FLUOMETURON BEHAVIOR IN LONG-TERM TILLAGE PLOTS

Abstract

Fluometuron (N, N-dimethyl-N'-[3-(trifluoromethyl) phenyl] urea) is a preemergence herbicide used widely for weed control in cotton (Gossypium hirsutum L.). Field research results suggest that many herbicides, including fluometuron, may be subject to preferential transport and leaching through the soil profile. The objectives of this study were to evaluate the distribution and leaching of fluometuron in a Lexington silt loam (Typic Paleudalf) through 3 years of study by analyzing depth incremented soil samples and tension-free pan lysimeter leachates collected at a soil depth of 90 cm under no-till (NT) and conventional tillage (CT, disc 2x - roller harrowed) management. Fluometuron depth distribution following application and subsequent rainfall was related to soil organic carbon content, with tillage effects restricted to the surface 2-cm. The amount of herbicide remaining in the soil profile after application decreased with increasing rainfall amounts. On average, 11% of applied herbicide in NT and 6% in CT remained in the surface 15 cm after 1 year. Fluometuron was detected in all lysimeter leachates during the 3-year period. Peak herbicide leachate concentrations ranged from 68 mg L -1 to 1700 mg L -1 , depending on herbicide application in relation to rainfall timing. Annual leaching losses ranged from 5 to 53% of applied fluometuron during the study. Of the amounts lost to leaching, 68 to 100% (averaging 87% of the total lost to leaching) were lost during the first two or three significant rainfall events after treatment. Rainfall timing in relation to herbicide application was the dominant effect in herbicide mobilization through the soil profile. Antecedent soil moisture content (predicted from rainfall amounts prior to fluometuron application) also influenced the appearance of leachate and herbicide in the lysimeters, with lower soil moisture conditions at application producing lower preferential flow. Depth-incremented soil sampling after initial rainfall events was a poor mechanism for detecting fluometuron mobility through the soil profile. The rapidity at which fluometuron was detected in the lysimeter leachates, the exceedingly high fluometuron concentrations, and the variability in leachate collection by the lysimeters, illustrated the significance of preferential flow. The influence of tillage practice on the volume of drainage collected by the lysimeters and on fluometuron leaching was of little significance when compared with the influence of natural infiltration heterogeneity.