Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 19, 2006
Publication Date: October 27, 2006
Citation: Shipitalo, M.J., Owens, L.B. 2006. Tillage system, application rate, and extreme event effects on herbicide losses in surface runoff. Journal of Environmental Quality. 35:2186-2194. 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. In this field study, we investigated whether herbicide losses could be reduced by substituting tillage for some of the herbicides used for corn and soybean production with chisel-till and no-till conservation practices. In general, we found that herbicide losses as a percentage of application were small (<5%) for all tillage practices, but tended to be greatest with no-till. Even with half-rate applications, however, the concentrations of some of the herbicides in runoff were greater that drinking water standards. Thus, while banding and half-rate applications as part of a reduced-input practice reduced herbicide loss in runoff, concentrations of some herbicides may still be a concern. Moreover, the risk of soil and yield loss was greater with the reduced-input practice than with chisel-till or no-till conservation practices. Farmers and the regulatory community will benefit by knowing the tradeoffs that occur from increasing tillage in order to reduce herbicide use. For all tillage practices, most of the herbicide loss was the result of rainstorms that produced runoff shortly after herbicide application. Techniques to minimize or mitigate these losses need to be developed in order to reduce herbicide contamination of surface waters.
Technical Abstract: 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 7 small (0.45-0.79 ha) watersheds for 9 years (1993 to 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 to chisel and no-till. Herbicide losses varied considerably among watersheds and years as a result of variation in rainfall timing and amounts. As a percent 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 frequently exceeded the drinking water standard of 3 ug/L. Averaged for 9 years, yearly flow-weighted atrazine concentrations were 26.3, 9.6 and 8.3 ug/L for no-till, chisel, and reduced-input, respectively. Similarly, flow-weighted concentrations of alachlor frequently exceeded the drinking water standard of 2 ug/L for all treatments. Thus, while banding and half-rate applications as part of a reduced-input management practice reduced herbicide loss in runoff, concentrations of some herbicides may still be a concern. For all watersheds, 60 to 99% of herbicide loss was due to the 5 largest transport events during the 9-year period. Thus, regardless of tillage practice, a small number of runoff events, usually shortly after herbicide application, dominated herbicide transport.