The development of two glyphosate-tolerant PRG cultivars, JS501 and Replay, provides a unique avenue for weed control (Samudio et al., 2011). A previous field trial established these cultivars can tolerate glyphosate rates up to 0.81 kg·ha−1 with minimal stand injury when applied to actively growing PRG (Baldwin et al., 2012). However, tolerance levels are reduced to ≈0.27 kg·ha−1 when applied during the fall season and applications at rates >0.29 kg·ha−1 should be avoided until the plants are beyond the four-leaf stage (Baldwin et al., 2015). In a greenhouse trial, Flessner et al. (2014) noted a glyphosate rate of 2.60 kg·ha−1 was required to achieve 50% visible injury, which was four times higher when compared with standard, nontolerant PRG cultivars.
The use of glyphosate applied over actively growing turfgrass has received little attention due to the lack of glyphosate-tolerant cultivars commercially available. Primarily, the focus has determined thresholds for tolerance (Baldwin et al., 2012; Dant et al., 2005; Flessner et al., 2014; Hart et al., 2005) with little published data regarding possible benefits of glyphosate applications, in particular, growth regulation. Although glyphosate is considered a class D PGR (Ervin and Zhang, 2008), its use as a PGR for growth regulation has historically been limited to low-maintenance sites where injury can be tolerated. Previous work in canola (Brassica napus L.) and soybean (Glycine max L.) demonstrated a reduction in shoot weight following an application of glyphosate (Cakmak et al., 2009; Schilling et al., 2006).
Combining various products, such as herbicides, PGRs, or foliar fertilizers into one tank has become a common practice in the turfgrass industry. Regarding herbicides, combining different modes of action can potentially increase the spectrum of weed control and prevent the development of herbicide resistant weeds by altering selection pressure. This is relevant when considering the use of glyphosate-tolerant cultivars in fine turf management for annual bluegrass (ABG) control since recent reports have noted resistant ABG biotypes in golf course fairways due to repeated glyphosate applications (Binkholder et al., 2011; Brosnan et al., 2012).
Several studies in turfgrass have documented the benefits of tank mixing herbicides, PGRs, and N sources. Jeffries et al. (2013) noted beneficial effects of tank mixing an early gibberellic inhibiting PGR, paclobutrazol (PB), with a photosystem II-inhibiting herbicide, amicarbazone. The authors reported increased ABG control, safety to the desirable species, and fewer overall applications were necessary for acceptable weed control. Elmore et al. (2013) also noted improved ABG control when tank mixing mesotrione and amicarbazone compared with each herbicide applied alone. Finally, Kaminski and Putman (2009) reported tank mixing an iron + N product with bispyribac-sodium herbicide masked chlorosis on ‘Allister’ colonial bentgrass (Agrostis capillary L.) compared with bispyribac-sodium applied alone.
With the emergence of glyphosate-tolerant cultivars, growth regulation and herbicide options must be revisited to identify management strategies not applicable with older cultivars. Young et al. (2003) applied glyphosate at 0.84 kg·ha−1 with and without AMS at 20 g·L−1. This study showed no improvement in glyphosate absorption and translocation by adding AMS to glyphosate when applied to common lambsquarters (Chenopodium album L.); however, similar applications resulted in a significant increase in glyphosate efficiency when applied to velvetleaf (Abutilon theophrasti Medik.). It appears, the addition of AMS to glyphosate is species dependent regarding glyphosate efficiency. Bradley et al. (2000) sprayed glyphosate at 0.43 and 0.84 kg·ha−1 with or without the addition of AMS at 3.8 kg·ha−1. The authors noted increased control of ‘Rox Orange’ forage sorghum (Sorghum bicolor L.) with the addition of AMS at the 0.43 kg·ha−1 rate; however, no differences were noted at the higher application rate.
Since reduced inputs, such as reduced mowing, is an important management consideration and tank mixing multiple products is a common practice in turfgrass management, the objectives of this study were to quantify potential growth regulation following a glyphosate application to a glyphosate-tolerant cultivar and determine the safety of tank mixing an alternative active ingredient, various N sources, and a PGR with glyphosate on a glyphosate-tolerant PRG cultivar.
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