Author(s): S. ROY, A. BHATTACHARYYA AND A. CHOWDHURY

Abstract: A laboratory experiment was performed to understand the persistence behaviour of the herbicide dithiopyr in three different soils of West Bengal, India. Estimation of dithiopyr residues revealed its dissipation following first-order kinetics. The half-life (T_1/2) values were in the range of 13.75-31.03 days for T₁ and 21.20-41.24 days for T₂ . The dissipation of 1/2 1 2 dithiopyr was maximum in coastal soil with the shortest T_1/2 (13.75 and 21.20 days) and the higher persistence was 1/2 recorded in Gangetic alluvial soil. Formation of three metabolites of dithiopyr viz. dithiopyr monoacid, reverse monoacid and diacid were also quantified and their structures were confirmed by Gas chromatography-Mass spectrometry (GC-MS). The metabolites were detected in soils at 7-30 days after application with varying rates of formation reaching maximum between 30-90 days followed by a decreasing trend. Among the metabolites, only diacid could be detected till 120 days. The degradation pathway of dithiopyr in soil involved pH induced abiotic hydrolysis of the methylthioester groups to form the monoacids and diacid which was more favourable under alkaline condition as observed in coastal soil (pH = 8.50) compared to the Terai soil (pH = 5.20) and the least in Gangetic alluvial soil (pH = 6.90).

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