Propanil resistant barnyardgrass in Arkansas: Competitive ability, distribution, and mechanism of resistance

Propanil-resistant barnyardgrass was verified in Poinsett County, Arkansas in 1990. Research was conducted to determine the distribution of this biotype in Arkansas, the competitive ability of the resistant barnyardgrass biotype, and the mechanism of resistance in the propanil-resistant barnyardgrass.;Barnyardgrass seed samples were collected from rice fields which had been treated with propanil in 1991 and 1992. Greenhouse assays were used to determine if plants from these seeds were resistant to propanil at 4.5 kg/ha. Over the 2-year period, 118 of 143 samples representing 16 of the 38 rice growing counties in Arkansas were found to be resistant to propanil. Resistance was highly correlated with reduced rotational practices where rice was grown multiple years.;Field studies were conducted in 1992 and 1993 to compare the competitiveness of the resistant biotype to that of the susceptible biotype. No difference was observed between the biotypes in plant height, dry weight, or rice yield reduction. Panicle production was slightly increased (20-30%) in the susceptible biotype at densities of 2 and 5 plants/m;Propanil absorption by barnyardgrass leaves averaged 6% at 48 hr and did not differ between the two biotypes. Ninety percent of the absorbed propanil generally remained in the treated leaf in both barnyardgrass biotypes. Chlorophyll fluorescence comparisons indicated that the site of action in the photosynthetic electron transport chain for propanil was not modified in the resistant biotype. Propanil was hydrolyzed in the resistant barnyardgrass to produce 3...