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TECHNICAL REPORTS

Surface Water Quality

Impact of Glyphosate-Tolerant Soybean and Glufosinate-Tolerant Corn Production on Herbicide Losses in Surface Runoff

^a USDA-ARS, North Appalachian Experimental Watershed, P.O. Box 488, Coshocton, OH 43812-0488
^b USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable

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

Received for publication December 15, 2006. Residual herbicides used in the production of soybean [Glycine max (L.) Merr] and corn (Zea mays L.) are often detected in surface runoff at concentrations exceeding their maximum contaminant levels (MCL) or health advisory levels (HAL). With the advent of transgenic, glyphosate-tolerant soybean and glufosinate-tolerant corn this concern might be reduced by replacing some of the residual herbicides with short half-life, strongly sorbed, contact herbicides. We applied both herbicide types to two chiseled 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 (Triticum aestivum L.)-red clover (Trifolium pratense L.) rotation and monitored herbicide losses in runoff water for four crop years. In soybean years, average glyphosate loss (0.07%) was 1/7 that of metribuzin (0.48%) and about one-half that of alachlor (0.12%), residual herbicides it can replace. Maximum, annual, flow-weighted concentration of glyphosate (9.2 µg L^–1) was well below its 700 µg L^–1 MCL and metribuzin (9.5 µg L^–1) was well below its 200 µg L^–1 HAL, whereas alachlor (44.5 µg L^–1) was well above its 2 µg L^–1 MCL. In corn years, average glufosinate loss (0.10%) was similar to losses of alachlor (0.07%) and linuron (0.15%), but about one-fourth that of atrazine (0.37%). Maximum, annual, flow-weighted concentration of glufosinate (no MCL) was 3.5 µg L^–1, whereas atrazine (31.5 µg L^–1) and alachlor (9.8 µg L^–1) substantially exceeded their MCLs of 3 and 2 µg L^–1, respectively. Regardless of tillage system, flow-weighted atrazine and alachlor concentrations exceeded their MCLs in at least one crop year. Replacing these herbicides with glyphosate and glufosinate can reduce the occurrence of dissolved herbicide concentrations in runoff exceeding drinking water standards.

Abbreviations: AGR, aminomethylphosphonic acid to glyphosate ratio • AMPA, aminomethylphosphonic acid • DEA, deethylatrazine • DIA, deisopropylatrazine • DWLOC, drinking water levels of comparison • HAL, health advisory level • MCL, maximum contaminant level • NAEW, North Appalachian Experimental Watershed