The dinitroaniline herbicides, such as trifluralin and pendimethalin, are used to reduce weed populations and aid in the establishment and production of many crops including groundnut, soybean, and grain sorghum (Dotray et al., 2004; Grichar & Colburn, 1993; Grichar et al., 2005a, b; Grichar, 2006). The dinitroaniline herbicides provide excellent control of annual grasses (Buchanan et al., 1982; Chamblee et al., 1982; Wilcut et al., 1995) and are the only soil-applied herbicides registered for use in peanut that will provide full-season control of Texas millet (Wilcut et al., 1987a,b; Wilcut et al., 1995).
Uptake of dinitroaniline herbicides is primarily through roots and emerging shoots (Ashton & Crafts, 1981; Appleby & Valverde, 1989). Parker (1966) showed that trifluralin was more inhibitory to Sorghum bicolor when absorbed through roots than emerging shoots. It is possible that the dinitroaniline herbicides will be concentrated in the extreme upper portions of the soil profile and weed seed may be able to germinate below the zone where dinitroaniline herbicides are located (Johnson et al., 2002). In this case, emerging shoots pass through treated soil, whereas developing roots would be below the herbicide treated soil. The dinitroaniline herbicides have very low water solubility and are subject to losses due to photodecomposition and volatilization (Weber, 1990). Therefore, incorporation soon after herbicide application is important for effective weed control.
The effectiveness of soil-applied herbicides is dependent upon several factors, including movement of the herbicide into the soil either through water provided by rainfall or irrigation, or by mechanical incorporation (Prostko et al., 2001; Ross & Lembi, 1999). Chenault et al. (1992) reported that pendimethalin or trifluralin provided greater than 78% control of barnyardgrass (Echinochloa crus-galli (L.) P. Beauv) depending on incorporation method. Tolerance to the dinitroaniline herbicides has been evaluated extensively in many crops. These herbicides injure susceptible plants by binding to |3-tublin molecules, which ultimately leads to an inhibition of cell mitosis (Appleby & Valverde, 1989). Information on absorption and translocation within plants is less clearly defined; however, direct entry into plant tissue is considered limited, and unless the dinitroaniline herbicide enters meristematic tissues, the herbicide will have little effect on plant growth (Miller et al., 2003). Previous research by Grichar et al. (2001a; 2009) reported sesame injury following the use of dinitroaniline herbicides applied preplant incorporated using various incorporation methods. Grichar et al. (2001a) reported that ethalfluralin, pendimethalin, and trifluralin reduced sesame stand numbers when compared with the untreated check. In that study the dinitroaniline herbicides were incorporated 2.5 cm deep with a tractor-driven power tiller. In another study, Grichar et al. (2009) reported that a spring-tooth harrow, with the lack of the ability to adjust incorporation depth, caused similar problems. However, the rolling cultivator mixing wheels, which were set to a depth of less than 2.5 cm, resulted in excellent sesame stands. Therefore, only a shallow incorporation of the dinitroaniline herbicides must be done when used in sesame to ensure a good stand. They concluded that it was best if the dinitroaniline herbicides were applied preemergence. Of the dinitroaniline herbicides, only pendimethalin formulated as Prowl H20® can be applied preemergence (Anonymous 2004b); however, annual grass control following pendimethalin applied preemergence is often poor (Byrd & York, 1987; Culpepper, 1996).
Commercial uses of trifluralin in sesame include: in Honduras, a grower guide (Anonymous, 2002) states that use of trifluralin applied preemergence has proved to be very efficient in the control of weeds in sesame while in Costa Rica, a grower guide (Anonymous, 2007d) recommends using a preemergence application of trifluralin at 2.0 L/ha. Martin and Crawford (1963) and Martin (1964) reported that trifluralin at 1.1 to 1.8 kg/ha was effective and non-toxic; however, trifluralin at 2.8 kg/ha killed sesame. In Venezuela, Montilla (1964) tried trifluralin at 1, 2, 3 L/ha, and the sesame did not germinate. In Ethiopia, Moore (1974) reported that trifluralin applied preplant incorporated at 0.75 and 1.4 kg/ha provided the greatest yields in sesame. Hussien et al. (1983) reported that trifluralin at rates greater than 0.84 kg/ha was harmful to sesame. However, it controlled annual grasses and increased the yield over the weedy check by 45%. Schrodter and Rawson (1984) reported that pendimethalin at 1.5 and 3.0 kg/ha and trifluralin at 0.84 kg/ha reduced sesame plant populations. Plant selectivity by herbicide placement is influenced greatly by the movement of the herbicide in soils (Ennis, 1964). If the dinitroaniline herbicides move, they may come in contact with the absorptive sites of sesame and cause sesame injury (Grichar et al., 2001a). In India, Shukla (1984) found that pendimethalin was toxic to sesame. In Israel, Graph et al. (1985) reported that preplant incorporation of trifluralin at 0.125 to 0.188 kg/ha was selective to sesame when the crop was sown on relatively warm soil, but early sowing resulted in inhibited root growth, retardation, and crop damage. In Korea, Kim et al. (1986) found that pendimethalin provided effective weed control using 1.27 kg/ha, but caused crop damage and yield reductions. In Egypt, Ibrahim et al. (1988) found that the best weed control and significantly greater seed yields and seed and yield components resulted from treatment with pendimethalin alone or in tank mixtures with linuron or diuron. In Somalia, Malik and Muhammed-Ramzak (1992) reported that pendimethalin at 3.7 L/ha provided the greatest weed control and significantly higher yield over the weedy check with no phytotoxic effects on sesame. Grichar et al. (2001a) reported yield increases over the untreated check with pendimethalin and trifluralin. They concluded that lack of yield differences among herbicide treatments which injured or reduced sesame stands could be attributed to the ability of the sesame plant to compensate for reduced stands. Sesame can produce excellent yields with only six to ten plants/m of row (author's personal observation). The rate of a dinitroaniline herbicide can affect sesame stand establishment. The one-half rate of ethalfluralin, pendimethalin EC, and trifluralin or the 3/4X dose of pendimethalin (Prowl H20® ) resulted in greater stand counts than the 1 to 2X rate of these herbicides when incorporated with rolling cultivator mixing wheels (Grichar & Dotray, 2007).
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