Golf Course Management

OCT 2013

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research V v v The research says ➔ Replay and JS501 are perennial ryegrass cultivars that have been conventionally bred for glyphosate tolerance. ➔ Once the perennial ryegrass is mature, glyphosate can be applied to these cultivars for weed control without injury. ➔ January is the best time to apply glyphosate following a midOctober overseeding; the perennial ryegrass has had time to mature, but the annual bluegrass is young enough to be controlled. ➔ Roundup ProMax at 7.1 fluid ounces/acre is sufficient to control annual bluegrass when applied at the appropriate timing. ➔ No bermudagrass injury or delays in spring green-up were observed; be sure bermudagrass is dormant before applying glyphosate. after initial treatment. Sequential Roundup ProMax applications in January all resulted in greater than 90% annual bluegrass control. However, all rates except 3.56 fuid ounces/acre followed by 3.56 fuid 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 fuid 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 fuid ounces/acre followed by 3.56 fuid ounces/acre, which resulted in 94% annual bluegrass control and a maximum of 23% perennial ryegrass injury. February applications Te February application timing was the safest of the timings evaluated. Tis fnding 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 fuid 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 fuid ounces/acre followed by 7.1 fuid 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 fuid ounces/acre resulted in 73% annual bluegrass control, but sequential applications at 7.1 fuid ounces/acre followed by 7.1 fuid ounces/acre resulted in 84% control. Prograss treatment Te comparison treatment with Prograss resulted in 73% annual bluegrass control with less than 15% perennial ryegrass injury (characterized by growth reduction and mild of-coloring). Te 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 efects 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 94 GCM October 2013 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. Tese two treatments were a single Roundup ProMax application at 7.1 fuid ounces/acre and 3.56 fuid ounces/acre followed by 3.56 fuid ounces/acre. Terefore, 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. GCM Michael L. Flessner is a research associate in the department of agronomy and soils and J. Scott McElroy (jsm0010@auburn. edu) is an associate professor in the department of crop, soil and environmental sciences at Auburn University, Auburn, Ala.

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