Ryegrass biotypes sensitivity to glyphosate, iodosulfuron-methyl and clethodim

Abstract

Ryegrass (Lolium multiflorum) is an annual grass, traditionally used as forage that has become weed in crops in the south of Brazil.  It has been observed that there are ryegrass biotypes resistant to glyphosate that survive the application of herbicides that inhibit ALS and ACCase. Therefore, the objective was to evaluate the sensitivity of different ryegrass biotypes to glyphosate, iodosulfuron-methyl and clethodim herbicides. The experiment was carried out in a greenhouse, in randomized blocks, in a triple factorial arrangement with three repetitions. Four ryegrass biotypes were used, where: biotype 1 (B1), susceptible to glyphosate; biotype 2 (B2), resistant to glyphosate; biotype 3 (B3) and biotype 4 (B4), biotypes suspected to be resistant to iodosulfuron-methyl. The doses were standardized in 0; 0.25; 0.5; 1; 2; 4 and 8 times the commercial doses of glyphosate (1080 g ha^-1 a.i.) and clethodim (96 g ha^-1 a.i.). The variables evaluated were control, in a percentage scale, and dry shoot biomass (DSB) of the plants at 28 days after the application of treatments (DAT). With that data, we calculated the parameters of the equation, from which the necessary dose was determined to reduce the DSB in 50% comparing to the non-treated plants (GR₅₀) and the resistance factor (F). Biotypes B2, B3 and B4 obtained Resistance factors (F) above 10 comparing to glyphosate and iodosulfuron-methyl herbicides, B3 being the highlighted biotype because, besides being resistant to those two herbicides, also showed an F value close to the clethodim threshold, which can characterize a low resistance level to this herbicide. All biotypes, except the control one, were resistant to the iodosulfuron-methyl and glyphosate herbicides, which characterizes multiple resistance to those herbicides. The clethodim herbicide can be an alternative for the control of biotypes resistant to glyphosate and iodosulfuron-methyl; however, attention should be paid to the chemical handling of these populations, because we identified biotypes with low resistance level to this herbicide.