Low-level resistance of Cyperus iria L. to ALS-inhibiting herbicides occurring in the State of Rio Grande do Sul

Resumo

This work aimed at determining possible resistance of three biotypes of Cyperus iria to inhibiting herbicides of acetolactate synthase enzyme (ALS). An experiment at greenhouse was conducted out where suspected resistance biotypes were multiplied through seeds in pots filled with soil. The treatments were arranged in a trifactorial (AxBxC) design, where factor A consisted of three biotypes of Cyperus iria, from three locations of the state of Rio Grande do Sul (named Santa Maria 1, São Borja 3 and Cachoeira do Sul 7). Factor B corresponds to three herbicides (pyrazosulfuron-ethyl, ethoxysulfuron and bentazon). Factor C refers to herbicide doses (zero, 50% of registered dose; registered dose, two, four and eight times registered dose of herbicides). For resistance measurement, it was used resistance factor calculation (RF) and for dose-response curves through regression analysis, RF on dry mass (DM) of plants. It was observed that São Borja 3 and Cachoeira do Sul 7 biotypes have a low resistance factor (1 < RF < 10) to pyrazosulfuron-ethyl and ethoxysulfuron herbicides. These showed lower susceptibility to ethoxysulfuron (RF = 5.49-6.76) than pyrazosulfuron-ethyl (RF = 2.21-2.47). The clear distinction in the susceptibility of biotypes, include responses of chemical control to field due to the long period of use of ethoxysulfuron herbicide. It is concluded that this factor characterizes low-level resistance, being a cross-re­sistance type, thus demonstrating the ineffectiveness of ALS - inhibiting herbicides to control this species.