Resistance Mechanism of Propanil-Resistant Barnyardgrass: II. In-vivo Metabolism of the Propanil Molecule

Propanil‐resistant barnyardgrass populations, previously verified in Arkansas rice fields and in greenhouse tests, were examined in the laboratory to ascertain if the resistance mechanism in this weed biotype was herbicide metabolism. Propanil‐resistant barnyardgrass was controlled >95% in the greenhouse when carbaryl (an aryl acylamidase inhibitor) was applied two days prior to propanil. Laboratory studies with 14C‐radiolabelled propanil indicated that the herbicide was hydrolysed in propanil‐resistant barnyardgrass and rice to form 3,4‐dichloroaniline, but no detectable hydrolysis occurred in susceptible barnyardgrass. Two additional polar metabolites were detected in propanil‐resistant barnyardgrass and rice and tentatively identified by thin layer chromatography. Overall, metabolites in the resistant barnyardgrass had Rf values similar to those in rice, indicating similar metabolism for both species. These data, coupled with data from a previous report on the resistant biotype showing no differential absorption/translocation or molecular modification of the herbicide binding site in the resistant biotype, indicate that the resistance mechanism is metabolic degradation of propanil. © of SCI.