Khakiweed (Alternanthera pungens Kunth) is a perennial broadleaf weed that readily invades turfgrass throughout the arid and semiarid regions of the United States (McCarty et al., 2008). The presence of this weed significantly reduces the aesthetic value and playability of desired turfgrass species. Morphological characteristics and the ability to adapt to a wide range of environments have led to the increase in khakiweed prevalence in managed turf. Long-term reductions in turfgrass density often result from the aggressive spread of dense, prostrate khakiweed mats throughout the summer (Sholedice and Renz, 2006; Umeda and Towers, 2004). A deep taproot and a thick waxy cuticle help khakiweed avoid acute drought stress (Parsons and Cuthbertson, 2001) and may contribute to salinity tolerance. Furthermore, turfgrass managers often observe khakiweed in areas of heavy traffic and wear (e.g., cart paths, athletic fields) where desired turfgrass species are thin and less vigorous (Umeda and Towers, 2004).
Limited options exist for selective khakiweed control in managed turfgrass. Trifloxysulfuron is the only herbicide labeled for the postemergence control of khakiweed in turf (Anonymous, 2009). However, turfgrass managers also use metsulfuron for selective control of khakiweed on athletic fields and golf courses (personal observation). Trifloxysulfuron and metsulfuron both belong to the sulfonylurea herbicide family. Good crop selectivity, very low acute and chronic animal toxicity, and broad-spectrum weed control at very low use rates (as low as 2 to 75 g·ha−1) make these chemistries very beneficial from a use and environmental risk standpoint (Beyer et al., 1988).
Sulfonylurea herbicides inhibit the enzyme acetolactate synthase, thereby blocking the biosynthesis of the branched-chain amino acids valine, leucine, and isoleucine (Brown, 1990; Senseman and Armbrust, 2007). Inhibition leads to the termination of plant cell division and growth. Cessation of plant growth in conjunction with long-term reductions in new leaves and reproductive organs may be attributed to the accumulation of acetolactate synthase substrate (e.g., a-ke-tobutyrate) in leaf tissue (Brown, 1990; LaRossa and Van Dyk, 1987).
Trifloxysulfuron and metsulfuron are both labeled for the postemergence control of broadleaf weeds in warm-season turf (Anonymous, 2009, 2011). Minimal to no bermudagrass phytotoxicity was observed after applications of metsulfuron and trifloxysulfuron for weed control (Butler et al., 2005; Davis et al., 1997; Johnson, 1987; McElroy et al., 2005). Busey (2009) reported 77% control of dollarweed (Hydrocotyle umbellata L.) 10 WAIT in St. Augustinegrass [Stenotaphrum secundatum S. (Walt.) Kuntze] with applications of metsulfuron at 0.021 kg a.i./ha. Metsulfuron at 0.017, 0.034, and 0.05 kg a.i./ha exhibited 95% to 100% control of broom snakeweed [Gutierrezia sarothrae (Pursh) Britt. & Rusby] present in pasture land 7 WAIT (McDaniel and Duncan, 1987). Singh and Singh (2004) observed 80% to 88% control of redroot pigweed (Amaranthus retroflexus L.), Florida beggarweed [Desmodium tortuosum (Sw.) DC.], hairy beggarticks (Bidens pilosa L.), tall morning glory [Ipomoea purpurea (L.) Roth], and ivyleaf morningglory (Ipomoea hederacea Jacq.) in the greenhouse with applications of trifloxysulfuron at 0.063 kg a.i./ha. Trifloxysulfuron at 0.005 kg a.i./ha exhibited 73% control of sicklepod [Senna obtusifolia (L.) H.S. Irwin Barneby] 3 WAIT in the greenhouse (Koger et al., 2005).
Postemergence control of khakiweed with herbicides has received limited investigation. Trifloxysulfuron at 0.017, 0.029, and 0.053 kg a.i./ha exhibited 95%, 94%, and 98% control, respectively, 40 d after treatment (DAT) (Kopec et al., 2004a). Single applications of metsulfuron at 0.021 and 0.042 kg a.i./ha exhibited 99% khakiweed control 40 DAT (Kopec et al., 2004a). Penoxsulam at 0.1 kg a.i./ha exhibited 69% khakiweed control 40 d after initial treatment (DAIT) (Kopec et al., 2004b). Umeda (2008) observed 86% khakiweed control 36 DAIT with applications of carfentrazone + 2,4-D + mecoprop + dicamba at 0.45 kg a.i./ha. Tank-mixing carfentrazone + 2,4-D + mecoprop + dicamba at 0.45 kg a.i./ha with fluroxypyr at 0.056 kg a.i./ha provided 85% khakiweed control 40 DAIT (Umeda and Towers, 2004). Applications of 2,4-D + mecoprop + dicamba at 0.14 kg/a.i. only controlled khakiweed 1% 27 DAIT (Kopec et al., 2004a). Thus, carfentrazone may have potential for khakiweed control and warrants further investigation in bermudagrass.
The presence of a deep taproot enables khakiweed to regenerate from abundant carbohydrate reserves after defoliation (Umeda, 2008). Minimal research has examined sequential applications for the long-term control of khakiweed. Umeda (2008) reported 94% khakiweed control 36 d after sequential treatment of carfentrazone + 2,4-D + mecoprop + dicamba at 0.45 kg a.i./ha tank-mixed with isoxaben at 0.84 kg a.i./ha. Sequential applications may be necessary to reduce the photosynthetic capability of khakiweed and associated carbohydrate production to obtain long-term control of perennial plant portions. Therefore, the objective of our research was to examine single vs. sequential applications of various herbicides for long-term control of khakiweed in bermudagrass turf.
Anonymous 2009 Monument™ herbicide product label. Syngenta Crop Protection, Inc., Greensboro, NC.
Anonymous 2011 Blade™ herbicide product label. PBI/Gordon Corp., Kansas City, MO.
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