Pecan production is increasing in the United States as exports to foreign markets have stimulated value and prompted newly planted groves (Wells, 2014). Between 2009 and 2014, the U.S. in-shell pecan production (nuts from improved, native, and seedling trees) averaged more than 181,400 t each year, valued at more than $479 million (NASS, 2015). In the United States, Georgia consistently ranks number one in pecan production generating 36,000 t on 63,464 ha, with a value of $361 million in 2015 (Wolfe and Stubbs, 2016). With increased demand and value, growers in Georgia have begun to interplant older groves with new cultivars (Wells, 2014). This can expose newly planted trees to herbicides that have been applied in previous seasons and are presently labeled for bearing crops. In addition, greater than 6000 ha of new groves were planted from 2010 to 2014, to more than 391,000 trees in Georgia (Wells, 2014). It is forecast that production in Georgia could exceed 121,000 t annually by 2025 with newly planted trees and renovated groves.
Pecan trees in commercial production are often irrigated with solid set risers or emitters near each tree (Wells, 2015). Nearly all commercial groves are irrigated, although exact data from Georgia are not available. Although irrigation is necessary for maximum yields, it also can promote growth of weeds (Faircloth et al., 2007). Weed competition can reduce growth in pecan groves by more than 50%. Newly planted trees are especially sensitive to competition for sunlight, moisture, and nutrients (Smith, 2011). In established groves, weeds also serve as inoculum for diseases and alternate hosts for insects (Lee, 1994). Establishing a weed-free strip by applying herbicides between pecan trees to increase survival, water use efficiency, and growth is a common practice (Faircloth et al., 2007). This reduces the time required for pecan trees to begin bearing nuts and producing the first commercially viable yield (Smith, 2011).
Indaziflam is an alkylazine herbicide assigned to group 29 by the Weed Science Society of America (Shaner, 2014). It is an effective soil residual herbicide registered for citrus (Citrus L.), apple (Malus Mill.), pear (Pyrus L.), stone (Prunus L.) and pome (Punica L.) fruits, grape (Vitis L.), olive (Olea europaea L. ssp. europaea), and tree nuts. Indaziflam is classified as a cellulose biosynthesis inhibitor (Brabham et al., 2014) with no reports of any resistant weeds (Brabham et al., 2014; Shaner, 2014).
Indaziflam’s soil persistence has reported half-lives of >150 d (U.S. EPA, 2010), with others indicating 22–176 d (González-Delgado et al., 2015). The water solubility of indaziflam increases with increasing pH from 2.0 g·L−1 at pH 7 to 18.3 g·L−1 at pH 9 (Shaner, 2014). Using Italian ryegrass [Lolium perenne L. subsp. multiflorum (Lam.) Husnot] as a bioassay indicator, Jhala et al. (2012) determined that indaziflam did not leach beyond 30 cm deep in sand soil, when evaluated in column studies. Indaziflam desorption was hysteric for multiple soils examined, indicating decreased potential for mobility (Alonso et al., 2015). Turf weed control with indaziflam was noted for up to 28 weeks (McCullough et al., 2013; Perry et al., 2011) and at least 6 months of weed control noted in pistachio (Pistacia vera L.), pome fruit, stone fruit, and citrus (Allen, 2011). For Florida citrus, indaziflam has a 24(c) supplemental label for potted trees planted for a minimum of 1 year (Anonymous, 2015a; Jhala et al., 2013). Total seasonal rates for perennial crops can range from 50 to 150 g a.i./ha.
Halosulfuron is also registered to control perennial nutsedges and broadleaf weeds in perennial crops including pecan, with total seasonal use rates of 35–70 g a.i./ha (Anonymous, 2015b). Halosulfuron has multiple registrations, including perennial crops. Soil longevity of halosulfuron varies with adsorption to soil colloids and soil organic carbon, with availability often inversely related to soil pH (Dermiyati and Yamamoto, 1997a). Degradation can increase with increasing soil temperature and lower soil pHs, with soil moisture content and soil type further affecting carryover (Dermiyati and Yamamoto, 1997b). Dissipation is primarily by chemical hydrolysis and microbial degradation and is much faster in acid and basic solutions and slower under neutral conditions (Zheng et al., 2008). Halosulfuron half-life ranges from 6 to 98 d, depending on soil moisture and temperature regimes (Dermiyati and Yamamoto, 1997b; Grey et al., 2007a) and can exhibit hysteric effects (Carpenter et al., 1999). Injury from halosulfuron carryover to rotational crops has occurred as a result of its variable soil behavior (Grey et al., 2007b). Although halosulfuron has been extensively evaluated in annual crops for tolerance and carryover (Dittmar et al., 2008; Haar et al., 2002; Jennings, 2010; Sikkema et al., 2008; Webster et al., 2003), little information about pecan tree tolerance has been reported.
As a nonselective cellulose biosynthesis inhibitor, having low soil mobility, along with a long soil half-life, indaziflam has a niche for broad-spectrum control of annual grasses and broadleaf weeds in pecan groves for maintaining bare ground in the tree row. The sulfonylurea residual herbicide halosulfuron provides excellent control of nutsedge and broadleaf weed species and could be used to assist in maintaining a weed-free environment, especially in irrigated groves where these weed species can proliferate. Although pecan production in Georgia continues to increase in hectares and value, limited information about these herbicides’ effects on newly planted trees in newly planted or established groves settings is available. Therefore, the objective of this research was to determine the effects on in-field establishment and growth of pecan trees with multiple applications of indaziflam and halosulfuron over time.
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