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Abstract:Azimsulfuron (DPX-A8947) is a new selective sulf onylurea herbicide used in rice for control of annual and perennial broadleaf weeds and sedges. We investigated the mechanism of selectivity by comparing the behavior of this herbicide in rice with that of the sensitive weed, flat sedge (Cyperus serotinus) using 14C-azimsulfuron.Azimsulfuron afforded high selectivity between rice and C. serotinus in the growth response studies by a hydroponic solution method. Studies of absorption and translocation indicated that the differential sensitivities of rice and C. serotinus to azimsulfuron are not due to its differential absorption and translocation from their subterranean parts. In metabolism studies, rice metabolized azimsulfuron very rapidly in its shoots and roots, while C. serotinus metabolized it very slowly in its shoots, roots and tubers.The metabolism of azimsulfuron in rice within 24 hours involved mostly O-demethylation of pyrimidine ring. The metabolite formed by 0-demethylation, metabolite 1, was herbicidally much less active on rice and weeds tested. The results obtained suggest that the selectivity of azimsulfuron between rice and C. serotinus mainly results from the rapid 0-demethylation in rice.