Environmental Fate and Dietary Risk Assessment Associated with Triafamone, Ethoxysulfuron and Their Metabolites in Rice Ecosystem

Abstract

ABSTRACT Triafamone and ethoxysulfuron are ALS inhibitor herbicides belonging to sulfonamide and sulfonylurea group of herbicides used for control of weeds in rice ecosystem. Considering the environmental risk of these herbicides, the fate of triafamone and ethoxysulfuron was assessed to understand the best use of these herbicides. Degradation studies were carried out under field conditions in direct seeded rice (DSR) and transplanted rice (TPR) at an application rate of 67.5 and 135 g ha−1 and adsorption-desorption studies were carried out using batch equilibration technique. Residues of triafamone, ethoxysulfuron and their metabolites were extracted by ultrasonic assisted extraction method from soil and rice grain samples and liquid liquid extraction from straw samples and quantified using HPLC. Degradation half-life (DT50) of triafamone varied from 13.02–20.93 and 12.09–14.67 days under DSR and TPR conditions, respectively while for ethoxysulfuron DT50 ranged from 11.71–15.64 and 9.90–12.19 days. Among both the herbicides, triafamone was more persistent in the soil and at harvest, concentrations of triafamone and ethoxysulfuron in soil and rice were below the acceptable limit (<0.01 mg kg−1) imposed by principal regulatory agencies. Appearance and persistence of studied metabolites corroborated the degradation of parent molecule. Hazard quotient (HQ) of triafamone and ethoxysulfuron was 0.0013 and 0.000657 suggesting that application of these herbicides was safe for use in rice. Triafamone and ethoxysulfuron had a transient effect on soil biochemical and microbiological parameters depending on herbicide dose and moisture conditions. Freundlich adsorption coefficient (Kf) was 1.81 and 1.06 µg1–1/ng−1ml1/n for triafamone and ethoxysulfuron, respectively. GUS index for triafamone and ethoxysulfuron were 1.4 and 2.0 indicating low and moderate mobility, respectively. Principal component analysis (PCA) indicated that ethoxysulfuron has higher leachability than triafamone. Low to moderate persistence, adsorption and leaching of triafamone and ethoxysulfuron and transient ecological impacts on soil microorganisms may pave the way for safe and efficient weed control in rice cultivation.