Distribuição geográfica e caracterização de biótipos de Lolium multiflorum Lam. resistentes ao herbicida clethodim.

Abstract

The ryegrass (Lolium multiflorum) is a weed of annual cycle, growing in a winter fields, orchards and vineyards of Brazilian southern. The specie normally is controlled with clethodim herbicide, however, the continuous use of this product selected resistant biotypes. Therefore, the objectives of this research were evaluating the distribution of ryegrass biotypes resistant to clethodim in Rio Grande do Sul (RS); determine the required dose to control 50% of population (C 50 ) and reduce 50% of dry matter (GR 50 ) of resistant biotypes, relative to susceptible biotype; examine the mechanism of resistance by application of cyt-P450 inhibitors; and indicate alternative herbicides to control biotypes resistant to clethodim. Thereunto, trials were conducted in greenhouse, lab and field, using seeds from plants that survived to clethodim application, suspected of resistance, collected in RS state fields. According to results, there are not ryegrass biotypes resistant to clethodim when applied 120g a.i. ha -1 (maximum register dose) and phenological stage recomendate, however, there are biotypes with less susceptible that survived to half commercial dose (60g a.i. ha -1 ). As for survived plants of field application, the biotypes were controlled 50% with herbicide dose between 28,4 and 29,5 times larger compared to susceptible biotype, and 50% of dry matter was reduced with doses between 540 and 575 times larger than necessary to control the susceptible biotype. Field trials revealed that paraquat and glufosinate-ammonium herbicides are efficient alternatives to ryegrass control. The biotypes showed clethodim metabolism by cyt-P450, as regards to inhibition by piperonyl butoxide (PBO), indicating that metabolism is probably the causes of control failures observed in the field.