Bermudagrass (Cynodon spp.) is commonly produced as sod throughout the southern United States and other temperate or transition climate zones throughout the world, with most of the growers producing vegetatively propagated hybrid bermudagrass [C. dactylon (L.) Pers. × C. transvaalensis Burtt-Davy] varieties (McCalla et al., 2008). To meet the demand for quality sod, producers must account for the aesthetic, harvesting, and establishment characteristics of the hybrid bermudagrass (Sharpe et al., 1989). Annual grassy weeds, such as crabgrass (Digitaria spp.) and goosegrass [Eleusine indica (L.) Gaertn.], commonly germinate during hybrid bermudagrass establishment, which can reduce turfgrass quality and harvesting characteristics (Fishel and Coats, 1994; Johnson, 1996). Many sod producers use preemergence herbicides, such as prodiamine, pendimethalin, and dithiopyr, to control these grassy weeds. However, many preemergence herbicides may hinder root growth and establishment of hybrid bermudagrass (Bingham and Hall, 1985; Brosnan et al., 2014; Fagerness et al., 2002). Prodiamine and pendimethalin are mitotic-inhibiting herbicides that commonly cause malformed and stubby roots on hybrid bermudagrass (Vaughn and Vaughn, 1990; Weber, 1990). Fishel and Coats (1994) determined that prodiamine, pendimethalin, and dithiopyr also increased the number of roots that were considered abnormal (less than 2.5 cm, lacked secondary root formation, or were swollen). In the same study, labeled rates of dithiopyr and prodiamine significantly reduced root fresh weight of hybrid bermudagrass plugs when compared with pendimethalin and oxadiazon (Fishel and Coats, 1994).
Producing new roots quickly is critical to establishing hybrid bermudagrass sod. For example, Bingham and Hall (1985) determined that ‘Tifway’ hybrid bermudagrass was tolerant to oxadiazon because the herbicide did not negatively affect shoot or root growth throughout establishment. However, Bingham and Schmidt (1983) determined that a high rate of oxadiazon (6.7 kg·ha−1) slightly reduced hybrid bermudagrass root growth, although adequate root growth was apparent for proper plant function. McCullough et al. (2012) determined that S-metolachlor (1.1 and 2.2 kg a.i./ha) and oxadiazon (1.1, 2.2, and 4.4 kg a.i./ha) did not reduce hybrid bermudagrass cover relative to the nontreated 8 WAT. However, the higher rate of S-metolachlor (4.4 kg a.i./ha) did significantly reduce hybrid bermudagrass cover measured at the same time interval. Brosnan et al. (2014) determined that pendimethalin (3.36 kg a.i./ha), dithiopyr (0.56 kg a.i./ha), prodiamine (0.6 kg a.i./ha), oxadiazon (3.36 kg a.i./ha), prodiamine + sulfentrazone (0.84 + 0.41 kg a.i./ha), dimethenamid-P (1.68 kg a.i./ha), and indaziflam (0.03 and 0.05 kg a.i./ha) all increased the days to reach 50% hybrid bermudagrass cover when compared with the nontreated. However, only the two rates of indaziflam (0.03 and 0.05 kg a.i./ha) reduced sod tensile strength compared with the nontreated when harvested 377 d after sprigging.
Preemergence herbicides may affect the amounts of nonstructural carbohydrates within the roots, which may have an impact on plant function, transplant success, and spring green-up (Macolino et al., 2010; Schiavon et al., 2016). White (1973) describes nonstructural carbohydrates as the total amount of reducing and nonreducing sugars, as well as fructosans and starch. The levels of these sugars may be used to measure the physiological stresses experienced by the plant, including possible herbicidal injury (Hull, 1992; Sheffer et al., 1979). However, research to determine the amount of nonstructural carbohydrates within hybrid bermudagrass roots in response to herbicide application is lacking. Therefore, research was conducted to determine the effects of preemergence herbicides commonly used on sod farms on hybrid bermudagrass root architecture and to provide some insight into the effects of these preemergence herbicides on the amounts of nonstructural carbohydrates within those roots.
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