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TECHNICAL REPORTS

Surface Water Quality

Interactions of Tillage and Cover Crop on Water, Sediment, and Pre-emergence Herbicide Loss in Glyphosate-Resistant Cotton: Implications for the Control of Glyphosate-Resistant Weed Biotypes

^a United States Department of Agriculture, Agriculture Research Service, Southern Weed Science Research Unit, P.O. Box 350, Stoneville, MS 38776
^b United States Department of Agriculture, Agriculture Research Service, National Sedimentation Laboratory, Water Quality and Ecology Research Unit, P.O. Box 1157, Oxford, MS 38655

^* Corresponding author (jason.krutz{at}ars.usda.gov).

Received for publication July 28, 2008. The need to control glyphosate [N-(phosphonomethyl)glycine]-resistant weed biotypes with tillage and preemergence herbicides in glyphosate-resistant crops (GRCs) is causing a reduction in no-tillage hectarage thereby threatening the advances made in water quality over the past decade. Consequently, if environmental gains afforded by GRCs are to be maintained, then an in-field best management practice (BMP) compatible with tillage is required for hectarage infested with glyphosate-resistant weed biotypes. Thus, 1 d after a preemergent application of fluometuron [N,N-dimethyl-N'-(3-(trifluoromethyl)phenyl)urea] (1.02 kg ha^–1) and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] (1.18 kg ha^–1) to a Dundee silt loam (fine-silty, mixed, active, thermic Typic Endoaqualf), simulated rainfall (60 mm h^–1) was applied to 0.0002-ha microplots for approximately 1.25 h to elucidate tillage (no tillage [NT] and reduced tillage [RT])and cover crop (no cover [NC] and rye cover [RC]) effects on water, sediment, and herbicide loss in surface runoff. Regardless of tillage, RC delayed time-to-runoff 1.3-fold, reduced cumulative runoff volume 1.4-fold, and decreased cumulative sediment loss 4.7-fold. Cumulative fluometuron loss was not affected by tillage or cover crop. Conversely, total metolachlor loss was 1.3-fold lower in NT than RT and 1.4-fold lower in RC than NC. These data indicate that RC can be established in hectarage requiring tillage and potentially curtail water, sediment, and preemergence herbicide losses in the spring to levels equivalent to or better than that of NT, thereby protecting environmental gains provided by GRCs.

Abbreviations: BMPs, best management practices • GRCs, glyphosate-resistant crops • K_oc, organic carbon partitioning coefficient • MDL, method detection limit • NC, no cover • NT, no tillage • RC, rye cover • RT, reduced tillage