Resistance characterization of Italian ryegrass (Lolium multiflorum) biotypes to clethodim herbicide

The objective of this study is to determine the dose required to control 50% population (C50) and to reduce 50% dry matter production (GR50) of resistant biotypes of ryegrass in comparison to that of a susceptible biotype as well as to evaluate the mechanism of resistance by cyt-P450 inhibitor application. The study was conducted in a greenhouse in Rio Grande do Sul, Brazil on plants that survived clethodim herbicide application, which were suspected of possessing resistance. For plants surviving field application, the biotypes were 50% controlled with herbicide dose of 28.4- and 29.5-times greater compared to that of susceptible biotypes; 50% of dry matter reduction occurred with doses of 540- and 574-times greater than the susceptibility dose of a biotype, since the dose required to reduce 50% of susceptible biotype was 0.2 g a.i. ha-1. The biotypes showed metabolism of clethodim herbicide as regards the inhibition by piperonyl butoxide, indicating that metabolism is the probable cause of control failures in the field. Key words: Lolium multiflorum, acetyl coenzyme A carboxilase, weed, mechanism of resistance; metabolization.