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 metabolites 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 μg 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 detection. 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 metolachlor and metolachlor metabolites, respectively. These results demonstrate that adsorption 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 preferentially 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-methylphenyl)(2-methoxy-1-methylethyl-1)amino]-2-oxoethanesulfonic acid); metolachlor oxanilic acid (2-[(2-ethyl-6-methylphenyl)(2-methoxy-1-methylethyl)amino]-2-oxoacetic acid); buffalograss, Buchloe dactyloides (Nutt.) Engelm.