Characterization of the cross-resistance mechanism to herbicides inhibiting acetyl coenzyme-A carboxylase in itchgrass (Rottboellia cochinchinensis) biotypes from Bolivia

Whole-plant greenhouse bioassays and absorption, translocation, and metabolism experiments were conducted to investigate the resistance and cross resistance patterns and resistance mechanism of itchgrass (Rottboellia cochinchinensis) biotypes from Bolivia to herbicides that inhibit the enzyme acetyl coenzyme-A carboxylase. Varying levels of resistance to haloxyfop-R-methyl and sethoxydim were found in the biotypes designated as San Pedro and Yapacaní and cross-resistance among graminicides was confirmed. The resistance indices (RI) for haloxyfop-R-methyl based on the GR50 values (herbicide dose required to inhibit growth by 50%) for these biotypes were 33.4 and 44.3, respectively. For the sethoxydim, the RI values were 9.3 and 4.7, respectively. The rate of [14C]sethoxydim absorption was similar in the resistant and susceptible biotypes at 6, 12, 48 and 96 h after treatment (HAT); there was no significant difference in the amount of [14C]sethoxydim uptake among biotypes. No differences in the translocation or metabolism of [14C]sethoxydim were observed between resistant and susceptible biotypes at any interval after application. In in vitro ACCase assays, the concentrations of sethoxydim required to inhibit ACCase activity by 50% (I50) were substantially higher (about 11 times) for the two resistant biotypes compared to the reference biotype, indicating that the resistant itchgrass biotypes have an ACCase that is relatively insensitive to the graminicides. These results suggest that cross resistance in itchgrass biotypes is conferred by a reduced sensitivity of the target enzyme.