GLYPHOSATE SUSCEPTIBILITY OSTARTF ANNUAL RYEGRASS (LOLIUM MULTIFLORUM L.) POPULATIONS IN NO-TILLAGE SYSTEMS IN URUGUAY

The response of annual ryegrass (Lolium multiflorum L.) populations to different glyphosate rates was assessed in highly infested sites of the Western agricultural area of Uruguay. Six sites with previous information regarding crop rotation history, years under no-tillage and glyphosate usage were selected for this study. Seven glyphosate rates were tested: 0.5, 1.0, 1.5, 2.0, 4.0, 6.0, 8.0 kg a.i./ha and a control treatment without herbicide treatment was also included. Visual estimations at 10, 20, 30 and 40 days after the applications were conducted to determine control efficiency of glyphosate rates. Results showed significant differences in the speed of control among different rates. No satisfactory controls were achieved at the lowest herbicide rates at any of the evaluation times. Furthermore, the increase of glyphosate rates reduced the differences on control efficiency between sites. Ten days after herbicide treatments the control efficiency was extremely variable across sites. However, after 30 days from the herbicide treatments, rates of 4.0, 6.0, 8.0 kg a.i./ha achieved more than 90% of control efficiency at all sites. No glyphosate-resistant ryegrass populations were found in this study. The susceptibility of annual ryegrass populations was highly dependent on phenological stage when the herbicide treatments were applied, as well as dry matter per hectare present in each of the sites. It was observed that some ryegrass seedlings survived high glyphosate rates showing a tolerance response and suggesting the need to frequently monitor these sites in future.