Pollutants; D000097797:Glyphosate