Conservation cotton production in the southern United States: herbicide dissipation in soil and cover crops

Soil and surface residues from cotton field studies in Stoneville, MS (1994 through 1996) and Florence, SC (1995 through 1996) were sampled to evaluate effects of cover crop and tillage on herbicide dissipation. Mississippi treatments included tillage (conventional [CT]; none [NT]) and cover crop (ryegrass; none [NC]). South Carolina treatments included tillage (CT; reduced tillage [RT]) and cover crop (rye; NC). Fluometuron was applied preemergence (PRE) in both Mississippi and South Carolina, and norflurazon was applied PRE in Mississippi. Soils were sampled various times during the growing season (depths: 0 to 2 cm, 2 to 10 cm). Cover crop residues were sampled from RT or NT cover crop areas. Soil and cover crop sample extracts were analyzed for herbicides. Soil organic carbon tended to increase with tillage reduction and presence of cover crop and was positively correlated with herbicide sorption, especially in the surface. Across locations, herbicide half-lives ranged from 7 to 15 d in the soil surface. Tillage had mixed effects on herbicide persistence in surface soil, with higher herbicide concentrations in CT at early samplings, but differences were insignificant later on. The most consistent effects were observed in RUNT with cover crops, where cover crop residues intercepted applied herbicide, impeding subsequent movement into soil. Herbicide dissipation in cover crop residues was often more rapid than in soil, with half-lives from 3 to 11 d. Herbicide retention in cover crop residues and rapid dissipation were attributed to strong herbicide affinity to cover crop residues (e.g., fluometuron K-d = 7.1 [in rye]; K-d = 1.65 [in Mississippi Dundee soil CT, NC]) and herbicide co-metabolism as cover crop residues decomposed. A fluometuron metabolite, desmethyl-fluometuron, was observed in most soil and cover crop samples after 1 wk. Only minimal herbicide or metabolite moved into the subsurface, and little treatment effect could be ascribed to herbicide or metabolite movement below 2 cm.