The use of glyphosate tolerant perennial ryegrass (Lolium perenne L.) cultivars, JS501 and Replay, could provide turfgrass managers with a new option for Poa annua control.  The objectives of these trials were to determine 1) glyphosate application safety rates and 2) glyphosate rates for effective Poa annua control.

In the safety rate trial, Replay perennial ryegrass was seeded on 5 May 2010 in Post Falls, ID at 390 kg ha^-1.  Glyphosate (Departure brand, Syngenta) was applied on 15 Sep. 2010 at 0, 0.29, 0.58, 1.16, 1.74, 2.32, and 3.48 kg ae ha^‑1. In the Poa annua control trial, one block of JS501 and one block of Replay were seeded on 15 Sep. 2008 at 390 kg ha^-1.  Glyphosate rates were randomized within each block at rates of 0, 0.15, 0.29, 0.44 and 0.58 kg ae ha^-1.  Glyphosate was applied on 17 June and 19 Aug. 2009 and 25 June and 25 Aug. 2010.  Data collected for both trials included turfgrass color, percent leaf firing, and percent Poa annua.

In the safety rate trial, an application of 0.85 kg ae ha^-1 (air temperature at application was 26^oC) was required to cause 20% leaf firing.  In the Poa annua control trial, after four glyphosate applications over a 2-year period, plots treated at 0.29 kg ha^-1 or greater resulted in less than 10% Poa annua.  Meanwhile, plots that were untreated or sprayed at 0.15 kg ae ha^-1 had 83% and 28% Poa annua, respectively.

Based on the environmental conditions of each trial, results suggest a recommended application rate should be 0.29 kg ae ha^-1.  This rate is sufficient for Poa annua control and also provides protection in case a spray overlap occurs during an application.