| Italian ryegrass populations have evolved resistance to herbicides that producers rely on for weed control both in wheat and burn-down. The objectives of this research were to: test populations of Italian ryegrass from across Arkansas for resistance to glyphosate, diclofop, pinoxaden, and pyroxsulam; determine if there were any differences in control of 12 glyphosate-resistant populations in relation to glyphosate rate or application timing; determine the level of glyphosate resistance in one selected population versus a susceptible standard and a previously discovered glyphosate-resistant population; and determine the best options for controlling Italian ryegrass prior to planting crops. A total of 215 population samples were tested. On average 17% of the samples were resistant to glyphosate, 95% were resistant to diclofop, 64% were resistant to pyroxsulam, and 12% were resistant to pinoxaden. A few were resistant to all four chemistries tested. Control of glyphosate-resistant populations was improved with the high rate of glyphosate at the three- to four-tiller growth stage; however, results for individual populations were variable. When averaged across populations, no rate or timing of glyphosate controlled these resistant populations greater than 62%. One population was found to be 23 times more tolerant to glyphosate than a susceptible standard. Three field experiments were conducted for Italian ryegrass control in the spring, in no-till production in the fall, and following fall tillage. Herbicide applications in the spring were unsuccessful, especially when glyphosate is not an option. Even when postemergence (POST) treatments visually controlled ryegrass at least 80%, enough ryegrass residue remained that would cause problems with spring tillage, planting, and overall crop stand establishment. In the fall-tilled study, the residual herbicides flumioxazin plus S-metolachlor, S-metolachlor, clomazone, and pyroxasulfone applied immediately following fall tillage reduced Italian ryegrass biomass by 83 to 95% at 200 days after treatment. |
