Several factors influence the efficacy of foliar applied herbicide, e.g., water quality, adjuvant, tank-mixed application of two or more herbicides, and environmental conditions (Colby, 1967; Foy, 1989; McWhorter, 1982). Adjuvants often improve efficacy of foliar applied herbicides (Hart and Wax, 1996; Sharma and Singh, 1999). When two or more herbicides are tank-mixed and applied together and the resulting effect is greater than the expected control of the individual herbicides applied alone, the combination is said to be synergistic (Colby, 1967). However, a tank-mixed application could also result in reduced control under certain circumstances; such combinations are said to be antagonistic.
Glyphosate has been used for vegetation management in citrus groves and is the most commonly used herbicide for effective postemergence (POST) weed management in the Florida (Jackson and Davies, 1999). Although glyphosate is a broad-spectrum herbicide, not all weeds are equally susceptible (Culpepper and York, 2000; Payne and Oliver, 2000). Taylor (1996) reported that some grasses, such as barnyardgrass (Echinochloa crus-galli) and broadleaf signalgrass (Brachiaria platyphylla), are more sensitive to glyphosate than broadleaf species, such as velvetleaf (Abutilon theophrasti) and morning-glory. These differences in susceptibility to glyphosate may be attributed to lower absorption of glyphosate due to physicochemical differences in the plant cuticle and reduced translocation from the treated leaf (Taylor, 1996). Pitted morning-glory is among the most troublesome and common weed species found in roadsides, fence rows, noncrop areas, pastures, and utility poles (Anonymous 2000, 2001; Uva et al., 1997). Norsworthy et al. (2001) reported only 59% and 69% control of 3- to 4-leaf pitted morning-glory using glyphosate as active ingredient at 0.85 and 1.25 kg·ha−1, respectively, compared with at least 98% control of more sensitive weeds, such as barnyardgrass [Echinochloa crus-galli (L.)] and prickly sida (Sida spinosa L.).
Reduced efficacy of glyphosate on some weed species has been instrumental in their increased infestation due to its continuous use in many groves. It has been observed that several broadleaf weeds have less phytotoxic effect/control by glyphosate application (Singh and Singh, 2005). For example, Brazil pusley (Richardia brasiliensis) is a problematic weed in Florida citrus plantations, and effective doses of glyphosate do not provide adequate control of various weeds, including Brazil pusley (personal observation). Carfentrazone-ethyl has been found effective against certain broadleaf weeds but has no activity on grasses (Vencill, 2002). Carfentrazone-ethyl inhibits the protoporphyrinogen oxidase enzyme and therefore provides control of selected broadleaf weeds, including kochia (Dayan et al., 1997). Observed symptoms of carfentrazone-ethyl treatments are a rapid necrosis of plant tissues. This fast activity makes carfentrazone an ideal candidate for postemergence and burn-down applications (Mize et al., 2002). Carfentrazone-ethyl applied at recommended growth stages provided 99% control of kochia (Kochia scoparia [L.]), including resistant biotypes (Nandula and Manthey, 2002). Thus carfentrazone-ethyl is a relatively new broadleaf herbicide in the phenyl triazolinone class of chemicals (Dayan et al., 1997). Alternate methods, such as tank-mixed application, may provide improved weed control under various cropping systems. No single herbicide can provide adequate control of all the weed species, even in herbicide-tolerant (e.g., Roundup-Ready) crops. Herbicide mixtures application may be beneficial for better weed management. Scanty research has been conducted to evaluate the potential of carfentrazone as a tank-mixed partner with glyphosate. Therefore, the objective of the study was to evaluate carfentrazone as a potential tank-mixed partner for glyphosate to improve the control of hard-to-control weeds. The study was conducted in greenhouse as well as in the field.
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