VERIFICATION AND DISTRIBUTION OF PROPANIL-RESISTANT BARNYARDGRASS (ECHINOCHLOA-CRUS-GALLI) IN ARKANSAS

Propanil-resistant barnyardgrass was reported in Poinsett County, AR, in 1990. Greenhouse studies were initiated to determine the distributi on of propanil-resistant barnyardgrass in the state and to characteriz e the resistance. Barnyardgrass seeds were obtained in 1991 and 1992 f rom fields in 19 of the 38 rice producing counties in Arkansas where p ropanil treatment at recommended rates gave unsatisfactory barnyardgra ss control. Barnyardgrass seedlings from the various sources were trea ted with propanil at 4.5 kg ai/ha and seedling injury response was com pared to the response of seedlings collected from known resistant and susceptible barnyardgrass populations. Propanil-resistance of varying levels was confirmed in 115 (16 counties) out of the 138 Arkansas barn yardgrass seed sources. Propanil I-50 values (rate of herbicide requir ed to provide 50% injury/control) were determined to be 14, 20, and 39 kg/ha for slightly, moderately, and highly resistant barnyardgrass, r espectively. a resistance factor of 20x was found in the highly resist ant barnyardgrass category. Development of resistance was highly corre lated with crop rotations where rice was grown one out of two, or two out of three years,