Submitted to: Weed Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/7/2005
Publication Date: 5/20/2005
Citation: Locke, M.A., Zablotowicz, R.M., Bauer, P.J., Steinriede Jr, R.W., Gaston, L.A. 2005. Conservation cotton production in the southern United States: Herbicide dissipation in soil and cover crops. Weed Science. 53:717-727. Interpretive Summary: An increasing number of farmers are using conservation practices in cotton production systems, but information on the environmental impact of these management practices is limited. Multiple year field studies were conducted at two locations to assess the effects of tillage and cover crop management on herbicide dissipation in soil. These studies showed that increased residues and soil organic carbon resulting from conservation practices influenced dissipation of herbicides such as fluometuron and norflurazon in cotton production systems. The crop residues accumulated on the surface of reduced tillage soils intercepted and retained herbicides, enhancing their dissipation and impeding movement into soil. These studies are important because they demonstrated that using conservation practices helped to mitigate off-target movement of herbicides.
Technical Abstract: 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. Treatments in MS included conventional tillage vs. no-tillage and ryegrass cover vs. no cover. Treatments in SC included conventional tillage vs. reduced tillage and rye cover vs. no cover. Fluometuron herbicide was applied as a preemergence in MS and SC, and norflurazon was applied in MS. Soils were sampled various times during the growing season at depths of 0 to 2 and 2 to 10 cm. Cover crop residues were sampled from the reduced- or no-tillage cover crop areas. Soil and cover crop residues were extracted and analyzed for herbicides using HPLC. Soil organic C levels tended to increase with reduction of tillage and presence of a cover crop. Fluometuron and norflurazon sorption indicated a strong positive relationship with soil organic C content, especially in the 0 to 2 cm depth. Including both locations, herbicide half-lives ranged from 7 to 15 d in the soil surface. Tillage had mixed effects on herbicide persistence in the soil surface, with higher herbicide concentrations in conventional tillage at early samplings, but differences diminishing at later samplings. The most consistent effects were observed in reduced- or no-tillage treatments with cover crops. In those areas, cover crop residues intercepted much of the applied herbicide and impeded subsequent movement into the soil. Herbicide dissipation in cover crop residues was often more rapid than in soil, with half-lives ranging from 3 to 11 d. Retention of herbicide in cover crop residues and rapid dissipation were attributed to strong herbicide affinity to cover crop residues (e.g., Kd rye = 7.1 vs. Kd Dundee conventional tillage, no cover = 1.65) and co-metabolism of herbicide as cover crop residues decomposed. The fluometuron metabolite, desmethyl fluometuron, was observed in most soil and cover crop samples one wk after application. There was minimal herbicide or metabolite movement into or accumulation within the 2 to 10 cm zone, and little treatment effect could be ascribed to herbicide or metabolite movement below two cm.