Residual Fluometuron Levels in Three Arkansas Soils under Continuous Cotton (Gossypium hirsutum) Production

Evidence has shown that fluometuron {N,N-dimethyl-N′-[3-(trifluoromethyl)phenyl]urea} persists beyond the end of the growing season when used in continuous cotton (Gossypium hirsutumL.) production. Samples were taken from three soils following cotton production in 1980, 1981, and 1982. All three soils had been in production under the same herbicide use regime, fluometuron preemergence followed by fluometuron plus MSMA (monosodium methanearsonate), since either 1976 or 1977. The fluometuron remaining in each soil was quantified using a greenhouse bioassay and a chemical extraction technique followed by high-performance liquid chromatography determinations. The fluometuron concentrations determined by bioassay and chemical extraction methods had partial correlation coefficients of 0.62, 0.91, and 0.72 for a Sharkey silty clay, a Dundee silt loam, and a Loring silt loam, respectively. Predictive equations were determined for each soil to relate chemical extraction findings to plant response. Bioassay analysis indicated nearly 2 ppmw of fluometuron in the Sharkey silty clay in October 1980, with 1 ppmw in the Dundee silt loam, and approximately 0.27 ppmw in the Loring silt loam with annual application rates of 4.0, 2.9, and 3.5 kg/ha, respectively. Fluometuron concentrations as determined by chemical analysis were 0.83, 0.34, and 0.14 ppmw, respectively. Fluometuron concentrations declined over the winter in all three soils. Samples taken in March of 1981, 1982, and 1983 showed little difference in carryover levels in the Sharkey silty clay but more yearly variation in the other two soils. Fluometuron was found in all three soils to depths of 60 cm, but more than 55% of the fluometuron was found in the upper 15 cm of each soil. A controlled laboratory study conducted with the three soils showed that both cold and dry conditions reduced fluometuron dissipation rates. In the laboratory under conditions favorable for dissipation, fluometuron had a half-life of 26 days in the Dundee silt loam, 43 days in the Loring silt loam, and 73 days in the Sharkey silty clay. In the field, dissipation was very rapid in the Loring silt loam compared to the Dundee silt loam and the Sharkey silty clay.