High levels of cover-crop residue can suppress weed emergence and also can intercept preemergence herbicides and potentially reduce their effectiveness. This research was conducted in continuous no-tillage corn to compare the effect of residue from a hairy vetch cover crop with that of background crop residue on the soil solution concentration of atrazine and metolachlor and on the emergence of weeds with and without herbicide treatment. In a 3-yr field experiment, 5-cm-deep soil samples were taken and the weed density measured in paired microplots with and without herbicide at approximately weekly intervals after application of atrazine and metolachlor. High levels of residue were present in both treatments; the percentage of soil covered by residue ranged from 91 to 99 in the no–cover-crop treatment and from 99 to 100 in the hairy vetch treatment. Initial metolachlor concentration was lower and degradation rate higher in two of the 3 yr with a hairy vetch cover crop than without a cover crop. Cover-crop treatment had little effect on atrazine concentration or degradation. Annual grass weeds (predominantly fall panicum) were the major species in this field. Hairy vetch alone reduced grass emergence by 50 to 90%, and preemergence herbicides alone reduced emergence by 72 to 93% compared with the treatment without cover crop and herbicide. The combination of preemergence herbicides with hairy vetch provided only 24 to 61% control of grass weeds compared with control by hairy vetch alone and 23 to 52% compared with control by herbicide alone, suggesting an antagonism probably resulting from reduced metolachlor concentration by hairy vetch residue. Metolachlor with hairy vetch delayed emergence of weeds and reduced the concentration of metolachlor required to prevent emergence initiation compared with metolachlor without a cover crop.

Sorption and desorption of cyanazine with three Mississippi Delta soils (two silt loams and one silty clay) were studied under laboratory conditions. Cyanazine sorption calculated using the Freundlich equation was greatest for the Sharkey silty clay soil. Partition coefficients (Kd values) for cyanazine sorption ranged from 1.67 to 1.82, 1.92 to 2.15, and 3.65 to 3.96 ml g−1 for the Bosket silt loam, Dubbs silt loam, and Sharkey silty clay soils, respectively. Differences in sorption and Kd values were attributed to clay content. At a given initial cyanazine concentration, cyanazine was desorbed more readily from the silt loam soils than from the Sharkey clay after the first 4-h desorption cycle. Desorption from the Sharkey clay continued for a longer period than that from the silt loam soils, with up to 6% cyanazine desorption from the Sharkey clay after a 16-h desorption cycle compared with 0% for the silt loam soils. Cyanazine losses increased with decreasing clay content, Dubbs = Bosket > Sharkey. This implies a potential relationship between cyanazine desorption and surface runoff losses of cyanazine.