166 • Weed Science 52, January–February 2004 Weed Science, 52:166–171. 2004 Infiltration and adsorption of dissolved metolachlor, metolachlor oxanilic acid, and metolachlor ethanesulfonic acid by buffalograss (Buchloe dactyloides) filter strips Larry J. Krutz Corresponding author. Department of Soil and Crop Sciences, Texas Agricultural Experiment Station, Texas A&M University, College Station, TX 77843-2474; lkrutz@ag.tamu.edu Scott A. Senseman Department of Soil and Crop Sciences, Texas Agricultural Experiment Station, Texas A&M University, College Station, TX 77843-2474 Monty C. Dozier Department of Soil and Crop Sciences, Texas Cooperative Extension, Texas A&M University, College Station, TX 77843 Dennis W. Hoffman Blackland Research Center, Texas Agricultural Experiment Station, Temple, TX 76502 Dennis P. Tierney Environmental Stewardship and Regulatory Policy, Syngenta Crop Protection, P.O. Box 18300, Greensboro, NC 27419-8300 Vegetative ﬁlter strips (VFS) potentially reduce herbicide transport from agricultural ﬁelds by increasing herbicide mass inﬁltrated (M inf ) and herbicide mass adsorbed (M as ) compared with bare ﬁeld soil. However, there are conﬂicting reports in the literature concerning the contribution of M as to herbicide trapping efﬁciency (TE). Moreover, no study has evaluated TE among metolachlor and metolachlor metab- olites in a VFS. This experiment was conducted to compare TE, M inf , and M as among metolachlor, metolachlor oxanilic acid (OA), and metolachlor ethanesulfonic acid (ESA) in buffalograss ﬁlter strips. Runoff was applied as a point source upslope of a 1-  3-m microwatershed at a rate of 750 L h -1 . The point source was fortiﬁed with metolachlor, metolachlor OA, and metolachlor ESA, each at 0.12 g ml -1 . After moving through the plot, water samples were collected at 5-min intervals and stored at 5 C until analysis. Water samples were extracted using solid-phase extraction and analyzed by high-performance liquid chromatography–photodiode array detec- tion. TE was signiﬁcantly greater for metolachlor (25.3%) as compared with the OA (15.5%) and ESA metabolites (14.2%). The average M inf was 8.5% and was not signiﬁcantly different among compounds. Signiﬁcantly more metolachlor (17.3%) was retained as M as compared with either metolachlor OA (7.0%) or metolachlor ESA (5.5%). Moreover, M as accounted for 68 and 42% of the total TE for meto- lachlor and metolachlor metabolites, respectively. These results demonstrate that ad- sorption to the VFS grass, grass thatch, or soil surface (or all) is an important retention mechanism for metolachlor and metolachlor metabolites, especially under saturated conditions. Moreover, the M as data indicate that metolachlor is preferen- tially retained by the VFS grass, grass thatch, or soil surface (or all) compared with the OA and ESA metabolites. Greater metolachlor retention in VFS compared with the OA and ESA metabolites may partially explain why metolachlor metabolites are frequently measured at higher concentrations than metolachlor is in surface water. Nomenclature: Metolachlor; metolachlor ethanesulfonic acid, (2-[(2-ethyl-6-meth- ylphenyl)(2-methoxy-1-methylethyl-1)amino]-2-oxoethanesulfonic acid); metolach- lor oxanilic acid (2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2- oxoacetic acid); buffalograss, Buchloe dactyloides (Nutt.) Engelm. Key words: Vegetative ﬁlter strips, metabolites, trapping efﬁciency. Metolachlor is widely used for weed control on corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] and has been shown to degrade to metolachlor ethanesulfonic acid (ESA) and metolachlor oxanilic acid (OA). Metolachlor ESA for- mation is a biologically mediated process that occurs through glutathione conjugation, a common detoxiﬁcation process in plants, animals, and microorganisms (Aga et al. 1996; Field and Thurman 1996). Pathway(s) describing the degradation of metolachlor to metolachlor OA are not avail- able. Runoff water can transport ﬁeld-applied metolachlor and metolachlor metabolites to rivers, lakes, and streams, resulting in deterioration of surface water quality. Research has been conducted regarding the occurrence and environ- mental fate of metolachlor in hydrologic systems. Few stud- ies have considered metolachlor metabolites. However, both metolachlor and metolachlor metabolites have been detected in surface and groundwater (Aga and Thurman 2001; Kol- pin et al. 2000; Lambropoulou et al. 2002; Lerch et al. 1998; Phillips et al. 1999; Senseman et al. 1997). Moreover, metolachlor metabolites frequently constitute a majority of metolachlor’s measured concentration in hydrologic systems (Kolpin et al. 1996, 2000; Thurman et al. 1996). Vegetative ﬁlter strips (VFS) are a best-management practice recom- mended by the United States Department of Agriculture (USDA) to reduce herbicide-runoff losses from agricultural production areas. Data indicate that greater inﬁltration (M inf ) in VFS compared with bare ﬁeld soil is the governing process for the retention of moderately adsorbed herbicides (Arora et al. 1996; Barﬁeld et al. 1998; Kloppel et al. 1997; Misra et al. 1996; Schmitt et al. 1999; Seybold et al. 2001). However, herbicide adsorption to the VFS grass, grass thatch, or soil surface (M as ) is also a proposed retention mechanism (Arora et al. 1996; Asmussen et al. 1977; Bar- ﬁeld et al. 1998; Seybold et al. 2001). There are conﬂicting reports regarding the contribution of M as to the overall her- bicide trapping efﬁciency (TE). Kloppel et al. (1997) re- ported that differences between concentrations of the highly soluble dichlorprop-p and the moderately soluble terbuthy- lazine between VFS inﬂow and VFS outﬂow indicated that sorption to grass thatch and soil surfaces did not signiﬁ-