Abstract:Herbicide resistance is currently one of the most important problems in agricultural systems, threatening their sustainability. In Uruguay, there have been increasing reports of herbicide failures to control LOLMU (Lolium multiflorum Lam.) populations. However, methodical characterization of herbicide resistance cases is lacking. Seeds from 66 LOLMU populations were collected from cropping fields and screened in greenhouse conditions with the recommended (1x) and half the recommended (1/2x) label dose of glyphosate, clethodim, pinoxaden and a formulation with iodosulfuron-methyl-sodium and mesosulfuron-methyl (IodoMeso). Populations were classified as resistant when mortality was equal to or lower than 80% in response to 1x doses, assessed 28 days after treatment (DAT). Results show that 80% of LOLMU populations were resistant to glyphosate, followed by 49%, 36% and 24% resistance to IodoMeso, pinoxaden and clethodim respectively. Moreover, 52% of the tested populations were resistant to more than one herbicide, with an important frequency of populations resistant to glyphosate and IodoMeso (17%) and to all tested herbicides (16%). In addition, a high correlation of resistance levels was found between pinoxaden and clethodim resistant populations (R2 = 0.84). Geographic distribution of tested populations suggests that multiple herbicide resistance is concentrated in fields with a longer history of herbicide use. Overall, these results confirm that herbicide resistance in LOLMU is widespread in agricultural regions of Uruguay and that significant number of multiple resistance cases already exist and are expected to increase rapidly if management practices are not adjusted quickly and substantially.This survey represents a key input to improve herbicide-resistant LOLMU management strategies.