Zoysiagrass (Zoysia japonica Steud.) is a popular turfgrass species used in golf course fairways in Missouri and surrounding states (Lyman et al., 2007). Throughout this region, zoysiagrass enters dormancy typically in November and resumes growth in late March or early April (Dunn and Diesburg, 2004). Weed established in dormant zoysiagrass reduces the aesthetic value as well as the playability during winter and spring months. Various pre- and post-emergent herbicides can be effective for control of winter weeds on zoysiagrass turf (Harrell et al., 2005; Johnson, 1980; Vargas and Turgeon, 2004). However, the main strategy adopted by golf course superintendents in this region is winter application of the non-selective herbicide glyphosate (Velsor et al., 1989), which is an economical and effective tool for control of a broad spectrum of weeds.
One of the concerns with winter applications of glyphosate in dormant warm-season turf is possible/potential delay of spring green-up (Johnson, 1977; Johnson and Burns, 1985; Johnson and Ware, 1978). Besides variation in zoysiagrass species and environmental conditions (Rimi et al., 2012; Velsor et al., 1989), application timing is a critical factor influencing herbicide safety on zoysiagrass turf. A 2-year study conducted in Missouri found that glyphosate at 2.24 kg·ha−1 on ‘Meyer’ zoysiagrass (Z. japonica) applied on 1 Mar. had no impact on turf quality, but the same applications made on 1 Apr. caused significant injury in late April and May (Velsor et al., 1989). At both application dates, zoysiagrass turf was apparently dormant despite the presence of green tissue at the base of stems for 3 to 5 or 5 to 8 mm in length for March and April application dates, respectively (Velsor et al., 1989). This indicates that characterization of zoysiagrass as dormant is difficult, complicating identification of a safe window for application of non-selective herbicides. For golf course superintendents, applications closer to spring green-up are more desirable than earlier applications, because the limited residual activity of non-selective herbicides such as glyphosate allows continued emergence of winter annuals. Application before spring is more desirable also because there is limited demand for use of zoysiagrass turf in the winter months (Ryan Sears, personal communication). Therefore, it is critical to determine the proper timing window toward the end of winter for both effective weed control and optimum zoysiagrass safety.
Another concern for winter application of glyphosate on dormant warm-season turf is selection of herbicide-resistant weeds. Since the first report of resistant rigid ryegrass (Lolium rigidum Gaudin) in 1996 (Powles et al., 1998), there are 24 species worldwide today with reported resistance to glyphosate (Heap, 2012). A common factor that links the selection of resistant species is repeated application. On turf, the first reported glyphosate-resistant annual bluegrass (Poa annua L.) was identified in 2011 from a golf course fairway in Missouri (Binkholder et al., 2011). After more than 10 years of glyphosate applications in dormant ‘Meyer’ zoysiagrass at rates up to 0.62 kg·ha−1, a biotype of annual bluegrass survived glyphosate at 6.27 kg·ha−1, which is eight times the labeled rate. Similarly, after winter application of glyphosate at 0.84 kg·ha−1 for 20 years, a population of glyphosate-resistant annual bluegrass was identified from a bermudagrass [Cynodon dactylon (L.) Pers.] turf in Tennessee (Brosnan et al., 2012).
Because winter application of non-selective herbicides for weed control on dormant warm-season turf is a common practice, alternative herbicides are desirable. Glufosinate, a non-selective, foliar-applied herbicide, can be used to control winter weeds in dormant bermudagrass turf (Tharp et al., 1999). Despite differences in toxicity for glyphosate (Carlson and Burnside, 1984) compared with glufosinate, most annual weed species, including barnyardgrass (Echinochloa crusgalli L.), fall panicum (Panicum dichotomiflorum L.), giant foxtail (Setaria faberi L.), and large crabgrass (Digitaria sanguinalis L.), responded similarly to glufosinate and glyphosate (Tharp et al., 1999). When applied on dormant bermudagrass turf, glufosinate at 1.68 kg·ha−1 controlled annual bluegrass up to 93%, which was similar to the efficacy of 0.56 kg·ha−1 glyphosate (Toler et al., 2007). Unlike glyphosate, which is translocated to the sink tissues including stolons and rhizomes in zoysiagrass, glufosinate is mainly directed to the apical-developing tissues (Grangeot et al., 2006). This characteristic could potentially improve the safety of glufosinate compared with glyphosate when applied to dormant zoysiagrass turf, especially when complete dormancy is difficult to determine.
Currently, the label for glufosinate does not include zoysiagrass for winter applications, and no report has documented the safety of glufosinate on dormant zoysiagrass. The objectives of this study were to: 1) evaluate the efficacy of glufosinate applied at two rates for control of winter weeds; and 2) determine the safety of glufosinate when applications were made within weeks or days before zoysiagrass breaking dormancy.
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