Soil adsorption studies of a rice herbicide, cyhalofop-butyl, in two texturally different soils of India.

The ability of herbicides to be adsorbed by the soil and sediment and their tendency to be desorbed are some of the most important factors affecting soil and water contamination. Therefore, a sorption study was conducted to evaluate the adsorption of cyhalofop-butyl, butyl (2R)-2-[4-(4-cyano-2-fluorophenoxy) phenoxy] propanoate, in the sandy clay loam and clayey soils using a batch equilibrium method. The adsorption of cyhalofop-butyl was found positively related with the clay and organic carbon content. Freundlich constants (Kf) of cyhalofop-butyl in the clayey and sandy clay loam were found to be 13.39 and 2.21, respectively. Sorption coefficients (Koc) and distribution coefficients (Kd) were found to be 265.38 and 2,092.79, and 1.38 and 11.48, for sandy clay loam and clayey soils, respectively. The adsorption isotherm suggested a relatively higher affinity of cyhalofop-butyl to the adsorption sites at low equilibrium concentrations. The low value of the soil organic carbon partition coefficient (Koc) of cyhalofop-butyl in the sandy loam soil suggested its weaker adsorption in soil and thus increased its risk of mobility into water sources; hence, it should be used judiciously to prevent groundwater contamination.

Cyhalofop-p-butyl mobility and distribution of residues in soil at various depths.

This study was conducted to evaluate cyhalofop-p-butyl mobility in a sandy loam soil and subsequent distribution of residues at various depths under field conditions. Soil samples were taken from 0 to 150 cm depths at 3-90 d after rains in lysemeter of 1 and 2 m depths. Cyhalofop-p-butyl application at two rates and subsequent precipitation had a significant impact on soil, physico-chemical properties and herbicide mobility. Precipitation caused substantial mobility of cyhalofop-p-butyl in the soil and 1.1-7.6 µg L(-1) of cyhalofop-p-butyl was found in leachates. Cyhalofop-p-butyl residues in the leachates were probably due to preferential flow through the soil. Cyhalofop-p-butyl residues were detected in significant amounts from the soil up to 10 d, later, residues were found below the detection limit but its three transformation products viz., cyhalofop acid, diacid, and phenol were detected.