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TECHNICAL REPORTS

Surface Water Quality

Tillage System, Application Rate, and Extreme Event Effects on Herbicide Losses in Surface Runoff

USDA–Agricultural Research Service, North Appalachian Experimental Watershed, P.O. Box 488, Coshocton, OH 43812-0488

^* Corresponding author (martin{at}coshocton.ars.usda.gov)

Received for publication December 22, 2005. Conservation tillage can reduce soil loss; however, the residual herbicides normally used to control weeds are often detected in surface runoff at high levels, particularly if runoff-producing storms occur shortly after application. Therefore, we measured losses of alachlor, atrazine, linuron, and metribuzin from seven small (0.45–0.79-ha) watersheds for 9 yr (1993–2001) to investigate whether a reduced-input system for corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production with light disking, cultivation, and half-rate herbicide applications could reduce losses compared with chisel and no-till. As a percentage of application, annual losses were highest for all herbicides for no-till and similar for chisel and reduced-input. Atrazine was the most frequently detected herbicide and yearly flow-weighted concentrations exceeded the drinking water standard of 3 µg L^–1 in 20 out of 27 watershed years that it was applied. Averaged for 9 corn yr, yearly flow-weighted atrazine concentrations were 26.3, 9.6, and 8.3 µg L^–1 for no-till, chisel, and reduced-input, respectively. Similarly, flow-weighted concentrations of alachlor exceeded the drinking water standard of 2 µg L^–1 in 23 out of 54 application years and in all treatments. Thus, while banding and half-rate applications as part of a reduced-input management practice reduced herbicide loss, concentrations of some herbicides may still be a concern. For all watersheds, 60 to 99% of herbicide loss was due to the five largest transport events during the 9-yr period. Thus, regardless of tillage practice, a small number of runoff events, usually shortly after herbicide application, dominated herbicide transport.

Abbreviations: DAA, days after application • HAL, health advisory level • MCL, maximum contaminant level • NAEW, North Appalachian Experimental Watershed

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