EchinochloaResistance to Herbicides Continues to Increase in Arkansas Rice Fields

Abstract Herbicide-resistantEchinochloaspp. pose a significant threat to U.S. rice production. Two surveys were conducted to characterizeEchinochloaresistance to common rice herbicides and provide important demographic information on the populations in Arkansas: one was theEchinochloa Herbicide Resistance Confirmation Surveyconducted annually since 2006; the other was theEchinochloa Herbicide Resistance Demographics Surveyconducted since 2010. TheResistance Confirmation Surveyshowed that resistance to propanil (50%) was most prevalent, followed by quinclorac (23%), imazethapyr (13%), and cyhalofop (3%). Multiple resistance increased with time, with 27% of accessions being multiple-resistant, mostly to propanil+quinclorac (12%). The parallelResistance Demographics Surveytested resistance by species. Of the 264 accessions collected, 73% were junglerice, 14% were rough barnyardgrass, and 11% were barnyardgrass. Overall, this survey also showed resistance to propanil (53%) and quinclorac (28%) being most prevalent, with low frequencies of resistance to cyhalofop (12%) and imazethapyr (6%). Resistance to herbicides was less frequent with barnyardgrass (54%) and rough barnyardgrass (28%) than with junglerice (73%). Multiple resistance was most frequent with junglerice (33%) and least frequent with rough barnyardgrass (8%). Across both surveys, the resistance cases were clustered in the northeast and Grand Prairie regions of the state. Herbicide resistance amongEchinochloapopulations in rice fields is continuing to increase in frequency and complexity. This is a consequence of sequential selection with different major herbicide sites of action, starting with propanil followed by quinclorac and others.