St. augustinegrass is the most widely used turfgrass in commercial and home landscapes in Florida, and also is commonly used in Alabama, LA, MS, and Texas. It grows throughout the year in southern Florida, but usually senesces during the winter in the northern range of its distribution. Bermudagrass (Cynodon dactylon) is a fine-textured high-maintenance turfgrass that is most often used on golf courses, athletic fields, and other high-profile areas. There are many bermudagrass varieties and hybrids grown throughout the southern portion of the United States. Bahiagrass is grown as a pasture crop for forage and hay and is also used in low-maintenance landscapes, roadsides, and similar areas, but is much less common than st. augustinegrass and bermudagrass in traditional urban landscapes. Bahiagrass is grown in the southeastern United States from Virginia to Texas, but is most frequently used in South Carolina, GA, FL, AL, MS, and Louisiana. These three species are considered warm-season grasses because they all undergo significant winter senescence in colder regions (Trenholm et al., 2011, 2014, 2015).
Topramezone is a carotenoid biosynthesis inhibitor that interferes with the production of the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD) in the carotenoid pigment synthesis pathway (Shaner, 2014). It is sold under a number of trade names, including Armezon, Pylex, and Frequency® (BASF, Research Triangle Park, NC). Armezon is labeled at application rates of up to 25 g·ha−1 for postemergence control of grasses and broadleaf weeds in corn [Zea mays (BASF, 2012)], and Pylex is labeled for use in cool-season turfgrasses such as golf courses, sod farms, and residential landscapes (BASF, 2015). The Pylex label specifically states that applications will control or suppress st. augustinegrass, bahiagrass, and bermudagrass (BASF, 2015). The herbicide Frequency® allows applications of up to 50 g·ha−1 topramezone in pine (Pinus sp.) plantations, rights of way, and other noncrop sites (BASF, 2013). Grasses listed as tolerant of topramezone include cool-season grasses such as bluestem (Andropogon sp.), fescue (Festuca sp.), bluegrass (Poa sp.), and centipedegrass (Eremochloa sp.).
Topramezone was studied for several years under an experimental use permit to evaluate its efficacy on the fluridone-resistant submersed aquatic weed hydrilla. Puri et ;al. (2009) found that topramezone provided effective control of fluridone-resistant hydrilla at concentrations of 25–40 ppb; it was highly selective at these rates and caused little damage to most native nontarget plants. It was registered for aquatic use by the U.S. Environmental Protection Agency (USEPA) in 2013 under the trade name Oasis® (SePRO Corporation, 2014).
Bispyribac-sodium is an acetolactate synthase (ALS) inhibitor, which is used for pre- and postflood grass and broadleaf weed control in rice (Oryza sativa). It is sold under the trade name Regiment® for use in rice at rates of up to 32 g·ha−1 (Valent U.S.A. Corporation, 2012). It is also labeled under the trade name Velocity® for weed control in creeping bentgrass (Agrostis stolonifera) and established perennial ryegrass turf (Lolium perenne) at application rates of up to 74 g·ha−1 (Valent U.S.A. Corporation, 2010). Bispyribac-sodium was registered for aquatic use by the USEPA in 2011 under the trade name Tradewind (Valent U.S.A. Corporation, 2011).
The USEPA evaluates the impacts and risks associated with aquatic herbicides in water and may impose water use restrictions on treated water to protect human health and the environment. Water use restrictions typically prohibit the use of treated water for drinking and irrigation of food crops for a specified length of time or until concentrations of the herbicide are reduced to a particular level. Regulatory agencies are less concerned about the effects of using herbicide-treated water to irrigate turfgrasses and other ornamental species that are not destined for human consumption, but registrants must consider potential phytotoxic effects on these plants if treated water is used to irrigate landscape species.
Topramezone and bispyribac-sodium provide aquatic resource managers with alternate modes of action to control fluridone-resistant hydrilla and are consequently expected to be used intensively in ponds and lakes in Florida. Many waterbodies are multiuse and are commonly used as a source of irrigation water by riparian owners. Therefore, these studies were designed to determine the effects of topramezone and bispyribac-sodium in water used to irrigate three warm-season grasses that are typically grown in the southeastern United States.
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