Peanut (Arachis hypogaea L.) production decreased from 582,000 hectares in 1968 to a forecasted 451,170 hectares in 2011 (Anonymous, 2011a). Peanut yield nearly doubled over this period in part due to use of more effective herbicides, peanut genetics with improved disease resistance and greater yield potential, improved irrigation efficiencies, and more efficient cultural practices. However, weeds continue to be a major problem in all peanut growing regions. Weeds can reduce peanut yield 60 to 80% through competition and reduced harvest efficiency (Buchanan et al., 1982; Wilcut et al., 1995).

Peanut has several unique features that contribute to challenging weed management. First, peanut produced in the United States require a fairly long growing season (140 to 160 d), depending on cultivar and geographical region (Henning et al., 1982; Wilcut et al., 1995). Consequently, soil-applied herbicides do not provide season-long control and mid-to-late season weed problems are common. Secondly, peanut has a prostrate growth habit, a relatively shallow canopy, and is slow to shade inter-rows allowing weeds to be more competitive with the peanut plant (Walker et al., 1989; Wilcut et al., 1995). Additionally, peanut fruit develops underground on pegs originating from branches that grow along the soil surface. This prostrate growth habit and pattern of fruit development restricts cultivation to an early season control option (Brecke and Colvin, 1991; Wilcut et al., 1995). With conventional row spacing (91 to 102 cm), complete ground cover may not be attained until 8 to 10 wk after planting. In some areas of the U.S. peanut growing region, complete canopy closure may never be attained.

Control of annual grasses and small-seeded broadleaf weeds can be achieved with a dinitroaniline herbicide applied preplant incorporated (PPI) (Wilcut et al., 1994a); however, control of larger-seeded weeds such as ivyleaf morningglory (Ipomoea hederacea (L.) Jacq.) must occur with the addition of other herbicides such as imazethapyr and imazapic (Cole et al., 1989, Grey et al., 1995, Grichar et al., 1994, Wilcut et al., 1991a, 1994c, 1995). Imazethapyr applied postemergence (POST) provided broad-spectrum and most consistent control when applied within 10 d of weed emergence (Cole et al., 1989; Grey et al., 1995; Grichar et al., 1992; Wilcut et al. 1991a, b; 1994b, c). Imazapic applied POST controls the same spectrum of weeds as imazethapyr (Nester and Grichar, 1993; Grichar et al., 1994; Wilcut et al., 1993, 1994c, 1995). In addition, imazapic provided control and suppression of Florida beggarweed [Desmodium tortuosum (S.W.) D.C.] and sicklepod [Senna obtusifolia (L.) Irwin & Barneby], which are not adequately controlled by imazethapyr (Grey et al., 2001). The limiting factors on the use of imazethapyr and imazapic are the rotational restriction (18 mo.) to crops such as cotton (Gossypium hirsutum L.) and sorghum (Sorghum bicolor L. Moench) and the potential development of weeds resistant to the ALS-inhibiting class of herbicides (Grey et al., 2005; Matocha et al., 2003; Wilcut et al., 1995; York and Wilcut, 1995).

Lactofen was registered for use POST in peanut in 2005 for control of several annual broadleaf weeds including annual morningglory. Lactofen is classified as a diphenyl ether (cell membrane disruptor) which interferes with protoporphyrinogen IX oxidase synthesis and causes accumulation of protoporphyrin IX (Duke et al., 1991). Protoporphyrinogen IX is a potent photosensitizer that generates high levels of singlet oxygen in the presence of molecular oxygen and light, leading to light-induced oxidative breakdown of cell constituents (Duke et al., 1991). In general, herbicides classified as cell membrane disruptors (contact inhibitors) must be applied to small weeds. In previous weed control studies, lactofen controlled 99% Palmer amaranth (Amaranthus palmeri S. Wats.) in two of three yrs, while controlling this weed 80% in the third yr (Grichar, 1997). Grichar (1994) reported that lactofen at 0.28 kg/ha controlled 92% spiny amaranth (Amaranth spinosus L.) when applied early postemergence (EPOST), but control was less than 80% when applied late postemergence (LPOST); however, Jordan et al., (1993) reported that lactofen applied LPOST controlled prickly sida (Sida spinosa L.) and morningglory species (Ipomoea species) as well as lactofen applied EPOST. Lactofen controlled 86% common lambsquarters (Chenopodium album L.) when applied at ground-crack (GC), but control declined to 34% when applications were delayed 2 wks (Wilcut, 1991). Lactofen controlled 87 to 95% prickly sida and 83 to 86% morningglory species when applied at GC and 2 wks after GC, respectively, but control declined to 0% when applications were made at 4 wks after GC. Wilcut et al. (1990) reported that broadleaf weed control systems containing sequential lactofen systems control common lambsquarters 100% and morningglory species 86%.

Peanut and soybean (Glycine max L.) tolerance to lactofen is based on the plants ability to metabolize the herbicide, which often results in some leaf bronzing and spotting and plant growth is often temporarily reduced (Harris et al., 1991). Since injury following paraquat plus bentazon is often viewed as unacceptable by peanut growers in the southwest, the potential phytotoxicity caused by lactofen may be considered unacceptable as well regardless of its effect on yield and grade. The objective of this research was to evaluate peanut response in weed-free conditions following lactofen applied in single and sequential applications over several environments.

Field experiments were conducted at four locations in 2005 and five locations in 2006. Locations included Lamesa (Texas High Plains, 2005–06), Lockett (Texas Rolling Plains, 2005–06), Yoakum (south Texas, 2005–06), Ty Ty (Tift County, GA, 2005–06), and Plains (Sumter County, GA, 2006). The experimental design was a randomized complete block with three or four replications depending on location. The plot size was two or four rows by 7.6 to 9 m. Lactofen at 220 g ai/ha plus crop oil concentrate7 at 1% v/v was applied to peanut at 6-leaf (lf), 6 lf followed by (fb) 15 days after the initial 6 lf treatment (DAIT), 15 DAIT alone, 6 lf fb 30 DAIT, 30 DAIT alone, 6 lf fb 45 DAIT, 45 DAIT alone, 6 lf fb 60 DAIT, and 60 DAIT alone. Herbicides were applied using water as a carrier with a CO₂-pressurized backpack sprayer that delivered 93 L/ha at 206 kPa using 110015 TurboTee8 flat fan nozzles (Lamesa), 117 L/ha at 177 kPa using 11002 XR flat fan nozzles (Lockett), 187 L/ha at 205 kPa using 11002 Drift Guard flat fan nozzles (Yoakum), 140 L/ha at 237 kPa using 11002 Drift Guard flat fan nozzles (Ty Ty), and 140 L/ha at 170 kPa using 11001 Drift Guard flat fan nozzles (Plains). Planting dates, varieties, spray timings, digging, and harvest dates are shown in Table 1. Peanut injury was estimated visually throughout the season using a scale of 0  =  no peanut injury to 100  =  complete injury or peanut plant death. Experimental sites received a dinitroaniline herbicide applied PPI (all locations), diclosulam PRE (Ty Ty, 2005), diclosulam plus flumioxazin PRE (Ty Ty, 2006), clethodim POST (Ty Ty), cultivation (Lamesa), and hand-weeding at all locations throughout the growing season to maintain weed-free conditions to prevent any stress due to competition. Peanut yield was determined by digging, air-drying in the field for 6 to 10 d, and harvesting individual plots with a mechanical combine. Yield samples were adjusted to 10% moisture. Pod, shell, and peanut kernel weight were determined from each sample. Grades were determined from a 250-g pod sample from each plot following procedures described by the Federal-State Inspection Service (USDA, 1986).

In 9 tests from Georgia and Texas in 2005 and 2006, lactofen caused visible peanut injury at all locations. Mature peanut leaves exposed to lactofen will exhibit bronzing and necrotic lesions, and growth of the next two true leaves may exhibit some cupping and crinkling of leaf margins. Subsequent growth should be normal and plants should quickly outgrow the initial temporary injury conditions. Yield reductions occurred when applications were made at 45 DAIT, which corresponded to beginning to full seed fill. No grade reductions were apparent in any of these studies. Thus, growers who apply lactofen according with manufacturer's label recommendations and prior to seed fill should have confidence that yields will not be negatively impacted despite dramatic above-ground injury symptoms; however, applications made during seed fill may adversely affect yield.