Effect of cover crop and tillage on fluometuron bioavailability

Studies were conducted in 1991 and 1992 to determine the effects of long-term no-tillage and cover crops on efficacy and degradation of fluometuron in the field. Experiments were established on an area which had been planted no-till (NT) or conventional-till (CT) into either no cover crop (NC) or vetch (VE) continuously since 1981. The study utilized a two-by-two factorial arrangement of treatments in which the factors consisted of tillage systems (CT or NT) and cover crop (VE or NC).

Fluometuron was applied preemergence (PRE) at 1.7 kg ai ha⁻¹ broadcast. Soil samples were collected immediately after fluometuron application and at approximately 1, 2, 3, 4, 6, 8, and 12 wk after treatment (WAT). Fluometuron was quantified by High Pressure Liquid Chromatography (HPLC). Visual evaluations of weed control were made at approximately 3, 7, and 9 WAT.

Control of Palmer amaranth was lower in NT than in CT both years. Control of Palmer amaranth was not affected by cover crops.

Degradation of fluometuron did not differ among treatments in either surface soils (0 to 4 cm) or subsurface soils (4 to 8 and 8 to 15 cm). Therefore the differences in Palmer amaranth control could not be explained by differences in degradation of fluometuron among treatments.

Additional studies were conducted in 1992 to determine the effects of no-tillage and cover crops on adsorption and degradation of fluometuron in soil under controlled conditions. Soil from each treatment-by-depth combination in the previous study was collected prior to application of fluometuron. Adsorption of fluometuron on soil was determined using a slurry technique, and a distribution coefficient (Kd) was calculated for each treatment-by-depth combination.

Adsorption of fluometuron on soil was affected by tillage and cover crop. Adsorption was greatest in NT VE at a soil depth of 0 to 4 cm. At 4 to 8 cm adsorption in CT VE was higher than in NT VE.

Adsorption of fluometuron on soil correlated with various soil characteristics, with organic matter content being the most important. Cation exchange capacity was also highly correlated with adsorption of fluometuron.

Degradation of fluometuron under controlled conditions was determined for each treatment-by-depth combination by fortifying soil with fluometuron and incubating at 30 C. Fluometuron concentrations were quantified by HPLC.

First-order kinetics explained degradation of fluometuron at each soil depth (r²=0.99). Degradation of fluometuron at 0 to 4 cm was more rapid in CT NC than in NT VE, although all treatments had half-lives <49 d. No differences in degradation of fluometuron were observed between treatments at 4 to 8 or 8 to 15 cm soil depths.

Dissipation of fluometuron was slowest under NT VE at all soil depths. Dissipation also tended to be slower under NT than under CT at the 0 to 4 and 4 to 8 cm depths regardless of the cover crop effect. Soil under NT may adsorb more fluometuron rendering it unavailable for degradation or plant uptake.