The objective was to determine the magnitude of resistance of A. quitensis H.B.K. biotypes to imazethapyr and chlorimuron-ethyl. Weed biotypes were collected at Zavalla, General Baldissera, Marcos Juárez y Las Rosas. The herbicides were applied at 1/8x, 1/4x, 1/2x, 1x, 5x, 10x and 20x the suggested rate. Weed mortality and reduction of biomass related with untreated plants were evaluated. Herbicides concentrations required to reduce growth by 50% (GR50) were determined. Resistance ratio (GR50 resistant/GR50 susceptible ) was calculated to indicate the degree of resistance. Zavalla showed high susceptibility, with 1/2 of the suggested rate of both herbicides mortality reached 95%. General Baldissera was cross-resistant; with rates 20 times superior to the recommended field use rate mortality was 57.5% and 20% for imazethapyr and clhorimuron-ethyl, respectively. Resistance ratios of this biotype compared with Zavalla were 165 and 246 for imazethapyr and chlorimuron-ethyl, respectively. Las Rosas y Marcos Juárez were resistant to imazethapyr but showed high susceptibility to chlorimuron-ethyl. The resistance patterns showed by the biotypes could be associated with differences in the selection pressure at each location.
biotypes; weeds; herbicide resistance
