Herbicide Losses in Surface Runoff from Conservation Tilled Watersheds Planted with Glyphosate-Tolerant Corn and Soybean.

Conservation tillage is frequently used for corn (Zea mays L.) and soybean [Glycine max (L.) Merr] production to reduce soil loss, improve soil quality, and maintain eligibility for commodity support payments. The residual herbicides normally used in the production of these crops, however, are often detected in surface runoff, rivers, and reservoirs at concentrations that exceed drinking water standards. With the advent of transgenic, glyphosate-tolerant [N-(phosphonomethyl)-glycine] (Roundup Ready) corn and soybean  this concern might be reduced by replacing residual herbicides with  glyphosate, a short-half life, strongly sorbed, contact herbicide. Therefore, we applied both herbicide types at the maximum recommended rates to 2 chisel-plowed and 2 no-till watersheds in a 2-yr corn/soybean rotation and at half rates to 3 reduced-input, disked watersheds in a 3-yr corn/soybean/wheat-red clover (Triticum aestivum L. - Trifolium pratense L.) rotation and monitored herbicide losses in surface runoff on an individual event basis for 3 crop years (2002-2004). Growing transgenic corn and soybean and replacing residual herbicides with glyphosate did not eliminate herbicide losses in surface runoff, but transport and concentrations where substantially reduced in most instances. Moreover, glyphosate is of lower toxicity than the residual herbicides it can replace and its presence in surface water should be of less environmental concern.