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TECHNICAL REPORTS

Organic Compounds in the Environment

Herbicide Incorporation by Irrigation and Tillage Impact on Runoff Loss

^a USDA-ARS, Southeast Watershed Research Lab., Box 748, Tifton, GA 31793
^b USDA-ARS, Crop Protection and Management Research Unit, Box 748, Tifton, GA 31793. Mention of trade names or commercial products in this article does not imply recommendation or endorsement by the authors or the USDA

^* Corresponding author (Tom.Potter{at}ars.usda.gov).

Received for publication June 12, 2007. Runoff from farm fields is a common source of herbicide residues in surface waters. Incorporation by irrigation has the potential to reduce herbicide runoff risks. To assess impacts, rainfall was simulated on plots located in a peanut (Arachis hypogaea L.) field in Georgia's Atlantic Coastal Plain region after pre-emergence application of metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl]-acetamide) and pendimethalin (N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitro-benzenamine). Runoff, sediment, and herbicide loss as function of strip tillage (ST) versus conventional tillage (CT) were compared with and without irrigation (12.5 mm) after application of an herbicide tank mixture. For the CT system, metolachlor runoff was reduced 2x and pendimethalin 1.2x when compared with the non-irrigated treatment. The difference in irrigated and non-irrigated metolachlor means was significant (P = 0.05). Irrigation reduced metolachlor runoff by 1.3x in the ST system, but there was a 1.4x increase for pendimethalin. Overall results indicated that irrigation incorporation reduces herbicide runoff with the greatest impact when CT is practiced and products like metolachlor, which have relatively low K_oc and high water solubility, are used. The lower ST system response was likely due to a combination of spray interception and retention by the ST system cover crop mulch and higher ST soil organic carbon content and less total runoff. During the study, the measured K_oc of both herbicides on runoff sediment was found to vary with tillage and irrigation after herbicide application. Generally, K_oc was higher for ST sediment and when irrigation incorporation was used with the CT system. These results have significant implications for simulation model parametization.

Abbreviations: AWC, antecedent soil water content • CsT, conservation tillage • CT, conventional tillage • DAT, days after treatment • K_oc, sediment organic carbon water partition coefficient • RZWQM, Root Zone Water Quality Model • SOC, soil organic carbon • ST, strip tillage • VWC, volume-weighted concentration