Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2011
Publication Date: 6/8/2011
Citation: Shipitalo, M.J., Owens, L.B. 2011. Comparative losses of glyphosate and selected residual herbicides in surface runoff from conservation-tilled watersheds planted with corn or soybean. Journal of Environmental Quality. 40(4):1281-1289. Interpretive Summary: Planting new crops through some or all of the residue of the previous crop, a farming practice known as conservation tillage, can help to reduce erosion, but the residual herbicides usually necessary for weed control can be lost in high concentrations in the water that runs off the fields. With the genetically modified, glyphosate-tolerant, corn and soybean varieties now available, farmers can replace some of the commonly used residual herbicides with the contact herbicide glyphosate, which break downs rapidly and is more strongly held by the soil and may be less likely to be transported in runoff. In this 3-yr field study, we found that loss of glyphosate in runoff was substantially less than the losses of the residual herbicides it can replace. Perhaps more importantly, in the 18 watershed-years investigated only once did glyphosate concentration exceed its drinking water standard while many of 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 regulators with corn and soybean herbicide losses in runoff can be reduced by planting glyphosate-tolerant versions of these crops and replacing some of the residual herbicides with glyphosate.
Technical Abstract: Conservation tillage is frequently used for corn (Zea mays L.) and soybean [Glycine max (L.) Merr] production to reduce soil loss and improve soil quality. The residual herbicides regularly used in conjunction with conservation tillage to produce these crops, however, are often detected in surface waters at concentrations that exceed their maximum contaminant level (MCL). This concern might be reduced by planting glyphosate-tolerant corn and soybean and totally or partially replacing residual herbicides with glyphosate, a short-half life, strongly sorbed, contact herbicide. 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 (Triticum aestivum L. - Trifolium pratense L.) rotation and monitored herbicide losses in surface runoff for three crop years. Average glyphosate loss for all tillage practices, as a percentage of the amount applied, was 21.4x less than atrazine, 3.5x less than alachlor, and 8.7x less than linuron in corn-crop years. Annual, flow-weighted, concentration of atrazine was as high as 41.3 ug/L, well above its 3 ug/L MCL. Likewise, annual, flow-weighted alachlor concentration (MCL = 2 ug/L) was as high as 11.2 and 4.9 ug/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 ug/L MCL and the highest, annual, flow-weighted concentration was 3.9 ug/L. Growing transgenic corn and soybean and replacing some of the residual herbicides with glyphosate should reduce the impact of the production of these crops on surface water quality.