RESISTANCE MECHANISM OF PROPANIL-RESISTANT BARNYARDGRASS .2. IN-VIVO METABOLISM OF THE PROPANIL MOLECULE

Propanil-resistant barnyardgrass populations, previously verified in A rkansas rice fields and in greenhouse tests, were examined in the labo ratory to ascertain if the resistance mechanism in this weed biotype w as herbicide metabolism. Propanil-resistant barnyardgrass was controll ed >95% in the greenhouse when carbaryl (an aryl acylamidase inhibitor ) was applied two days prior to propanil. Laboratory studies with C-14 -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 b arnyardgrass and rice and tentatively identified by thin layer chromat ography. Overall, metabolites in the resistant barnyardgrass had R(f), values similar to those in rice, indicating similar metabolism for bo th species. These data, coupled with data from a previous report on th e resistant biotype showing no differential absorption/translocation o r molecular modification of the herbicide binding site in the resistan t biotype, indicate that the resistance mechanism is metabolic degrada tion of propanil.