Verification and Distribution of Propanil-Resistant Barnyardgrass (Echinochloa crus-galli)in Arkansas

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Abstract
Propanil-resistant barnyardgrass was reported in Poinsett County, AR, in 1990. Greenhouse studies were initiated to determine the distribution of propanil-resistant barnyardgrass in the state and to characterize the resistance. Barnyardgrass seeds were obtained in 1991 and 1992 from fields in 19 of the 38 rice producing counties in Arkansas where propanil treatment at recommended rates gave unsatisfactory barnyardgrass control. Barnyardgrass seedlings from the various sources were treated with propanil at 4.5 kg ai/ha and seedling injury response was compared 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 barnyardgrass seed sources. Propanil I50values (rate of herbicide required to provide 50% injury/control) were determined to be 14, 20, and 39 kg/ha for slightly, moderately, and highly resistant barnyardgrass, respectively. A resistance factor of 20® was found in the highly resistant barnyardgrass category. Development of resistance was highly correlated with crop rotations where rice was grown one out of two, or two out of three years.