Resistance to Nine Herbicide Classes in a Population of Rigid Ryegrass (Lolium rigidum)

Abstract

Rigid ryegrass population VLR69 has become resistant to nine classes of herbicides after 21 yr of exposure to five herbicides in five different chemical classes. The population was exposed to diuron in 17 seasons and is resistant to diuron (4 fold) and chlorotoluron (8 fold) when compared with a reference susceptible population (VLR1). VLR69 had six seasons of exposure to chlorsulfuron and exhibits a high level of resistance to chlorsulfuron (> 20 fold) and triasulfuron (> 25 fold) and a lesser change in sensitivity to sulfometuron (7 fold); however, 4% of the population has a high level of resistance to sulfometuron. Resistance to atrazine (5 fold), simazine (6 fold), and ametryn (10 fold) was observed after five seasons of exposure to atrazine. There is a high level of resistance to all aryloxyphenoxypropionate herbicides after only two exposures to diclofop eight generations prior to testing the population. The population was cross-resistant to tralkoxydim (> 9.5 fold) and sethoxydim (1.8 fold). There was a small change in sensitivity to paraquat (1.4 fold) after three generations of exposure. The population displayed cross-resistance to: imazaquin (7 fold), imazapyr (2.5 fold), metribuzin (8.7 fold), and metolachlor (2 fold) but was susceptible to oxyfluorfen and dinitroaniline herbicides. There was also a small shift in sensitivity to tridiphane (1.6 fold).