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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.

Safening effect of two herbicides, dymron (1-(a, a-dimethylbenzyl)-3-ptolylurea and dimepiperate (S-(1-methyl-1-onyl]-urea)-treated rice and the effect of the two compounds on absorption and metabolism of azimsulfuron in the plant were determined. While treatment of azimsulfuron at very high dosage (64-125g a.i./ha) reduced the dry weight of rice shoots, simultaneous treatment with dymron at 450g a.i./ha or dimepiperate at 3,000g a.i./ha caused their recovery from the azimsulfuron injury. Dymron and dimepiperate significantly decreased 14C-azimsulfuron absorption from rice roots at 24 hours after exposure. 14C-Azimsulfuron was rapidly degraded in rice plant into a metabolite formed by O-demethylation of the pyrimidine methoxy substituent of the parent compound. This metabolism was remarkably enhanced by addition of dymron or dimepiperate. The results indicate that the reduction of absorption and the enhancement of detoxification (O-demethylation) of azimsulfuron in rice plants are primary factors in the safening effect of dymron and dimepiperate.

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Bensulfuron methyl is a sulfonylurea herbicide controlling a broad spectrum of broadleaf and sedge weeds in paddy rice. Safening effects with certain thiolcarbamate herbicides for grass control have been discovered in the process of combination products development. The results of mode of safening studies revealed that the enhancement of metabolic inactivation rate of bensulfuron methyl in plants was the basis of safening.Among these thiolcarbamates, dimepiperate was unique in exhibiting safening action even in direct water-seeded rice. Further studies on dimepiperate effects on root-applied bensulfu­ron methyl indicated that rice root elongation, enhanced metabolism in roots and in shoots, were the key to the safening effects. The translocation rate of bensulfuron methyl, from roots to shoots, on the other hand, was not significantly affected by dimepiperate.