Maturity and Temperature Sensitivity of Two Glyphosate Tolerant Perennial Ryegrass Cultivars, ‘JS501' and ‘Replay'.

Previous trials of JS501 and Replay perennial ryegrass (Lolium perenne L.) have shown that a glyphosate rate of 0.29 kg ae ha^-1 provides good Poa annua control, while also maintaining minimal ryegrass discoloration.  The objective of these trials was to determine the seedling tolerance and sensitivity of JS501 and Replay to cold temperatures following glyphosate applications.

Two maturity trials were conducted at the Jacklin Seed research farm in Post Falls, ID.  In the first maturity trial, JS501 and Replay were seeded on 7 May 2010 at 390 kg ha^-1.  Glyphosate (Departure, Syngenta Brand)) rate treatments were 0, 0.15, 0.29, and 0.58 kg ae ha^-1.  Glyphosate application timing treatments were 1, 2, 3, and 4 weeks after seeding (WAS).  The second maturity trial treatments and design were identical; however, seeding date was 13 July 2010.  Trials in Idaho and at Oregon State University (OSU) in Corvallis determined the temperature sensitivity of both cultivars.  In Idaho, JS501 and Replay were seeded on 7 May 2010 at 390 kg ha^-1.  Glyphosate was applied on 30 Sep. 2010 at 0, 0.29, and 0.58 kg ae ha^-1.  In the OSU trial, JS501 and Replay were seeded at the end of July, 2010.  Glyphosate was applied on 6 Oct. 2010 at 0, 0.15, 0.29, 0.44, 0.58, 1.16, 3.48 kg ae ha^-1.  For all trials, the existing stand of turfgrass was killed, leaving bare ground for seeding.  Data collected for all trials included turfgrass color, percent cover, percent injury, and percent Poa annua.

In the first maturity trial in Idaho, the 28 June cover rating revealed control plots and all glyphosate timing treatments at the 0.15 kg ae ha^-1 rate had reached 100% cover.  All plots that received 0.29 kg ae ha^-1 of glyphosate had greater than 80% coverage.  However, the 0.59 kg ae ha^-1 rate applied at 4WAS had 55% coverage.  In the second maturity trial in Idaho, the 11 August color rating was 8 or greater at glyphosate rates of 0, 0.15, and 0.29 kg ae ha^-1.  Glyphosate applied 3WAS at 0.58 kg ae ha^-1 had a color rating of 6.  On the 1 Sep. rating, the most injurious treatment occurred at the 0.58 kg ae ha^-1 rate applied at 3 and 4WAS with a color rating of 6 and 3, respectively.  In the Idaho temperature sensitivity trial, 41 days after an application at 0.29 and 0.58 kg ae ha^-1, plots had 18% and 60% leaf firing, respectively.  In Oregon, 36 days after treatment, JS501 percent injury was 3%, 25%, and 82% at rates of 0.15, 0.29, and 0.58 kg ae ha^-1, respectively.  In both trials, air temperatures within 1 month after application dropped below 10^oC.

Based on the environmental conditions of each trial, results suggest glyphosate applications should be avoided 3 to 4WAS if the application rate is greater than 0.29 kg ae ha^-1.  Also, glyphosate applications greater than 0.15 kg ae ha^-1 as temperatures drop below 10^oC should be avoided.