Numerous studies have discussed the negative impact weeds have on peanut production (Buchanan et al. 1982; Cardina and Brecke 1989; York and Coble 1977; Young et al. 1982; Wilcut et al. 1994). Weed control during peanut growth and development is critical to reduce inter-species competition and maintain optimum pod yields (Bridges et al. 1992; Burke et al. 2007; Cardina and Brecke 1989; Walker et al. 1989; York and Coble 1977). Weeds can dramatically inhibit digging and inversion procedures in peanut, leading to harvest losses and harvest inefficiency (Wilcut et al. 1994; Young et al. 1982). Additionally, weeds may serve as hosts for nematodes and diseases (Bird and Hogger 1973; Clewis et al. 2001; Hogger and Bird 1976; Martin 1958).

Growers use a combination of cultural and chemical control tactics to manage weeds; such as, promoting crop health through fertility and crop rotation, as well as preventing weeds from going to seed each year using herbicides or hand-weeding (Buchanan et al. 1982). In Georgia, Palmer amaranth (Amaranthus palmeri S. Wats) is considered the most troublesome weed in peanut production (Webster et al. 2005). Populations of Palmer amaranth exist in Georgia that are resistant to acetyl-lactate synthesis, glycine, and/or triazine herbicides (Heap 2013; Wise et al. 2009). Managing the weed seed bank within cropping systems has become challenging due to increased incidence of herbicide resistant Palmer amaranth. Keeley et al. (1987) reported that a single female Palmer amaranth can produce up to 600,000 seeds. Weed control failures due to resistance can directly influence weed control in subsequent years due to a buildup of weed seed in the soil (Dieleman et al. 1999; Hartzler and Roth 1993; Sparks et al. 2003; Webster et al. 1998).

Weed management in Georgia peanut production commonly requires a combination of residual and postemergence (POST) herbicides to maintain season long weed control and prevent the production of weed seed (Wilcut et al. 1995). Due to fewer hectares of peanut than other row crops, agrichemicals are rarely developed specifically for peanut. Thus, it is critical to evaluate herbicides developed for use in other row crops for their potential use in peanut.

Pyroxasulfone, formerly KIH-485 is a soil applied herbicide labeled for use in corn (Zea mays L.) and soybean (Glycine max L.) for the control of annual broadleaf weeds and grasses. Pyroxasulfone applied at rates between 60 and 180 g ai/ha has been documented to control: Amaranthus spp., Lolium spp., Urochloa spp., goosegrass (Eleusine indica L.), crowfootgrass (Dactyloctenium aegyptium L), and Digitaria spp. (Geier et al. 2006; Hulting et al. 2012; King and Garcia 2008; Koger et al. 2008; Knezevic et al. 2009; Nurse et al. 2011). Pyroxasulfone inhibits very long chain fatty acid synthesis similar to chloroacetamide, oxyacetamide, and tetrazolinone herbicides (Tanetani et al. 2009). Previous research has determined that peanut is adequately tolerant to pyroxasulfone applied 44 to 51 days after emergence (Prostko et al. 2011). Pyroxasulfone applied PRE to peanut has been documented to cause minimal early season stunting but no yield loss (Prostko et al. 2011).

Pyroxasulfone use in corn, cotton (Gossypium hirsutum L.), soybean, and wheat (Triticum aestivum L.) tolerance has been thoroughly described (Cahoon et al. 2012; Eure et al. 2013; Geier et al. 2006; Hulting et al. 2012; King and Garcia 2008; Koger et al. 2008). However, very little is known concerning peanut response to pyroxasulfone following PRE applications. Previous research indicates differential peanut cultivar response to herbicides (Jordan et al. 1998; McLean et al. 1994). Therefore, research was conducted to evaluate peanut cultivar response to pyroxasulfone applied PRE.

Field experiments were conducted during 2012 and 2013 at the University of Georgia Ponder Research Station near Ty Ty, GA on a Tifton loamy sand (fine-loamy, kaolinitic, thermic Plinthic Kandiudults) soil with 93% sand, 3% silt, 4% clay, 1% organic matter, and pH 6.0. Trials were arranged in a split-plot design with main plots consisting of three peanut cultivars [‘Georgia-06G’(Branch 2007a), ‘TifGuard’ (Holbrook et al. 2008), and ‘Georgia Greener’ (Branch 2007b)] and sub-plots consisting of three pyroxasulfone rates (0, 120, or 240 g ai/ha). All treatments were replicated 4 times. Peanut cultivars were planted in freshly tilled seed beds at a rate of 15 plants/m-row, in twin rows spaced 23 cm apart on a 91 cm center. Plots were 1.8 m (two sets of twin rows) wide and 9 m in length. Herbicide treatments were applied immediately following planting using a CO₂-pressurized backpack sprayer calibrated to deliver 140 L/ha at 275 kPa. Immediately following pyroxasulfone application, the trial area was irrigated with 1.3 cm of water using overhead irrigation. Plots were maintained weed-free throughout the season using commonly applied PRE herbicides (pendimethalin plus dimethenamid plus flumioxazin) in combination with cultivation between plots and hand-weeding. Production, irrigation, and pest management practices other than specific treatments were held constant over the entire experiment to optimize peanut growth and development (Anonymous 2013).

Visual estimates of peanut stunting were recorded 10, 80, and 120 d after planting (DAP) using a scale of 0 to 100% where 0 = no stunting and 100 = complete death. Peanut plant density was recorded 20 DAP from 1 meter of twin-rows. Additionally, canopy height was recorded prior to harvest at 120 DAP for 5 plants per plot. To determine treatment effects on peanut maturity, 100 pods per plot were randomly collected immediately after inversion. The hull-scrape method was used to remove the exocarp of the peanut pod. This practice is recommended to determine peanut maturity for harvest timing to ensure optimum peanut pod yield and grade (Johnson 1987; Williams and Drexler 1981). Peanut have indeterminate growth and commonly have pods with varying levels of maturity (Sholar et al. 1995). Peanut pod mesocarp darkens as pods mature. Pods with brown or black mesocarps are mature while pod mesocarps that are white, yellow, or orange in color are immature. The distribution of immature and mature peanut pods can serve as an indicator of delayed maturity caused by cultivar, irrigation, fertility, or herbicide injury (Johnson et al. 1987; Sholar et al. 1995; Mixon and Branch 1985; Mozingo et al. 1991). Once pod mesocarps were removed from each sample, pods were grouped by color: black, brown, and other (white, yellow, orange). These data were combined into two groups; immature (white, yellow, orange) and mature (brown and black).

Peanut were inverted and harvested 3 days later using commercial equipment. Peanut yields were recorded and adjusted to 10% moisture. Data for all parameters were analyzed as a split-plot design and subjected to ANOVA using the PROC MIXED procedure in SAS (SAS Institute Inc., Cary, NC 27513) with peanut cultivar and pyroxasulfone rate as fixed affects and 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.