Peanut Response to Postemergence Application of Pyroxasulfone

Over 428,000 ha of peanut were harvested in the United States in 2013 (Anonymous 2013a). Although economically important in the southern region, peanut acreage is significantly lower than many row crops grown in the United States. Consequently, secondary labels are often the only opportunity to obtain new herbicide labels for peanut growers. For example, lactofen was registered for use in soybean in 1989, and later labeled for use in peanut in 2004 (Anonymous 2004; Hagwood and Wilcut 1989; Wilcut et al. 1990). Similarly, pyroxasulfone, a preemergence (PRE) herbicide currently labeled for use in corn and soybean is being developed for use in wheat (Triticum aestivum L.) and sunflower (Helianthus annus L.). While research concerning peanut tolerance to pyroxasulfone is limited, Prostko et al. (2011) reported excellent tolerance when it was applied POST at 44 to 51 days after emergence. Pyroxasulfone may potentially have utility in peanut but manufacturers may never explore this use without third party research efforts.

Pyroxasulfone, is a member of the isoxazoline herbicide class and inhibits very-long-chain-fatty acid synthesis in susceptible plants (Tanetani et al. 2009). Other members with this mode of action used in peanut include dimethenamid-p and S-metolachlor (Johnson et al. 1994; Grichar et al. 1996; Baumann et al. 1999). Pyroxasulfone provides residual control of troublesome annual broadleaf weeds and grasses, including; browntop millet (Urochloa ramose L.), barnyardgrass (Echinochloa crus-galli L.), green foxtail (Setaria viridis L.), Amaranthus spp., velvetleaf (Abutilon theophrasti Medik.), large crabgrass (Digitaria sanguinalis L.), Italian ryegrass (Lolium multiflorum L.), and kochia (Kochia scoparia L.) (Geier et al. 2006; Gregory et al. 2005; Hulting et al. 2012; King and Garcia 2008; Koger et al. 2008; Nurse et al. 2011). Pyroxasulfone applied at 209 g ai/ha controlled broadleaf signalgrass (Urochloa platyphylla L.) similar to dimethenamid-p and S-metolachlor; however, dimethenamid-p and S-metolachlor provide poor residual control of Texas millet (Urochloa texana Buckl.) (Mueller and Steckel 2011). Pyroxasulfone applied at 208 g ai/ha resulted in greater than 90% control of Texas millet 4 weeks after treatment (WAT) (Gregory et al. 2005). At pyroxasulfone rates of 120 g ai/ha or less, Texas millet control is inconsistent. Originally, Knezevic et al. (2009) proposed that pyroxasulfone use rates range from 200 to 300 g ai/ha. Due to high manufacturing costs, pyroxasulfone use rates are projected to be between 60 and 120 g ai/ha.

Residual herbicides are often applied topically to peanut in combination with POST herbicides such as paraquat, bentazon, acifluorfen, imazapic, and lactofen. Bentazon and acifluorfen are commonly tank-mixed with paraquat to increase control of prickly sida (Sida spinosa L.), smallflower morningglory (Jacquemontia tamnifolia L.), sicklepod (Senna obtusifolia L.), and coffee senna (Senna occidentalis L.) (Wehtje et al. 1992). Peanut tolerance to imazapic is excellent (Faircloth and Prostko 2010; Richburg et al. 1994; Warren and Coble 1999; Wilcut et al. 1996). Following application of paraquat, peanut foliage becomes stunted and necrotic. Peanut tolerance to paraquat or in mixture with bentazon and/or acifluorfen has been thoroughly studied (Carley et al. 2009; Grichar and Dotray 2012; Johnson et al. 1993; Knauft et al. 1990; Tubbs et al. 2010; Wehtje et al. 1991; Wilcut and Swann 1990; Wilcut et al. 1994). Bentazon tank-mixed with paraquat has been documented to reduce paraquat injury in peanut (Wehtje et al. 1992). The addition of S-metolachlor to paraquat systems has shown to increase peanut stunting (Grichar and Dotray 2012). However, peanut yield loss has only been documented following a 3x rate of S-metolachlor (Grichar et al. 1996). Limited information is available regarding peanut tolerance to pyroxasulfone applied POST. Therefore, the objectives of this research were to determine the influence of pyroxasulfone applied POST from emergence to podset and to evaluate peanut response to pyroxasulfone applied POST with and without herbicide tank-mix partners.

Field experiments were conducted from 2009 through 2011 at the University of Georgia Ponder Research Station near Ty Ty, GA on a Tifton loamy sand (fine-loamy, kaolinitic, thermic Plinthic Kandiudults) with 93% sand, 2% silt, 4% clay, 1% organic matter, and pH 6.0. The cultivar ‘Georgia-06G’ was planted in freshly tilled seed beds at a rate of 13 seed/m of row, in twin rows spaced 23 cm apart on a 91 cm center. Production, irrigation, and pest management practices other than specific treatments were held constant over the entire experiment to optimize peanut growth and development (Anonymous 2013b). Plots were maintained weed-free throughout the season using a tank-mixture of commonly applied PRE herbicides [pendimethalin (1 kg ai/ha) plus diclosulam (25 g ai/ha) plus flumioxazin (105 g ai/ha)] in combination with cultivation between plots and hand-weeding. All treatments were applied using a CO₂-pressurized backpack sprayer calibrated to deliver 140 L/ha at 275 kPa with 11002DG nozzle tips. At maturity, peanut were inverted and harvested using commercial equipment. Peanut yields were adjusted to 10% moisture. Data were subjected to ANOVA using the PROC MIXED procedure in SAS (SAS Institute Inc., Cary, NC 27513) with years and replications as random effects. Means of significant main effects and interactions were separated using Fisher’s Protected LSD test at P ≤ 0.05.

A field experiment was conducted twice during 2010 and 2011. Herbicide treatments were arranged in a factorial treatment arrangement including three pyroxasulfone rates (0, 240, and 480 g ai/ha) and four POST application timings [10, 30, 60, and 90 days after planting (DAP)]. The experimental design was a randomized complete block with each treatment replicated 4 times. Treatments applied at 10, 30 60 and 90 DAP were applied at the V2 to V3, R1, R3 and R6 stages, respectively, as described by Boote et al. (1982). Visual estimates of peanut stunting were made 2 and 8 WAT using a scale of 0 to 100% where 0 = no stunting and 100 = complete plant death.

Two field experiments were conducted during 2009 and 2010. Herbicide treatments were arranged in a factorial arrangement that included two pyroxasulfone rates (0 and 240 g ai/ha) and six POST herbicide systems [none; paraquat (140 g ai/ha); paraquat (210 g ai/ha) plus bentazon (280 g ai/ha); paraquat (210 g ai/ha) plus bentazon (560 g ai/ha) plus acifluorfen (280 g ai/ha); imazapic (70 g ai/ha); and lactofen (220 g ai/ha)]. All herbicide treatments included non-ionic surfactant (80/20) at 0.25% v/v. Paraquat rate was increased from 140 g ai/ha to 210 g ai/ha when mixed with products containing bentazon due to antagonism (Wehtje et al. 1992). The experimental design was a randomized complete block with each treatment replicated 4 times. Treatments were applied 10 DAP to peanuts 5 to 10 cm in height at growth stages V4 to V5 (Boote 1982). Visual estimates of peanut stunting were made 2 and 9 WAT using methods previously discussed.

Peanut stunting (2 and 9 WAT) and yield were not influenced by the interaction of pyroxasulfone rate, application timing, and experiment. Peanut stunting 2 WAT was influenced by the interaction of pyroxasulfone rate and application timing. Pyroxasulfone applied at 240 g ai/ha 10 DAP caused 24% stunting 2 WAT (Table 1). Increasing the pyroxasulfone rate to 480 g ai/ha increased stunting to 33%. Regardless of pyroxasulfone rate, peanut stunting 2 WAT following pyroxasulfone applied 30, 60, or 90 DAP was less than 3%. By 8 WAT, peanut stunting was minimal, ranging from 0 to 3%. Peanut yield was not influenced by pyroxasulfone rate or application timing (Tables 2 and 3). These results are similar to Prostko et al. (2011), who reported excellent peanut tolerance to pyroxasulfone applied 44 to 51 days after emergence. These data provide evidence that pyroxasulfone may be applied throughout the peanut growing season with little concern of negative yield effects. However, application of pyroxasulfone 10 DAP may result in stunting following higher use rates.

Peanut stunting (2 and 9 WAT) and yield were not influenced by the interaction of pyroxasulfone rate, foliar herbicide system, and experiment. Peanut stunting 2 and 9 WAT was influenced by the main effect of pyroxasulfone rate. Treatments that included pyroxasulfone caused greater peanut stunting 2 and 9 WAT. When pooled over foliar herbicide systems and locations, peanut stunting 2 WAT was 22% without pyroxasulfone (Table 4). The addition of pyroxasulfone to foliar herbicide systems increased peanut stunting to 26%. By 9 WAT, peanut stunting with and without pyroxasulfone was 5 and 2%, respectively. Although the addition of pyroxasulfone to weed management systems increased peanut stunting throughout the season, peanut yield was not reduced.

Peanut stunting (2 and 9 WAT) and yield were influenced by the main effect of foliar herbicide systems. Peanut treated with systems that included paraquat were stunted 33 to 37% 2 WAT, while less severe injury was observed following treatment with lactofen or imazapic (Table 5). By 9 WAT, stunting ranged from 3 to 6% regardless of foliar herbicide system. Similar peanut tolerance has been observed when paraquat was applied in combination with other residual herbicides (Carley et al. 2009; Grichar and Dotray 2012).

Peanut yield was not reduced using common foliar herbicide systems when compared to systems that did not include foliar herbicides (Table 5). However, peanut yield was reduced in systems using paraquat plus bentazon plus acifluorfen (6,565 kg/ha) when compared to imazapic alone (7,255 kg/ha). Paraquat may cause injury that results in yield loss; however, yield loss is sporadic and rare (Grichar and Dotray 2012; Knauft et al. 1990; Wilcut and Swann 1990; Wilcut et al. 1994). Paraquat reduced yield of the runner type peanut cultivar ̀York̀ following application 21 days after cracking while application 7, 14, or 28 days after cracking did not reduce yield (Grichar and Dotray 2012). Lactofen may also cause leaf necrosis and bronzing in peanut; however, this injury does not result in yield loss (Dotray et al. 2012; Ferrell et al. 2013; Grichar and Dotray 2011; Wilcut et al. 1990).

Results from these field studies suggest potential POST uses for pyroxasulfone in peanut. While significant stunting occurred when pyroxasulfone alone was applied 10 DAP, peanut recovered and yield was not reduced. Pyroxasulfone tank-mixed with POST herbicides increased peanut stunting; however, yield was not reduced. Future research should focus on weed management using pyroxasulfone in peanut and determining weed species sensitivity to projected use rates of 60 to 120 g ai/ha.