Glyphosate-tolerant perennial ryegrass and Poa annua control

Using glyphosate-tolerant perennial ryegrass to overseed bermudagrass can give superintendents an edge on Poa annua.

Michael L. Flessner and J. Scott McElroy, Ph.D.

Read this story in GCM's digital edition

Two glyphosate-tolerant perennial ryegrass (Lolium perenne) cultivars, JS501 and Replay, have recently been released for commercial use. Once these perennial ryegrasses are mature, glyphosate can be applied to these cultivars for weed control without injury to the turf. These cultivars were produced through conventional plant breeding and are not genetically modified organisms (GMOs).

October 2013 research: Glyphosate photo 1

The research site was at the Auburn University Turfgrass Research and Education Center in Auburn, Ala. This photo was taken on Feb. 22, 2011, during the first year of the study.
Photos by M. Flessner

Managing glyphosate-resistant cultivars 
Research conducted in permanent perennial ryegrass areas has concluded that glyphosate at 0.26 pound acid equivalent (ae)/acre (0.29 kilogram ae/ hectare) is safe to JS501 and Replay and results in over 90% annual bluegrass (Poa annua) control. Glyphosate at 0.52 pound ae/acre resulted in less than 10% injury (2). Other published work (3,4) has also indicated that glyphosate at 0.26 pound ae/ acre is adequate to control annual bluegrass. 

Previous research has also indicated that both JS501 and Replay must be mature before glyphosate application or injury and stand reduction will result. Glyphosate application at rates greater than 0.26 pound ae/acre should not be used three to four weeks after seeding to avoid unacceptable injury and cover reduction (1). In the same study, the researchers found that glyphosate applications greater than 0.13 pound ae/acre should be avoided if the temperature is below 50 F.

Using glyphosate in overseeded bermudagrass 
Since JS501 and Replay must be mature before their glyphosate tolerance is sufficient for weed control without injury, it is not known whether these cultivars can be used for overseeding in bermudagrass. In overseeded bermudagrass, there are three competing interests: perennial ryegrass must be well developed to tolerate glyphosate, bermudagrass must be completely dormant so as not to be injured, and annual bluegrass must be young enough to be controlled with glyphosate. Research was recently conducted at Auburn University to determine the optimal timing and rate of glyphosate application in bermudagrass overseeded with Replay perennial ryegrass for annual bluegrass control.

Materials and methods

Replay (Jacklin Seed) was overseeded into bermudagrass in mid-October in 2010 and 2011 at 800 pounds seed/acre (896.68 kilograms/hectare). Roundup ProMax (glyphosate, Monsanto) was applied the first week of December, January and February for both overseedings.

October 2013 Research: Glyphosate table 1

Note. Data were pooled across 2010-2011 and 2011-2012 seasons. Means sharing a letter within a column are similar according to statistical analysis.
Abbreviations: WAIT, weeks after initial treatment; fb, followed by.
Table 1. Annual bluegrass control and perennial ryegrass injury resulting from single glyphosate applications.

Two application regimes were tested: a single Roundup ProMax application and two sequential applications, with the sequential application made three weeks after the initial application. At each application timing, single applications of Roundup ProMax were made at rates of 0 (nontreated), 3.56, 7.1, 14.2 and 28.4 fluid ounces/acre (0, 0.26, 0.51, 1.03 and 2.07 liters/hectare) (Table 1). Sequential applications were 3.56 followed by 3.56 fluid ounces/acre, 7.1 followed by 7.1 fluid ounces/acre, 14.2 followed by 14.2 fluid ounces/acre, and 28.4 followed by 28.4 fluid ounces/acre (Table 2). 

A comparison treatment of Prograss (ethofumesate, Bayer Environmental Science) was applied twice at 86 fluid ounces/acre (6.28 liters/ hectare or 1.0 pound ai/acre) the first week of December followed by the same amount in the third week of December. This treatment has resulted in greater than 90% annual bluegrass control in previous research (5).

Results

Data from Roundup ProMax and Prograss treatments were collected for perennial ryegrass injury and annual bluegrass control. Injury was evaluated visually on a scale of 0% to 100%, where 0% corresponds to no injury, 100% to complete death and 20% to the maximum commercially acceptable level of injury. Injury was evaluated three and six weeks after initial treatment, when maximum glyphosate injury would be expected. Annual bluegrass control was also evaluated visually on a scale of 0% to 100%, where 0% corresponds to no control and 100% to complete control. Annual bluegrass was evaluated in April in order to determine the level of season-long control.

 October 2013 Research: Glyphosate table 2

Note. Data were pooled across 2010-2011 and 2011-2012 seasons. Means sharing a letter within a column are similar according to statistical analysis.
Abbreviations: WAIT, weeks after initial treatment; fb, followed by.
Table 2. Annual bluegrass control and perennial ryegrass injury resulting from sequential glyphosate applications.

December applications
Results indicated that December applications of Roundup ProMax were too injurious to the perennial ryegrass. Single applications greater than 3.56 fluid ounces/acre resulted in 30% or greater injury three and six weeks after initial treatment. Similarly, all sequential glyphosate applications resulted in greater than 40% injury six weeks after initial treatment. Annual bluegrass control was generally poor from the December application timing because annual bluegrass germinated after applications were made.

January applications
The January application timing was much better for both perennial ryegrass safety and annual bluegrass control. Single glyphosate applications at 7.1 fluid ounces/acre or greater resulted in greater than 90% annual bluegrass control. However, rates greater than 7.1 fluid ounces/acre resulted in greater than 50% injury at six weeks after initial treatment. 

October 2013 research: Glyphosate photo 2

This overview of the second year of the study (taken Feb. 21, 2012) shows differences in perennial ryegrass injury and annual bluegrass control from different treatments and application timings.

Sequential Roundup ProMax applications in January all resulted in greater than 90% annual bluegrass control. However, all rates except 3.56 fluid ounces/acre followed by 3.56 fluid ounces/ acre resulted in greater than 40% injury. 

Overall, two of the treatments applied in January may be commercially viable: a single Roundup ProMax application at 7.1 fluid ounces per acre, which resulted in 91% annual bluegrass control and a maximum of 23% perennial ryegrass injury, and a sequential Roundup ProMax application at 3.56 fluid ounces/acre followed by 3.56 fluid ounces/acre, which resulted in 94% annual bluegrass control and a maximum of 23% perennial ryegrass injury.

February applications
The February application timing was the safest of the timings evaluated. This finding corroborates previous research that shows that the Replay cultivar needs time to grow and mature before maximum glyphosate tolerance is achieved. A single application of Roundup ProMax at 14.2 fluid ounces/acre or less resulted in acceptable injury levels (less than 20%). Sequential applications were safe (less than 10% injury was observed) up to 7.1 fluid ounces/acre followed by 7.1 fluid ounces/acre. However, annual bluegrass control was lower than that in the January treatments because, by February, the annual bluegrass was more mature and robust and could withstand higher glyphosate rates. Roundup ProMax at 14.2 fluid ounces/acre resulted in 73% annual bluegrass control, but sequential applications at 7.1 fluid ounces/acre followed by 7.1 fluid ounces/acre resulted in 84% control.

Prograss treatment
The comparison treatment with Prograss resulted in 73% annual bluegrass control with less than 15% perennial ryegrass injury (characterized by growth reduction and mild off-coloring). The best January treatments and some of the best February treatments were statistically similar (Tables 1, 2).

Conclusions

No bermudagrass injury, delays in spring green-up or other adverse effects were observed from any treatment in this trial. However, to avoid the risk of bermduagrass injury, care should be taken to ensure that bermduagrass is fully dormant before glyphosate is applied.

To balance the competing interests of turfgrass safety and adequate weed control, glyphosate application timing and rate are two factors that must be adjusted. Only two Roundup ProMax treatments — both in January — provided greater than 90% annual bluegrass control and resulted in less than 25% perennial ryegrass injury throughout the trial. These two treatments were a single Roundup ProMax application at 7.1 fluid ounces/acre and 3.56 fluid ounces/acre followed by 3.56 fluid ounces/acre. Therefore, these are the best glyphosate treatments evaluated to control annual bluegrass in bermudagrass overseeded with Replay perennial ryegrass.

Although these treatments are successful, they are not without risk. For example, a slight spray overlap would likely result in conspicuous, unacceptable injury. It is also important to note that glyphosate should not be used exclusively for weed control, because weeds have been shown to develop resistance (3). Superintendents should also be aware that optimal application timings will vary by geographic region.

Acknowledgments

The authors thank Christian Baldwin, Ph.D., for his assistance and Jacklin Seed by Simplot for providing seed. An article on this research, “Annual bluegrass (Poa annua) control in glyphosate-tolerant perennial ryegrass overseeding” by M.L. Flessner, J.S. McElroy and G.R. Whtje, has been provisionally accepted in the journal Weed Technology.

Literature cited

  1. Baldwin, C., A.D. Brede, R. Golembiewski, C. Mallory-Smith and J. Mayer. 2011. Maturity and temperature sensitivity of two glyphosate tolerant perennial ryegrass cultivars, ‘JS501’ and ‘Replay.’ Proceedings Crop Science Society of America No. 328-1.
  2. Baldwin, C.M., A.D. Brede and J.J. Mayer. 2012. ‘JS501’ and ‘Replay’ perennial ryegrass glyphosate tolerance and rates required for annual bluegrass (Poa annua L.) control. HortScience 47:932-935.
  3. Brosnan, J.T., G.K. Breeden and T.C. Mueller. 2012. A glyphosate-resistant biotype of annual bluegrass in Tennessee. Weed Science 60:97-100.
  4. Gossbard, E., and D. Atkinson, eds. 1985. The herbicide glyphosate. Butterworths, Boston.
  5. McElroy, J.S., G.K. Breeden and G.Wehtje. 2011. Evaluation of annual bluegrass control programs for bermudagrass turf overseeded with perennial ryegrass. Weed Technology 25:58-63.

Michael L. Flessner is a research associate in the department of agronomy and soils and J. Scott McElroy is an associate professor in the department of crop, soil and environmental sciences at Auburn University, Auburn, Ala.