The study delineates the effect of repeated application of butachlor to rice crop from 1997 onwards. Additionally, in 2014 and 2015, dissipation kinetics of butachlor in soil was studied under field and laboratory conditions. The average recovery of butachlor for soil, rice grain and rice straw ranged between 80.3%-93.2% and 82.8%-96.5% with quantification limit of 0.01 and 0.003 µg g-1 for HPLC and GC-MS/MS, respectively. The dissipation of butachlor followed first order kinetics and half-life under long term field trials in rice soil varied from 15.2 to 19.29 days and 25.94 to 29.79 days under field and laboratory conditions, respectively. The residue of butachlor in soil, rice grain and straw samples at harvest over the years was below the quantification limit and no quantifiable amount of metabolites were present in soil at harvest suggestive of its safe application.
Keywords: Butachlor; Continuous application; Herbicides residues; Persistence; Rice.