Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2007
Publication Date: 3/3/2008
Citation: Shipitalo, M.J., Malone, R.W., Owens, L.B. 2008. Impact of glyphosate-tolerant soybean and glufosinate-tolerant corn production on herbicide losses in surface runoff. Journal of Environmental Quality. 37:401-408. Interpretive Summary: Conservation tillage practices are often necessary to reduce soil loss, but the residual herbicides commonly used with these practices to control weeds are frequently lost in high concentrations in surface runoff. When genetically modified, herbicide-tolerant, soybean and corn became commercially available in the 1990s it became possible to replace some of the commonly used residual herbicides with contact herbicides that breakdown more rapidly, are more strongly held by the soil, and are more environmental benign. In this 4-yr field study, we found that losses of contact herbicides were generally less than those for the residual herbicides they can replace. More importantly, the concentrations of the contact herbicides in the runoff never exceeded their applicable drinking water standards while the residual herbicides frequently exceeded their standards, particularly in the first few runoff events after they were applied. Our results suggest that the concerns of farmers and the regulatory community with corn and soybean herbicide losses in runoff can be reduced by planting herbicide-tolerant versions of these crops and replacing some of the residual herbicides with contact herbicides.
Technical Abstract: Residual herbicides normally used in the production of soybean [Glycine max (L.) Merr] and corn (Zea mays L.) are often detected in surface runoff at concentrations that exceed drinking water standards. With the advent of transgenic, glyphosate-tolerant soybean and glufosinate-tolerant corn this concern might be reduced by replacing some of these herbicides with short-half life, strongly sorbed, contact herbicides. Therefore, we applied both herbicide types to 2 chiseled and 2 no-till watersheds in a 2-yr corn/soybean rotation and at half rates to 3 disked watersheds in a 3-yr corn/soybean/wheat-red clover (Triticum aestivum L. - Trifolium pratense L.) rotation and monitored herbicide losses for 4 crop years. In soybean years, average glyphosate loss (0.07%) was ~7x less than metribuzin loss (0.48%) and ~2x less than alachlor loss (0.12%), residual herbicides it can replace. Maximum, annual, flow-weighted concentrations of glyphosate (9.2 ug/L) and metribuzin (9.5 ug/L) were well below their drinking water standards of 700 ug/L and 200 ug/L, respectively, while alachlor (44.5 ug/L) was well above its 2 ug/L Maximum Contaminant Level (MCL). In corn years, average glufosinate loss (0.10%) was similar to the losses of alachlor (0.07%) and linuron (0.15%), but ~ 4x less than atrazine (0.37%). Maximum, annual, flow-weighted concentration of glufosinate (no MCL) was 3.5 ug/L, whereas atrazine (31.5 ug/L) and alachlor (9.8 ug/L) substantially exceeded their MCLs of 3 ug/L and 2 ug/L, 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 herbicide concentrations in runoff exceeding drinking water standards.