Over the past decade, the major emphasis of peanut breeding programs in the U.S has been on the development of cultivars resistant to tomato spotted wilt virus (TSWV) (Culbreath et al., 2003). Although these new varieties have outstanding resistance to TSWV and other diseases, some may have characteristics, such as reduced germination or poor seed vigor (Morton et al., 2008). In recent years, the cultivars DP-1 (Gorbet and Tillman, 2008) and Georgia-01R (Branch, 2002) had great potential due to their multiple disease resistances but were not widely planted by growers due to poor seed germination and vigor issues (Beasley, 2007).

Peanut seed quality and germination are influenced by many factors including water and calcium availability during pod fill, maturity at harvest, curing and storage at the recommended moisture and temperature levels, and shelling and conditioning with minimal mechanical damage (Guerke, 2005). Because of the recent problems with the vigor and germination of new cultivars, questions have been raised about the potential of peanut herbicides to have an influence on seed germination.

Numerous studies have been conducted on the effects of herbicides on soybean seed. Early research indicated that herbicides do not significantly alter oil content and cause only relatively small changes in fatty acid composition of soybean seed (Penner and Meggitt, 1970). Subsequent research confirmed that herbicides do not cause important alterations in soybean seed constituents (Stoller et al., 1973). More recently, Bradley et al. (2002) found that soybean herbicides had little or no effect on seed germination, protein, and oil content. However, herbicides used as preharvest desiccants, such as glyphosate, paraquat, and sodium chlorate, applied before soybean maturity (R7) have the potential to reduce soybean seed germination (Bennett and Shaw, 2000).

Twelve herbicide active ingredients are labeled for postemergence use in peanut, but only imazapic, imazethapyr, 2,4-DB, and chlorimuron are highly systemic in the peanut plant (Senseman, 2007) and thus have potential to influence seed characteristics. Of these four herbicides, only imazapic and 2,4-DB are used over significant acreages (NASS, 2009). The 2,4-DB molecule is not readily absorbed or phytotoxic, but the secondary metabolite (2,4-D) resulting from beta oxidation is both mobile and phytotoxic (Wathana et al., 1972; Wehtje et al., 1992) in many species. Much research can be found that documents peanut plant injury and/or yield response to 2,4-DB and imazapic (Brecke and Stephenson, 2006; Grey and Wehtje, 2005; Grichar, 2007; Hicks et al., 1998; Wehtje et al., 1993; York et al., 1993), however, little research has been reported on the effects that herbicides may have on seed quality. Lancaster et al. (2005) reported that 2,4-DB reduced seed germination of sickelpod (Senna obtusifolia L. Irwin and Barneby) 30 to 40% when applied just prior to flowering or during peak flowering. Ketchersid et al. (1978) studied the effects of 2,4-DB in peanut and noted enlarged pods with less total sound mature kernels (TSMK), but no yield reduction. Therefore, the objective of this research was to evaluate the effect of imazapic and 2,4-DB on peanut yield, grade, and subsequent seed properties (germination and vigor). TSWV incidence on peanut was also evaluated.

Field studies were conducted in 2005 and 2006 at the Ponder Farm near Tifton, GA and at the Bolton Research Farm near Dawson, GA. Soils were Tifton loamy sand (fine-loamy, kaolinitic, thermic Plinthic Kandiudults) and Red Bay sand (fine-loamy, kaolinitic, thermic Rhodic Kandiudults) at Tifton and Dawson, respectively. Organic matter ranged from 1.0 to 1.1% with pH from 6.0 to 6.5 at Tifton and 1.0% organic matter and pH 6.2 at Dawson.

Peanut was sown in twin rows, spaced 90 cm apart at both locations on conventionally prepared seedbeds. Distance between paired twin rows was 23 cm. Experimental plot size in Tifton was two twin rows by 7.6 m in length, while Dawson plots were six twin rows by 9 m in length. Planting dates were May 9, 2005 at both Tifton and Dawson, and May 10 and May 16, 2006 for Tifton and Dawson, respectively. Rainfall was supplemented with overhead linear irrigation at Tifton, while surface drip irrigation was used at Dawson. Pendimethalin or ethalfluralin plus diclosulam was applied preemergence to the entire test area at both locations to suppress grass and broadleaf weeds. Weed-free conditions were maintained at both locations through mechanical cultivation and hand-weeding. Clethodim was applied for postemergence control of annual and perennial grasses that escaped preemergence herbicides and cultivation. Standard cultural practices for peanut production were followed using University of Georgia Extension recommendations.

The treatments were arranged in a split-plot design with four replications at each location. Peanut cultivars (main plots) were Georgia Green (Branch, 1996), Georgia-01R, and C-99R (Gorbet and Shokes, 2002). Herbicides (sub-plots) were imazapic at 71 g ai ha⁻¹ applied at 30 or 45 d after planting (DAP); 2,4-DB at 270 g ai ha⁻¹ applied at 75 or 90 DAP; and a non-treated control. All herbicides were applied with CO₂-pressurized spray equipment calibrated to deliver 140 L ha⁻¹ total spray volume and included crop oil concentrate at 1% v/v.

Prior to digging, peanut was visually rated for incidence of TSWV. The number of 31 cm sections (a single pair of twin rows) of row containing symptomatic plants was counted for each experimental plot according to Culbreath et al. (1997). Data were transformed into percentage based on total row length and reported as percent infection. Maturity was determined using the hull scrape method (Williams and Drexler, 1981) for each variety and plots were mechanically dug and inverted accordingly. The cultivar Georgia Green was dug approx. 2 wk prior to either Georgia-01R or C-99R at both locations in both years. Plots were mechanically harvested using a stationary plot thresher to ensure sample integrity. Yields were transformed to kg ha⁻¹ at 10% moisture. A 500-g sample was collected randomly using a riffle divider then shelled to obtain medium-sized seed. Medium-sized seed were treated with commercial seed treatment fungicide3 and sent to the Georgia Department of Agriculture Seed Lab in Tifton, GA for germination analysis. Standard (25 C) and cold (15 C) germination tests were conducted. The cold germination test provides some indication of peanut seedling vigor (Guerke, 2005). A 1500-g sample was collected similarly and analyzed for grade, including total sound mature kernels (TSMK), sound splits, and damaged kernels4.

Yield, grade, TSWV incidence, standard germination and cold germination data were subjected to Mixed Models analysis of variance5. Year and year nested within location were used as random effects. When main effects or interactions were significant (P ≤ 0.05), Duncan's MRT test was used for mean separation. All percentage data were arcsine transformed prior to analysis. Data transformation did not improve homogeneity of variance, thus actual percentage data are presented.

Despite notable differences in cultivar response to growing conditions, neither imazapic nor 2,4-DB influenced peanut performance. Cultivar differences were the dominant effect in this series of studies as opposed to herbicides. As peanut germination and stand establishment concerns remain high, producers and researchers should focus on other production practices or post-harvest mechanisms that may be decreasing peanut performance rather than the negligible impacts of herbicides. Georgia-01R had reduced seed germination when compared to the other cultivars in this study. This would help explain the establishment problems that have been observed when this cultivar is planted.