Various factors including environmental conditions, seeding rate, plant growth regulators, and genetics dictate peanut growth and development. Virginia-type peanuts frequently produce vegetative growth in excess of what is required to maximize pod yield (Mitchem et al., 1996). Excessive vegetative growth can increase disease incidence (Phipps, 1995) and mechanical damage to vines associated with mid- and late-season pesticide applications, and reduce pesticide penetration in the bottom region of the canopy (Bauman and Norden, 1971; Wu and Santleman, 1977; Henning et al., 1982). Row visibility at harvest can also be reduced by excessive vegetative growth, adversely affecting harvest efficiency and yield (Beasley, 1970; Culpepper et al., 1977; Jordan et al., 2001).

Agrichemicals with plant growth regulator activity have been examined in peanut for vegetative growth suppression. Foliar applications of daminozide resulted in enhanced row visibility although yield response was inconsistent (Brown and Ethredge, 1974; Mozingo and Steele, 1984; and Hodges and Perry, 1970). This product is no longer commercially available as a plant growth regulator in peanut. Wu and Santleman (1977) reduced vine growth with several plant growth regulators in spanish-type peanuts and observed no significant yield enhancements. Bauman and Norden (1971) examined the effects of applying daminozide and TIBA (2,3,5,-triiodobenzoic acid) to peanut cultivars displaying various growth habits and reported reduced vegetative growth in some cultivars but no increases in yield. Prohexadione calcium is a commercially available plant growth regulator registered for vegetative growth suppression in peanut (Mitchem et al., 1996).

Prohexadione calcium is a plant growth regulator that inhibits gibberellin biosynthesis by blocking kaurene oxidase, and increasing abscissic acid and cytokinin levels in certain plant species (Grossman et al., 1994). It is registered as a plant growth regulator in peanut, apple (Malus spp.) and several grass species and has been reported to reduce vegetative growth in other crops including rice (Oryza sativa L.), tomato (Lycopersicon esculentum Mill.), grain sorghum [Sorghum bicolor (L.) Moench], oilseed rape (Brassica napus L.), and wheat (Triticum aestivum L.) (Byers and Yoder, 1999; Grossman et al., 1994; Lee et al., 1998; Nakayama et al., 1992; Yamaji et al., 1991). In peanut, the registration recommends making one application when 50% of the peanut vines are touching in the row middles (row closure) followed by a second application at 100% row closure. Increases in row visibility (measured by visual observation at harvest) and decreases in mainstem and cotyledonary lateral branch length have been reported following prohexadione calcium applications (Jordan et al., 2001; Beam et al., 2002; Culpepper et al., 1997; Mitchem et al., 1996). Mitchem et al. (1996) reported reduction in mainstem and cotyledonary lateral branch length up to 34 and 32% respectively, following prohexadione calcium applications to virginia-type peanuts.

Yield response of virginia-type peanuts to prohexadione calcium applications in the literature is inconsistent (Jordan et al., 2001; Culpepper et al., 1997; Jordan et al., 2000). Beam et al. (2002) reported increases in combined yield and reductions in pod loss at harvest, but no changes in maximum yield (measured as sum of pods combined and the pods remaining on the soil surface and in soil following harvest). Reports of yield reduction have been isolated to early application timings, specific cultivars, and locations within a study (Mitchem et al., 1996; Culpepper et al., 1997; Jordan et al., 2000).

Prohexadione calcium application can alter the grade and quality of peanut. Increases in extra large kernels (ELK) have been reported following application of prohexadione calcium at 50% row closure followed by a subsequent application several weeks later (Culpepper et al., 1997; Jordan et al., 2001). However, Mitchem et al. (1996) reported decreases in ELK percentage when prohexadione calcium was applied at the early pegging stage. Inconsistent changes in yield and grade may have been influenced by changes in maturity due to prohexadione calcium application. Culpepper et al. (1997) reported prohexadione calcium hastened pod maturity. However, research by Beam et al. (2002) suggested that a higher percentage of brown and black pods using mesocarp pod color determination may have been associated with greater pod retention rather than hastening maturity.

Peanut vine growth and yield responses to prohexadione calcium application can vary by cultivar. Culpepper et al. (1997) examined the response of six Virginia type cultivars to application of prohexadione calcium at row closure followed by one application several weeks later. Row visibility was enhanced by prohexadione application for all cultivars although response was greatest for NC-V 11 and NC 9. Yield response in plots where prohexadione calcium was applied ranged from 16% increase to 8% reduction depending on the cultivar. Beasley et al. (1998) also reported variability across runner-type peanut yield response to prohexadione calcium application.

Peanut is typically produced on single rows spaced 91 to 102 cm apart in the U.S. (Sholar et al., 1995). Early research demonstrated no yield advantage to planting peanut in rows spaced 45 cm versus 90 cm (Hodges and Perry, 1970). Some producers grow peanuts in twin row configuration, consisting of two rows planted on the same bed. Increased yield and reduced infection from tomato spotted wilt tospovirus (TSWV) has been reported in peanuts planted in twin-row relative to single-row configuration (Baldwin et al., 1998). Other researchers have reported no change in yield or grade in peanut planted to twin rows unless plant populations in the twin-rows were higher than single row plantings (Jordan et al., 2001; Mozingo and Coffelt, 1984). Increasing plant density through either increasing seeding rate or planting in twin-rows may impact the amount of above ground vegetative growth present in a peanut field.

While peanut response to prohexadione calcium in single and twin row planting patterns or response to prohexadione calcium by different cultivars have been evaluated, information is not available on the response of virginia-type cultivars planted in varying patterns to prohexadione calcium application. Additionally, past research evaluating response of virginia-type peanut to prohexadione calcium application utilized cultivars that are no longer planted on substantial hectarage. Response of the cultivars Gregory, Perry, VA 98R, and Wilson to prohexadione calcium has not been reported in the literature, and these cultivars comprised 60% of the planted hectarage in 2004 (Virginia Crop Improvement Association, Inc, pers. commun.). Therefore, research was conducted to determine if recently released, commonly grown virginia-type cultivars respond variably to prohexadione application and if planting pattern and plant population together influence response.

The experiment was conducted in Virginia at Tidewater Agricultural Research and Extension Center (TAREC) research farm located in Suffolk during 2001 and 2002. Soil at this location was a Kenansville loamy sand (loamy, siliceous, thermic Arenic Hapludults) with pH 5.4 and 1.2% organic matter (2001) and pH 5.7 and 1.03% organic matter (2002). The cultivars Gregory, Perry, VA 98R, and Wilson were tested in each year. Gregory and Perry display an intermediate growth habit between runner and bunch type while VA 98R and Wilson display runner growth habit (Swann, 2002).

Peanut was planted on 7 May 2001 and 6 May 2002 in conventionally prepared, elevated beds in single rows spaced 91 cm apart at a seeding rate of 145,000 seed/ha (145-single) or in twin rows spaced 18 cm apart on 91 cm centers at seeding rates of 145,000 seed/ha (145-twin) or 218,000 seed/ha (218-twin). Pest control and fertility practices were implemented based on recommendations of Virginia Cooperative Extension. Irrigation was applied in 2002 (2.54 cm on 9 July and 14 August) but not in 2001. Prohexadione calcium was applied with a CO₂-pressurized unicycle sprayer calibrated to deliver 140 L ha^-1 at 221 kPa. Crop oil concentrate (Agri-Dex, Helena Chemical Co., Memphis, TN) at 1.22 L/ha and 30% urea ammonium nitrate solution at 2.44 L/ha were included with prohexadione calcium. In both years, prohexadione calcium was applied at 140 g ai/ha at 50% row closure and 2 wk later. A no prohexadione calcium control was included for each cultivar/planting pattern combination. Fifty-percent row closure was defined when 50% of the vines from adjacent rows were touching and occurred on 13 July 2001 and 29 July 2002 for the twin row planting pattern and 20 July 2001 and 30 July 2002 for single row planting pattern.

The experimental design was a split split plot with four replications. Cultivar was the main plot, prohexadione calcium treatment was the subplot, and planting pattern was the sub subplot. Individual plots were two rows (1.8 m) wide by 12.2 m long. Visual estimates of row visibility were recorded on 28 August 2001 and 27 September 2002 using a scale of 1 to 10 where 1 = flat canopy with vines completely overlapping in the row middles with rows indistinguishable and 10 = triangular-shaped canopy with no vines from adjacent rows touching in the row middles (Mitchem et al., 1996). Both rows of all plots were mechanically dug and inverted on 12 October 2001 and 28 September 2002 and allowed to dry in the field for periods of 5 to 10 d. In the current study, plots were mechanically harvested and dried using conventional dryers. Yields were adjusted to 7% moisture.

Data for row visibility and pod yield were subjected to combined analysis of variance for the two years, four cultivars, two prohexadione calcium application levels, and three planting patterns using the Proc Mixed procedure of SAS software (Table 1) (SAS Institute 1997). All possible interactions were tested and treatment means were separated where appropriate using LSMEANS and the PDIFF option at the P ≤ 0.05 level. Where interactions were significant, contrast statements were used to compare row visibility response to prohexadione calcium for cultivars and planting patterns.

The results of this research demonstrate the potential for enhancing row visibility of virginia-type peanut through the application of prohexadione calcium at 50% row closure followed by an application 2 wk later. Delineating rows at the end of the season can be more difficult in twin-row planted peanut, when cultivars producing a flat canopy with excessive vegetative growth. Furthermore, the response to prohexadione calcium appears to be variable across cultivars, being greatest for those producing a flat canopy with excessive vegetative growth. Similar variation in cultivar response to prohexadione calcium has been previously reported (Culpepper et al., 1997). While row visibility can be improved by prohexadione calcium in either single- or twin-row plantings, twin-row planting patterns may respond to a greater extent.

These results document the potential for yield reduction following prohexadione calcium application in virginia-type peanut irrespective of planting pattern and cultivar. Further research is needed to elucidate the mechanism of yield reductions. In a two-year, 12 location study, Jordan et al. (2000) reported numerical yield reductions of 210 kg/ha in 1997 and 100 kg/ha in 1998 in prohexadione calcium-treated plots in two years at TAREC in Suffolk, Virginia. Data from the fields at this location were not analyzed separate from the other 10 locations, however. As noted by Jordan et al. (2001), digging peanuts in relatively short rows utilized in small-plot research may not adequately account for the yield benefit a producer would derive from enhanced row visibility. Furthermore, yield in the current and previous research may have been confounded where only one digging date was utilized. The number of days needed to reach maturity varies across virginia-type cultivars (Swann, 2002). Although Culpepper et al. (1997) suggested that maturity of pods was hastened by prohexadione calcium, Beam et al. (2002) suggested that higher percentages of brown and black pods following mesocarp color determination may have been associated with greater pod retention following prohexadione calcium. Variations in maturity can result in significant changes in peanut yield and grade (Jordan et al., 1998). More research is needed to further elucidate the mechanisms behind reported variable yield response to prohexadione calcium application.

Based on findings in the current study, there is no yield advantage to altering planting pattern of virginia-type peanut provided seeding rate is similar. However, increasing seeding rate from 145,000 seed/ha to 218,000 seed/ha in twin rows will likely enhance yield of virginia-type peanut.