Herbicide tolerance in rice cultivars

Weed competition is severe in rice under direct sown conditions. Manual weeding is constrained by excessive labour involvement and cost. Preemergence butachlor or oxyfluorfen controls weeds efficiently but often results in initial crop injury. 46 rice cultivars were screened for tolerance to these herbicides and their time and dose of application were optimized. Three cultivars, ADT-37, ASD-16 and ASD-18 were highly tolerant to butachlor whereas ADT-36, ADT-38 and PY-3 were highly susceptible. Increased cl-amylase activity was associated with butachlor tolerance. Oxyfluorfen tolerance was high in ASD-18 and AS-18696 and IR-50 was highly susceptible. Increased total chlorophyll content was linked with oxyfluorfen tolerance. Among the different dates and doses of application of these herbicides tested in field experiments, 1500 g /ha 2 Days after Sowing (DAS) was better for butachlor. For oxyfluorfen, 150 g/ha 4 DAS was found be optimum.