Use of Glyphosate-Tolerant Perennial Ryegrass Cultivars in the Southeastern United States.

Two research trials were initiated at the Rodney Foil Plant Science Research Center located in Starkville, MS.  The objective of the tolerance trial was to determine 'Replay' perennial ryegrass (PRG) tolerance to various glyphosate application rates and timings.  Treatments included application timing (February, March, April, February fb March, February fb April, March fb April, and February fb March fb April) and application rate (0, 260, 520, 780, and 1040 g ae ha^-1).  The objective of the traffic trial was to determine Replay PRG traffic tolerance following a glyphosate application.  Treatments included application timing (February, March, and February fb March), traffic (none and moderate) and application rate (0, 260, 520, and 780 g ae ha^-1).  Traffic was applied using a Brinkman Traffic Simulator.  Across both trials, ratings included PRG color, bermudagrass green-up, normalized difference vegetation index (NDVI), and PRG injury.  In the tolerance trial, PRG color remained above the acceptable threshold of 6 following single applications of glyphosate at 1040 g ae ha^-1.  The February fb March treatment resulted in unacceptable color ratings at the 520 g ae ha^-1 rate, however, the February fb April treatment was never below a color rating of 6.  In the traffic trial, traffic tolerance was not reduced following single glyphosate applications in February and March.  However, plots treated with glyphosate in February fb March showed significantly greater injury from traffic compared to untreated plots.  Based on the environmental conditions of each trial, results suggest application intervals of glyphosate rates higher than 260 g ae ha^-1 should be greater than 4 weeks.  Also, traffic should be minimized if a repeat application of glyphosate is expected within four weeks of an initial application.