Increasing pod yield while decreasing input costs is of major importance to peanut (Arachis hypogaea L.) growers. Past research has shown that planting peanut in twin versus single rows can increase pod yield by as much as 450 kg/ha and total sound mature kernels (TSMK) by one to two percentage points (Beasley et al., 2000; Baldwin et al., 2000, Beasley et al., 2004). In addition to yield and grade increases, the twin-row pattern has shown reductions in the incidence of tomato spotted wilt virus (TSWV). Though the relationship between reduction in TSWV and twin-row patterns is not fully understood, planting in twin-rows has become a standard recommendation to reduce the risk of TSWV incidence (Brown et al., 2002a, b).

Roy et al. (1980) showed that seeding rates for a final stand count of between 180,000 and 300,000 plants/ha produced higher pod yields than did lower or higher plant populations. Mozingo and Cofelt (1984) showed that Virginia type peanut (cv. VA81B) had higher yield when planted in twin rows at high plant populations compared with Florigiant variety. Mozingo and Wright (1994) showed that planting bunch type variety on a small diamond shaped pattern (15.2 × 15.2 cm) had higher yields than a runner type variety. Humphrey and Schupp (2000) reported reduced plant competition for water, nutrients, and light stemming from population reductions that permitted more plant energy to be diverted from survival and maintenance mechanisms to reproductive functions. Sternitzke et al. (2000) reported reduced plant populations decreased yield but increased peanut pod mass per plant. Analogous observations have been reported with other commodities. Granberry et al. (1999) reported decreasing population tended to increase fruit size for watermelon (Citrullus lanatus L.), cantaloupe (Cucumis melo reticulatus L.), cabbage (Brassica oleracea capitata L.), broccoli (Brassica rapa L.), and sweet corn (Zea mays rugosa L.). Reiners and Riggs (1999) reported sparser populations' decreased yield but increased pumpkin (Cucurbita pepo L.) size. Bakelana and Regnier (1991) reported domestic oat (Avena sativa L.) dry matter, leaf area, and tiller number per plant increased with decreasing population. Zadeh and Mirlohi (1998) reported reduced rice (Oryza sativa L.) population's decreased yield but increased grain mass per plant.

Producers in the Southeast normally plant 19.7 seed/m runner-type peanuts in single rows on 0.91 m raised beds (Wehtje et al., 1994). The relatively high seeding rate is a hedge against poor germination and emergence in the hope of attaining a stand of approximately 13.1 plants/ m or 144,000 plants/ha (Baldwin, 1997). In addition to better yield, closer spacing and higher population benefits include: a) enhanced weed suppression (Hauser and Buchanan, 1981; Buchanan et al., 1982), b) faster canopy coverage (Mozingo and Wright, 1994), and c) reduced incidence and severity of tomato spotted wilt virus (Brown et al., 1997; Brown et al., 2001; Brown et al., 2002a, b).

Decreasing inter-row plant spacing and increasing plant populations can increase yield. However, the cost of peanut seed is one of the major expenses to the grower. Recommendations suggest that plant populations greater than 13 plants/m of row would reduce the risk of TSWV while achieving high yields (Brown et al., 2002a, b). With the twin-row pattern that would be about 6.5 plants/m in each of the twin-rows, effectively spreading the plants out and allowing more space between plants, i.e. less plant competition. The TSWV Index was validated on non-irrigated and overhead sprinkler irrigation type systems. There has been little peanut yield data collected, especially concerning twin- and single-row patterns, on sites where SDI was used as the primary source of irrigation. Therefore, the objectives of this project were to determine the yield and grade response of peanut when planted in two row patterns (single and twin) at two plant populations (recommended, 1.0R and half recommended, 0.5R) and irrigated with subsurface drip irrigation.

One research site was installed 3 km north of Sasser, GA (Sasser) on Tifton loamy sand (fine-loamy, kaolinitic, thermic Plinthic Kandiudults) with 2 to 5% slope. A subsurface drip irrigation system (SDI) was installed in the spring of 2000. The SDI system had drip laterals buried at 0.3 m deep and spaced at 0.91m with emitters spaced at 0.3 m. Water flow rate was 5.6 L/m per 100 m or 1.0 L/hr per emitter. Peanut was planted during the 2001 and 2002 growing seasons following cotton.

The second site was installed 1 km south of Shellman, GA (Shellman) on a Greenville fine sandy loam (fine, kaolinitic, thermic Rhodic Kandiudults) with 1% slope. A subsurface drip irrigation system (SDI) was installed in the spring of 2001. The SDI system had drip laterals buried at 0.3 m deep and spaced at 0.91m with emitters spaced at 0.45 m. Water flow rate was 5.2 L/min per 100 m or 1.5 L/hr per emitter. Peanut was planted during the 2001 and 2002 growing seasons following cotton.

The experiment was a randomized complete block with two row patterns and two seeding rates replicated three times per treatment. Seeds were planted at the 20 seeds/m (1.0R) rate and at the 10 seeds/m (0.5R) rate. The higher seeding rate is the recommended rate (1.0R) for reducing the risk of TSWV with the final stand counts of 14.4 plants/m (Brown et al, 2002a, b). The lower seeding rate was half the recommended rate (0.5R).

Land preparation was the same for both sites and both years with disk harrowing two times followed by an experimental bedder (USDA-ARS-National Peanut Research Laboratory, Dawson, GA) used to make 1.83 m wide beds. The peanut cultivar ‘Georgia Green’ was planted both years with planters centered on the 1.83-m beds. The single and twin-row patterns were planted with a commercial vacuum type planter (Monosem planter, ATI Inc, Lenexa, KS). The single row planter placed seeds at 0.91 m row spacing for two rows on a bed. Twin-row pattern was planted at 1.17 m between the outside rows and 0.7 m between the inside rows with 0.22 m between the twin-rows with four rows on a bed. Pest management practices followed University of Georgia Agricultural Extension Service recommendations for peanut production (Harris, 2002). Irrigation water was applied daily based on replacement of crop water use for peanut described by Stansell et al. (1976) except when precipitation amounts exceeded estimated water use. If precipitation amounts were greater than 12.5 mm, an irrigation event would not occur for about 3 days to allow the soil to drain.

Crop maturity was determined by the hull scrape method (Williams and Drexler, 1981). Yield rows were dug with a 2-row inverter and combined with a 2-row combine. All plots were dug on the same day and harvested when conditions were acceptable. Yield samples were dried to 10% moisture content (wet basis) or less, weighed, and final yields adjusted to 7% moisture. Kernel size distribution was determined using screens specified in USDA grading procedures (USDA, 1993). Market value of peanut ($/ha) was calculated using the 2002 marketing loan rate ($0.432/kg) for farmer stock grade and pod yield. Seed cost ($/ha) was estimated by multiplying the seeding rate of 1.0R (130.3 kg/ha) and 0.5R (65.1 kg/ha) by a seed cost of $1.17/kg.

Yield data were analyzed by year, location, planting pattern, and seeding rate using the general AOV/AVOC procedures in Statistix8 (Analytical Software, 2003). Mean separation test (Tukey) was used to show differences among means (P  =  0.05) when ANOVA F-test showed significance.

Plant date, harvest date, season precipitation, and irrigation amounts are shown in Table 1. The planting date in 2001 was late at the Shellman site due to delayed irrigation system installation. However, the planting date was within the recommended planting period of 01 to 25 May (Baldwin, 1997). Also, irrigation applied in 2001 at the Shellman site was higher than recommended due to programming malfunctions that occurred with the electronic controller.

Final plant population for the two seeding rates was 9.3 and 15.2 plants/m for the 0.5R and 1.0R, respectively (data not shown). The TSWV Index recommends plant populations greater than 12.8 plants/m to reduce the risk of TSWV disease (Brown et al., 2002a, b). The 1.0R seeding rate exceeded the seeding standard by 18% and the 0.5R seeding rate was 27% less the standard.

This two-year study showed that twin-row planting pattern had 490 kg/ha higher pod yield compared with single row plant pattern when irrigated using subsurface drip irrigation. The twin-row pattern also had a one percentage point increase in grade value (TSMK) compared with the single row pattern. These yield and grade values for single and twin-row patterns correspond to literature values determined using overhead sprinkler. Yield data from this project imply that SDI could be used to irrigate peanut without reduction of yield or grade. There was no difference in kernel size distribution with row pattern. Twin-row pattern had a $213/ha higher market value ($2205/ha) compared with the single row pattern ($1992/ha). There was no difference in market value for seeding rate. This implies that it may be possible for a grower to plant in a twin-row pattern at half the recommended seeding rate without sacrificing net market returns but may increase the risk of loss of yield due to TSWV.