First case of resistance to bispyribac-sodium in barnyardgrass (Echinochloa crus-galli) from Ecuador

Abstract

Background: Echinochloa crus-galli is a weed that has developed resistance to multiple herbicides. In Ecuador, the selection pressure caused by the continued use of the acetolactate synthase-inhibiting herbicide bispyribac-sodium would have favored the selection of resistant accessions. Objective: The current study aimed to analyze the sensitivity to bispyribacsodium of E. crus-galli populations collected in the province of Guayas, the main rice-growing region of Ecuador. Methods: A total of 35 E. crus-galli populations were assessed. These were sprayed with bispyribac-sodium at a commercial dose of 40 g a.i. ha-1. The four most resistant accessions and the most susceptible population were submitted to dose-response curves. Data were adjusted using a log-logistic model and the mean lethal dose (LD50) for each population was calculated. Finally, response of the populations to the herbicide and their interaction with malathion, a cytochrome P450 monooxygenase inhibitor, was studied. Results: Seventy-two percent of the 35 accessions showed poor to deficient control (<70%) by bispyribacsodium. Neither population showed 100% mortality in response to the recommend dose the herbicide. The LD50 values for the R populations were at least 16.1 times higher than those for the S population. The interaction of bispyribac-sodium with malathion indicated that, for at least one of the R populations, resistance may be determined by xenobiotic metabolism. Conclusions: E. crus-galli populations collected from rice fields showed high level of resistance to bispyribac-sodium, even at the highest doses evaluated.