As a result of PRG’s bunch-type growth, susceptibility to cold and heat stress, and slow recovery from traffic, weed invasion is often problematic where permanent stands of PRG are grown. One weed that is particularly troublesome is ABG. This is perhaps the most problematic weed species to control in fine turfgrass management as a result of prolific seed head production and genetic diversity (Cline et al., 1993; Lush, 1988). Currently, few herbicides are labeled for ABG control in permanent stands of PRG. Bispyribac-sodium has shown promise, but efficacy depends on temperature, rate, and application timing for acceptable ABG control with minimal PRG injury (McCullough and Hart, 2009). Williams et al. (2009) reported minimal PRG toxicity after mesotrione applications in a greenhouse experiment; however, this herbicide is only labeled for ABG suppression. Ethofumesate is also labeled for ABG control in PRG; however, results can be variable depending on numerous environmental factors such as temperature, light intensity, and soil moisture.
Maintaining a weed-free stand of PRG in northern climates is often a challenge as a result of weed invasion, in particular, ABG. Although ABG is sensitive to cold, heat, disease, and drought (Beard, 1973), most biotypes of ABG are susceptible to glyphosate. Grossbard and Atkinson (1985) reported that a glyphosate rate of 0.30 kg·ha−1 a.e. was sufficient for ABG removal. However, Goss et al. (2009) reported a rate of 3.30 kg·ha−1 a.e. to kill 90% of ABG plants in a greenhouse experiment. Although these studies indicate ABG’s genetic diversity in response to glyphosate applications, previous research indicates a wide range of diversity between ABG biotypes. Three golf courses in Minnesota, within a 5-mile radius, displayed a diverse population of ABG biotypes (Cline et al., 1993). These included prolific seed producers, minimal seed producers with dark fall color, cold-sensitive biotypes, and dark blue–green coarse biotypes. Also, ABG from the same golf course can vary because Lush (1989) noted ABG biotypes sampled on a putting green had smaller seed size, delayed flowering, and did not require chilling to break seed dormancy compared with ABG biotypes from the adjacent fairway/rough. All of these reports indicate the complexity associated when attempting to control such a diverse species.
Few commercially available cultivars possess a sufficient amount of glyphosate tolerance to control ABG. Hart et al. (2005) reported that ‘Aurora Gold’ hard fescue (Festuca longifolia Thuill.) can tolerate a single glyphosate application of 0.6 to 0.8 kg·ha−1 a.e., which would be sufficient for ABG removal. To date, there have been no reports of a glyphosate-tolerant PRG cultivar in the literature. ‘JS501’ and ‘Replay’, glyphosate-tolerant PRG cultivars, were part of a population improvement program that included selection, paired crosses, and polycrosses, which originated from a greenhouse cross in 1992 of ‘APM’ PRG, which was pollinated by ‘Birdie II’ PRG. The glyphosate tolerance of the cultivars is associated with double mutations of the 5-enolpyruvylshikimate-3-phosphate synthase gene (EPSPS) and its encoded EPSPS protein (Samudio et al., 2011). The glyphosate tolerance is naturally occurring, which has been documented in other Lolium species (Lorraine-Colwell et al., 2001, 2003).
The introduction of a glyphosate-tolerant PRG cultivar in fine turfgrass management would provide a unique tool in weed control for cool-season turfgrass managers. To preserve the glyphosate tolerance trait, rates used in ‘JS501’ and ‘Replay’ seedstock production fields are annually sprayed at 1.16 kg·ha−1 a.e. with minimal PRG injury. However, determining glyphosate rates adequate for ABG control and testing the tolerance in a fine turfgrass environment are needed to refine application rates. Therefore, research objectives were to determine the glyphosate tolerance of ‘Replay’ PRG and to determine glyphosate application rates required for effective ABG control in a stand of ‘JS501’ and ‘Replay’ PRG.
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