Perennial ryegrass is used as a permanent stand in temperate zone climates for overseeding dormant bermudagrass (Cynodon dactylon L.) in subtropical climates. A major challenge when managing permanent stands of PRG is controlling ABG during the fall season or during establishment. The introduction of two glyphosate-tolerant perennial ryegrass cultivars provides turfgrass managers a new tool in weed control, specifically ABG (Samudio et al., 2011). However, the glyphosate tolerance of these cultivars at the seedling stage and after a late-season, cool-temperature application has received limited investigation.
Although PRG is considered a fast-germinating cool-season species, ABG has greater seedling vigor (Grime and Hunt, 1975). When establishing a newly seeded PRG area, a glyphosate application during the PRG seedling phase would be beneficial in controlling newly emerging ABG. There have been several reports detailing the seedling tolerance of various turfgrass species to numerous herbicides and plant growth regulators (PGRs). Kaminski et al. (2004) reported on the effects of siduron, bensulide, ethofumesate, prodiamine, paclobutrazol, and chlorsulfuron on young seedlings of creeping bentgrass (CBG) (Agrostis stolonifera L.). In general, a mature stand of CBG is tolerant to the previous products listed. However, the authors noted seedling tolerance varied depending on whether certain herbicides were applied 2, 4, or 7 weeks after emergence. Dernoeden et al. (2008) investigated CBG and PRG seedling tolerance to bispyribac-sodium and ethofumesate when both species were between the two to five LS. Applications made during early seedling emergence (two LS) resulted in more injury to both species than applications when plants were fully established.
Regarding a glyphosate application during the seedling stage of glyphosate-resistant and -tolerant cultivars, Dant et al. (2005) applied glyphosate over Roundup Ready® CBG seedlings (Scotts Company, Marysville, OH) within 4 weeks of seeding and did not note any injury to the seedlings. ‘Aurora Gold’ hard fescue (Festuca longifolia Thuill.) seedling response to glyphosate has received limited investigation because applications were made the next year after seeding (Hart et al., 2005). Similarly, the glyphosate tolerance of mature stands of ‘JS501’ and ‘Replay’ PRG has been established (Baldwin et al., 2012), yet the seedling tolerance has received limited investigation. This is an important management aspect of these PRG cultivars to minimize weed competition in a newly seeded area or to develop renovation programs that allow for minimal weed pressure.
Another important consideration in developing weed control programs is determining the time of year to make herbicide applications while minimizing injury on the desirable species. Although no reports on seasonal application timing of glyphosate-tolerant turfgrasses are known, timing of application when using other herbicides has been reported. Research has noted increased toxicity on CBG and PRG after amicarbazone applications when applied in the fall compared with the spring (McCullough et al., 2010; Perry, 2011). Also, fall applications of bispyribac-sodium and sulfosulfuron caused greater injury to kentucky bluegrass (Poa pratensis L.) compared with spring applications (Hart and McCullough, 2007).
Herbicide success is highly dependent on species maturity and application timing; therefore, the objectives of this research were to determine tolerance of ‘JS501’ and ‘Replay’ PRG to glyphosate during the seedling stage and after a late-season fall application.
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