Dallisgrass (Paspalum dilatatum Poir.) is a problematic rhizomatous perennial grass weed of managed turfgrass stands throughout the southern United States. Dallisgrass is often considered a desirable grass species in pastures and roadsides; however, its wide adaptability to different environmental conditions has likely contributed to it becoming a weed in turfgrass settings. The presence of dallisgrass reduces turfgrass aesthetics and functionality of recreational, commercial, and residential turfgrass stands (Elmore et al., 2013; Henry et al., 2007a). Tolerance of dallisgrass to low mowing heights (1.3 cm) has been reported, suggesting that it can be problematic in both golf course fairways and roughs (Henry et al., 2007a). Furthermore, dallisgrass growth is often encouraged in areas with high volumetric water content and its survival has been observed in regions prone to flooding (Henry et al., 2009; Loreti and Oesterheld, 1996; Rubio et al., 1995; Rubio and Lavado, 1999).
Chemical control options are currently limited for dallisgrass. One of the most common control programs is the use of sequential monosodium methanearsonate (MSMA) applications; however, this may present phytotoxicity concerns to warm-season turfgrasses (Henry et al., 2007b; Henry et al., 2008). Summer applications of MSMA have been reported to cause reductions in turf quality in both ‘Tifway’ hybrid bermudagrass (Cynodon dactylon × Cynodon transvaalensis Burtt-Davy) and common bermudagrass [C. dactylon (L.) Pers.] (McCarty et al., 1991). Additionally, commercial availability and continued use of MSMA has been uncertain following the 2009 decision by the Environmental Protection Agency, which eliminated the use of MSMA on home lawns and athletic fields, while restricting use on sod farms, golf courses, and highway rights of way and prohibiting use in all turfgrass environments after 31 Dec. 2013 (United States Environmental Protection Agency (EPA), 2009). The 2009 EPA decision on MSMA prohibition was recently delayed, pending a registration review that began in 2013 and is scheduled for completion in 2019 (United States Environmental Protection Agency, 2015).
Limitations on the efficacy of other herbicides for dallisgrass control have also been reported. Henry et al. (2007b) observed <60% dallisgrass control 1 month after initial treatment (MAIT) in response to sequential foramsulfuron applications made in summer; control was <5% 1 year after initial treatment. Bingham et al. (1993) also reported insufficient control of dallisgrass in tall fescue (Festuca arundinacea Schreb.) roadsides with fenoxaprop and imazethapyr. Herbicide application timing has also been determined to be a limiting factor for effective dallisgrass control, and previous research has illustrated the ineffectiveness of certain herbicides when applied after early spring. Brosnan et al. (2010) reported that single fluazifop applications made after the accumulation of 500 growing degree days provided 46% and 0% dallisgrass control 28 and 76 d after treatment, respectively, and sequential applications resulted in similar trends. The same research indicated that tank mixtures of fluazifop with mesotrione provided no better control than single or sequential fluazifop applications alone. In addition to a lack of efficacy for dallisgrass control when applied after early spring, it has also been well documented that fluazifop may cause phytotoxicity to bermudagrass turf (Bryson and Wills, 1985; Johnson, 1992; McElroy and Breeden, 2006;). Nonselectivity of glyphosate limits its use to spot spray applications that may cause nontarget damage to hybrid bermudagrass (Henry et al., 2008). The lack of noninjurious and effective herbicides for dallisgrass control warrants further investigation into chemical control options for use in warm-season turfgrasses.
A combination product containing the acetolactate synthase inhibitors TFH became commercially available in 2012 for the control of annual and perennial grass weeds, sedges, and broadleaf weeds in bermudagrass and zoysiagrass (Zoysia spp.) turf (Anonymous, 2016). Previous research examined the efficacy of this combination chemistry for the control of nutsedge (Cyperus sp.) in bermudagrass (Henry et al., 2013). May and June applications of TFH resulted in 89% to 93% yellow nutsedge (Cyperus esculentus L.) control 12 WAIT (Henry et al., 2013). Yellow nutsedge, like dallisgrass, is a difficult-to-control perennial weed with abundant vegetative tissue for carbohydrate storage (Wills, 1987). Perennial grass rhizomes act as sink tissue for carbohydrate reserves before the onset of dormancy; therefore, coinciding herbicide treatments with basipetal carbohydrate movement may maximize long-term perennial weed control (Davis et al., 1978; Smith et al., 1993). Research investigating this approach to dallisgrass control is lacking and highly warranted. Thus, the objectives of this research were to evaluate the efficacy of fall applications of TFH compared with other herbicides on dallisgrass control in bermudagrass turf, and to determine the effect of rate, application timing, and number of applications on efficacy of these herbicide treatments.
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