Peanut is a valuable commodity in the United States with approximately 634,800 ha harvested with an estimated value of $1.19 billion (USDA 2016a; USDA 2016b). Weeds compete with peanut for sunlight, moisture, and nutrients throughout the growing season (Wilcut et al., 1994), and negatively affect yield, quality, and economic value (Everman et al., 2008; Walker et al., 1989). Season-long interference from combinations of broadleaf and grass weeds can reduce peanut yield by 60 to 80% and can decrease harvest efficiency in some instances (Everman et al., 2008; Wilcut et al., 1994). In studies investigating the effect of season-long interference from individual weed species, common ragweed (Ambrosia artemisiifolia L.) (Clewis et al., 2001) and Palmer amaranth [Amaranthus palmeri (S.) Wats] (Burke et al., 2007) at a density of 1 plant/m of row, resulted in peanut yield losses of 40 and 28%, respectively. Competition from Texas millet [Urochloa texana (Buckl.)] and Florida beggarweed [Desmodium tortuosum (Sw.) DC.] can also severely reduce peanut yield (Buchanan et al., 1976; Cardina and Brecke, 1991; Wilcut, et al., 1987). The prostrate growth habit of peanut causes peanut to be vulnerable to interference from weeds throughout the season and requires effective season-long management to protect yield and promote efficient digging and vine inversion (Walker et al., 1989; Wilcut et al., 1994).

S-metolachlor is a chloroacetamide herbicide, labeled for either pre-plant incorporated (PPI), preemergence (PRE), postemergence (POST), or lay-by application in peanut (Anonymous, 2017a). Peanut stunting and delayed emergence with S-metolachlor has been noted in previous studies (Cardina and Swann, 1988; Clewis et al., 2007; Grichar and Dotray, 2012). Grichar and Dotray (2012) reported peanut stunting from 0 to 15% with S-metolachlor alone and in combination with paraquat, and stunting increased as application timing was delayed to 28 d compared to 7 d after peanut cracking. Peanut injury by S-metolachlor and the racemic form of metolachlor is dependent on combinations of factors, including herbicide rate, moisture conditions at planting, soil temperature, and rainfall (Cardina and Swann, 1988; Grichar et al., 2004). Cardina and Swann (1988) reported that peanut growth suppression was related to higher metolachlor rates followed by irrigation after planting. Although some level of injury was reported in peanut from metolachlor, negative effects were not observed on peanut market grades and yield unless rates higher than those recommended by the manufacturer were applied. S-metolachlor provides control of a number of troublesome annual broadleaf weeds and yellow nutsedge when applied alone or in combination with sulfentrazone, diclosulam, or flumioxazin (Clewis et al., 2007; Grichar et al., 2008).

Acetochlor is registered for use in field corn (Zea mays L.), cotton (Gossypium hirsutum L.), sorghum [Sorghum bicolor (L.) Moench.], and soybean [Glycine max (L.) Merr.] to control annual grasses and small-seeded broadleaf weeds (Anonymous, 2017b; Cahoon et al. 2015; Geier et al., 2009; Steckel et al., 2002). Acetochlor inhibits geranylgeranyl pyrophosphate cyclization enzymes, which are part of the gibberellin biosynthetic pathway and controls weeds by inhibiting growth of seedling shoots (Shaner, 2014). A microencapsulated (ME) formulation of acetochlor recently received registration for PPI, PRE, and POST application in peanut (Anonymous 2017b). In addition to improving crop safety, this formulation of acetochlor extends herbicide persistence in the soil compared with the emulsifiable concentrate formulation (Parker et al., 2005; Scher et al., 1998). Grichar et al. (2015) reported no negative effect of acetochlor ME application timing and rate on peanut grade and yield in runner and Spanish market type cultivars. Cahoon et al. (2015) documented that acetochlor ME applied PRE caused less than 8% early season injury to cotton and at least 90% control of Palmer amaranth. Early season injury caused by acetochlor was transient and did not negatively impact cotton yield.

One effective approach to manage weeds with long periods of emergence is to use sequential applications of PRE and POST herbicides. Grichar et al. (2008) reported > 80% yellow nutsedge (Cyperus esculentus L.) control at five of six locations in peanut with sequential application of diclosulam PRE followed by S-metolachlor applied POST. Control of Palmer amaranth, common lambsquarters (Chenopodium album L.), and annual morningglory (Ipomoea spp.) in peanut was improved when a POST application of imazapic plus 2,4-DB followed PRE S-metolachlor alone or in combination with diclosulam, flumioxazin, or sulfentrazone, compared with PRE- or POST-only treatments (Clewis et al. 2007). Grichar et al. (2015) reported that the addition of pendimethalin and lactofen to acetochlor, flumioxazin, or S-metolachlor programs improved weed control in peanut.

Peanut producers typically include PPI, PRE, EPOST, and POST herbicide applications in their weed management programs for effective season-long control of nutsedge, annual grass, and broadleaf weeds. Dinitroaniline herbicides, such as ethalfluralin, pendimethalin, or trifluralin, are applied PPI to control many annual grass and small-seeded broadleaf weeds (Grichar and Dotray, 2012; Wilcut et al., 1994). Diclosulam, S-metolachlor, flumioxazin, and sulfentrazone are registered PRE (Clewis et al., 2007; Jordan, 2016). In addition to these herbicides, acifluorfen, bentazon, clethodim, imazapic, imazethapyr, paraquat, sethoxydim, and 2,4-DB are used in peanut for POST weed control (Grey et al., 2003; Wilcut et al., 1994). Herbicide resistance in weeds has increased concerns about proper herbicide stewardship (Wise et al., 2009). The registration of acetochlor will increase herbicide options for weed control in peanut. Considering the need for soil-applied residual herbicides to manage weeds and the limited published information on acetochlor tolerance to peanut, field research was conducted in Georgia and North Carolina to define utility of acetochlor compared with S-metolachlor. The specific objectives of this research were: 1) to determine peanut response to the ME formulation of acetochlor with S-metolachlor when applied at different rates and timings under weed-free conditions and 2) to compare weed control with the ME formulation of acetochlor to control by S-metolachlor alone or in various herbicide programs which includes combinations with acifluorfen, bentazon, flumioxazin, imazapic, lactofen, paraquat, and pendimethalin.

Field experiments were conducted in Georgia at the Ponder Research Station near Ty Ty, the Attapulgus Research and Education Center near Attapulgus, the Southwest Georgia Research and Education Center near Plains, and in North Carolina at the Peanut Belt Research Station located near Lewiston-Woodville, and the Upper Coastal Plain Research Station near Rocky Mount during 2011 and 2012. Soil was a Tifton loamy sand soil (fine-loamy, kaolinitic, thermic Plinthic Kandiudult) with 1.07 to 1.39% organic matter (OM) and pH 6.0 at Ty Ty, an Orangeburg loamy sand soil (Fine-loamy, kaolinitic, thermic Typic Kandiudult) with 1.07 to 1.19% OM and pH 6.0 at Attapulgus, and a Greenville sandy clay loam soil (Fine, kaolinitic, thermic Rhodic Kandiudult) with 1.0% OM and pH 5.9 to 6.1 at Plains. Soil at Lewiston-Woodville was a Norfolk sandy loam soil (fine-loamy, siliceous, thermic, Aquic Paleudult) with 0.8 to 1.2% OM and pH 5.9. Goldsboro loamy sand soil (fine loamy, mixed, semiactive, thermic, Typic Hapludult) with 1.4% organic matter and pH 6.1 was at Rocky Mount. Soils at these locations are representative of the southeastern United States peanut production region. Experiments were classified as specific combinations of year and/or location within each state. Experiments were conducted in conventionally-prepared, raised seedbeds. Plot size was 4 rows spaced 96-cm apart by 9 m in North Carolina and 2 rows spaced 91-cm apart by 7.6 m in Georgia. Peanut was planted at a depth of 4 to 5 cm at all locations at a rate designed to achieve final in-row populations of 4 and 6 plants/m in North Carolina and Georgia, respectively. Production, irrigation, and pest management practices other than specific treatments were standard for peanut production in Georgia and North Carolina to optimize peanut growth and development (Anonymous, 2017c; Jordan et al., 2016).

Results from this research demonstrated that acetochlor exhibited similar phytotoxicity to peanut as S-metolachlor, a herbicide currently used in peanut production. This herbicide will provide growers with another option in their battle against hard-to-control weeds. In most instances, acetochlor or S-metolachlor are not stand-alone herbicides and should be included in a systems approach for the most effective weed control. The addition of herbicides with multiple sites of action in weed management program not only provided increased control of weeds in peanut but may also provide optimum resistance management (Heap, 2017).