Manganese (Mn) is an essential element needed for peanut (Arachis hypogaea L.) growth and development (Gascho and Davis, 1995) serving as a cofactor in kinase and transferase enzymatic reactions in plants (Horst, 1986). Deficiencies of Mn are often associated with production on high pH soils (Gascho and Davis, 1995) but can be corrected with Mn-containing fertilizers applied topically (Gascho and Davis, 1995).
Large seeded virginia type peanut is the preferred market type grown in the Virginia-Carolina (VC) region. The price of certified seed is approximately $2.05/kg making the cost of $277/ha, a significant input cost when planting the recommended seed density of 140 kg/ha or 200 thousand seeds/ha (Jordan et al., 2018). Research conducted in Virginia, on virginia type peanut, in the 1980's indicated that higher seeding density (215 thousand seeds/ha) produced significantly (P<0.01) higher yield and economic value than lower seeding density (144 thousand seeds/ha) (Mozingo and Coffelt, 1984). Similarly, studies on runner type peanut had positive relationships between seeding density and pod yield. For example, when increasing seeding density from 10 to 20 seeds/m, Sorenson et al. (2004) reported 8.5% pod yield increase and Sconyers et al. (2007) showed 16% higher yields when planting 22.6 seeds/m compared to 12.5 seeds/m. Sarver et al. (2016) reported that increasing seeding density from 3.3 plants/m to 13.1 plants/m increased pod yield from 5200 kg/ha to 6500 kg/ha, and decreased Tomato spotted wilt virus (genus Tospovirus; family Bunyaviridae) (TSWV).
Peanut yield response to seeding density is cultivar dependent in many crops including corn (Zea mays L.) (Nafziger, 1994), soybean [Glycine max (L.) Merr.] (Buerlein, 1988), and peanut (Sullivan, 1991). The new available virginia type cultivars showed improved yield, i.e. in Virginia, average state yield during the 1980's was 2976 kg/ha and during the last decade 4560 kg/ha, (USDA, 2019); and biomass (Simmons, personal communication) than the old cultivars. Therefore, to produce optimally, the new cultivars may require more nutrients and water, which could be supplied at no additional costs by decreasing plant population to make more resources available to individual plants. If newly released cultivars can produce similar yields with less plants per hectare, reducing the seeding density could greatly lower the cost incurred by growers in the Virginia-Carolina region.
Due to the indeterminate growth habit and the effect of weather on plant development, i.e., dry seasons delay while hot summers rush maturity, determining the optimum digging date is essential for maximizing yield, quality, and the economic return. Jordan et al. (2003) showed that digging within the optimum harvest maturity window (Williams and Drexler, 1981) did not affect yield or grade. Literature has consistently reported, however, that digging either too early or too late produced negative effects on peanut yield and quality (Mozingo et al., 1991; Wright and Porter, 1991; Jordan et al., 1998). For example, digging peanut two wk early reduced yield by 15% (Wright and Porter, 1991); and delayed digging caused decrease to both pod yield and gross value, with economic loss as high as $500/ha (Mozingo et al., 1991; Jordan et al., 1998). However, in North Carolina, early maturing cultivars responded differently to digging with some being more stable in terms of yield and economic value over digging dates than others (Jordan et al., 1998). Research is limited with respect to defining response to digging date of more recently released virginia market type cultivars. Generally, virginia market types require 135 to 155 DAP to reach maturity, while runners may need over 155 DAP (Balota et al., 2018).
The objective of this research was to determine the effect of seeding density and digging date on yield, market grade characteristics, and economic return of more recently released virginia market type peanut cultivars.
Materials and Methods
Experimental Site and Design
Field studies were conducted at four site-yr in 2016 and 2017 at the Tidewater Agricultural Research and Extension Center in Suffolk, VA (36.6658280 N, -76.7292940 W), and the Peanut Belt Research Station in Lewiston-Woodville, NC (36.1322040 N, -77.1690820 W). In 2016, experiments at Suffolk were conducted on a Suffolk loamy sand (Fine-loamy, siliceous, semiactive, thermic Typic Hapludults), while soils at Lewiston-Woodville were on Norfolk sandy loam (Fine-loamy, kaolinitic, thermic Typic Kandiudults). In 2017, experiments at Suffolk were conducted on Eunola loamy fine sand (Fine-loamy, siliceous, semiactive, thermic Aquic Hapludults) while at Lewiston-Woodville the experiment was conducted on the same Norfolk sandy loam soil. These soils are representative of soils across the region where peanut is grown. Plot size was 2 rows (91-cm spacing) by 10.7 m in length. Peanut was planted in conventionally-tilled, raised seedbeds in both years of the experiment. Agronomic and pest management practices other than the specific treatments compared in these experiments were administered uniformly across the entire test area based on Cooperative Extension recommendations for North Carolina and Virginia (Balota et al., 2018; Jordan et al., 2018).
The experimental design was a split-factorial plot arranged in a randomized complete block design with four replications. The main plots were the digging dates including early (130 DAP), physiological maturity (140 DAP), and late (150 DAP) digging. Sub-plots consisted of a factorial arrangement of seeding densities (109, 143, 180, and 200 thousand seeds/ha) and cultivars (Bailey, Sullivan, and Wynne). Plots were planted using a two-row Cole planter on May 19 in Suffolk and on May 16 in Lewiston-Woodville in 2016, and on May 8 in Suffolk and on May 18 in Lewiston-Woodville in 2017. All pertinent information on planting and digging dates are provided in Table 1. Air temperature and precipitation were recorded at each site-year within less than a mile from the plots, and used to calculate the growing degree days (GDD); GDD and precipitation are presented in Table 2. Cumulative GDD [GDD= Tavg – Tbase ] was calculated using 13 C as the base temperature, i.e. temperature below which growth ceases. Tavg is the average daily temperature. (Balota et al., 2018).
Yield and Grade Measurements
Harvest was conducted approximately seven to ten d after digging. Pod yield was determined from the plot weight adjusted to 7% moisture and percent foreign material in a 500 g subsample. The same sub-sample was used for grade evaluations. First, fancy pod percentage, pods that do not pass 13.5 mm × 76.2 mm spacing set on the pre-sizer, was determined. Then, pods were shelled and kernels were sorted by size including extra-large kernels (ELK), kernels passing the larger screen but did not pass a 25.4 mm (1-in) × 8.5 mm (21.5/64) screen; mediums, kernels passing the larger screens but did not pass a 25.4 mm × 7.1 mm (18/64-in) screen; and number 1's, kernels passing the larger screens but did not pass a 25.4 mm × 5.9 mm (15/64-in) screen. Lastly, percent of sound mature kernels (SMK) was determined as the sum of ELK, mediums and number 1's. (USDA, 2019). Farmer stock grade characteristics were used to calculate the crop economic value ($/ha) using the USDA Agricultural Marketing Service approach (USDA, 2019).
Statistical and Economic Analysis
The PROC Mixed procedure in SAS (SAS Institute, 2012) was used to determine if there were differences between site-year, seeding density, digging date, cultivar, and their interactions. Block was considered a random effect. When treatment effects were significant (p ≤0.05), predicted means for each treatment were obtained and a post hoc comparison was done. Fischer's protected least significant difference (LSD) was used to separate the means.
To determine which seeding density was the most economical, a cost analysis was performed. Seed weight (kg/ha) was calculated for each individual seeding density. Since there was no interaction with cultivar, seed weight was averaged across the three cultivars. Seed price was $0.38/kg (Jordan et al., 2018). Total cost of seed/ha was calculated using .
Gross return ($/ha) was also calculated for each seeding density using a selling price of $0.09/ha (Jordan et al., 2018).
Economic return for seeding density was calculated by subtracting the total seed cost from the gross return, and it was used to determine which seeding density was most profitable. A sensitivity analysis was performed in which the selling price was increased in increments of 10% until the highest yielding seeding density produced the highest economic return. In addition, we decreased seed cost in increments of 10% until the highest yielding seeding density produced the highest economic return. Finally, the seed cost was dropped simultaneously with increasing price until the highest yielding seeding density produced the highest economic return.
Results and Discussion
Peanut requires at least 600 mm precipitation from planting to physiological maturity (Rowland et al., 2012); and this standard was achieved in both years of this experiment. In 2016, peanuts received 917 mm cumulative precipitation from planting to 130 DAP in mid-Sep and 34% additional precipitation before dig at 150 DAP in mid-Oct, in Suffolk (Table 2). A similar precipitation pattern was recorded at Lewiston and, in both locations, the amount of rainfall received from mid-May to mid-Oct exceeded by 70% the 30-yr average precipitation of 685 mm in Suffolk and 691 mm in Lewiston-Woodville. Year 2017 was, however, close to normal and cumulative precipitation from planting to first dig (130 DAP) was 633 mm in Suffolk and 561 in Lewiston-Woodville; then cumulative precipitation slowly increased at both locations but not more than 644 mm in Suffolk and 604 mm in Lewiston-Woodville in mid-Oct.
Peanut is known to require 2200 GDD from planting to physiological maturity, but new cultivars are increasingly earlier maturing than older cultivars. For example, virginia market type cultivars developed at the turn of the century needed 1800 GDD to mature, in comparison with 2500 GDD needed by cultivars developed prior to that (Balota and Phipps, 2013; Caliskan et al., 2008; Jordan et al., 2018). The cultivars included in this work included Bailey, released in 2008, (Isleib et al., 2011); and Sullivan and Wynne released in 2013 (North Carolina Crop Improvement Association, 2020). They are among the most recent cultivars currently in production in the VC region and have optimum maturity at around 140 DAP (Balota et al., 2018; Jordan et al., 2018). Recorded GDD at 140 DAP was 1578 GDD in average of the two locations in 2016 and 1447 GDD in 2017 (Table 2). Even though more humid than 2017, 2016 was 2 to 3 C warmer in Aug and Sep than same months in 2016. The 30-year GDD average for both locations from 1 May through 30 Oct is 2400 GDD.
Main effect of site-year, cultivar, seeding density and digging date, and the interaction of site-year × digging date × cultivar were significant for pod yield at p<0.05 (Table 3). Because seeding density main effect was significant but none of the interactions of seeding density with the other factors were, the data for seeding density were combined for site-years, cultivars, and digging dates. Unlike in other reports (Sullivan, 1991), the cultivars used in this study responded similarly to the increase of seeding density. The highest seeding density of 200 thousand seeds/ha produced the highest pod yield, 5930 kg/ha (Figure 1). There was no significant difference between the densities of 180 thousand seeds/ha and 143 thousand seeds/ha, as they yielded 5740 kg/ha and 5690 kg/ha, respectively. The 109 thousand seeds/ha density yielded the lowest at 5580 kg/ha. The current seeding density recommendations in Virginia are for 109 thousand to 143 thousand plants/ha (Balota et al., 2018).
At Suffolk in 2016, among the three cultivars, there was no significant difference for pod yield between the 130 and 140 DAP digging dates, i.e. cultivar average was 4900 kg/ha for 130 DAP and 5090 kg/ha for 140 DAP; but the 150 DAP digging date had the lowest pod yield for all cultivars, 3890 kg/ha (Table 4). The cumulative GDD was 1475 GDD at 130 DAP, 1555 GDD at 140 DAP and 1583 GDD at 150 DAP digging dates. Small GDD differences between digging times cannot explain lower yield at 150 DAP compared with earlier digs, but rather heavy precipitation in Oct could have caused yield reduction when waiting 150 days to dig. In 2017, digging at 130 DAP when only 1303 C GDD were accumulated caused significant yield reduction for all cultivars, cultivar average yield was 6030 kg/ha. Instead, pod yield increased significantly when dig was performed at 140 DAP (∼1400 GDD) and even more when digging was performed at 150 DAP with the crop having available 1458 GDD, i.e. cultivar average was 8480 kg/ha when dug at 140 DAP and 9020 kg/ha when dug at 150 DAP. Yield data in both years suggested that optimum digging date for Suffolk, VA, is between 140 and 150 DAP, when a min of 1500 GDD are achieved and in absence of heavy precipitation close to or at digging time.
At Lewiston-Woodville in 2016, the 140 DAP digging date produced significantly greater pod yield for all three varieties than 130 and 150 DAP digging dates (Table 4). The least average yield, 4400 kg/ha, was for the last dig and, as in the case of Suffolk, this was mostly related with the heavy precipitation recorded in October 2016. In 2017, digging at 130 DAP or less than 1400 GDD resulted in yield decrease for all cultivars (Table 4). Cultivar Bailey produced similar pod yield at 140 DAP and 150 DAP digging dates, in average 5890 kg/ha, while for Sullivan and Wynne digging when 1495 GDD were accumulated ensured the highest yields in comparison with digging at 130 DAP or 150 DAP when less than 1400 GDD or more than 1600 GDD were recorded.
Market Grade Characteristics and Gross Economic Value
For grade characteristics, ELK, SMK, and fancy pods, main effects were significant for site-year, cultivar and digging date, but not for the seeding density. With the exception of digging date × cultivar for the ELK and site-year × digging date × cultivar interaction for all grades, the other interactions were not significant (Table 3). This indicates that regardless of cultivar and digging time, seeding density may have no effect on peanut grade; unlike for site-year, cultivar, and digging date that could significantly affect grade, resulting in differences for yield and economic value. For example, Sullivan produced the largest ELK percentage only when pods were dug at 140 DAP or between 1400 and 1600 GDD (Tables 2 and 5). For Bailey and Wynne, extending digging from 140 to 150 DAP did not result in ELK reduction; but digging too early, 130 DAP, produced significantly less ELK for both cultivars regardless the site-year.
Similarly, digging at 140 DAP resulted in the highest SMK for all three varieties at Suffolk in 2016 and Lewiston in 2017; but in other site-years, SMK was cultivar dependent (Table 6). Among cultivars, Bailey was less sensitive to the digging date; unlike for Sullivan and Wynne producing the highest SMK at the 150 DAP dig date. Nonetheless, data in Table 6 suggest that, unless there is a rainy end of the season. SMK production requires a minimum of 1500 GDD and longer time from planting to dig.
While ELK and SMK are major grade factors in calculation of the gross economic return, fancy pod content is not. However, for in-shell product commercialization, fancy pod content is important. Interestingly, and unlike for the ELK and SMK, fancy pods were highest when digging early, at 130 DAP, compared with later digs (Table 7). For this, it seems to be an inverse relationship between kernel and pod size, with immature pods, e.g. pods harvested early, containing higher amounts of water than mature pods when freshly dug. This characteristic is probably not maintained months after harvest and certainly will be lost through pod cooking of the in-shell products.
ANOVA for the economic value followed a similar pattern with pod yield, with main effect of site-year, cultivar, seeding density and digging date, and the interaction of site-year × digging date × cultivar being significant at p<0.05 (Table 3). Results were mixed depending on the site-year and digging date (Table 8); but clearly the rainy year 2016, which affected yield and at some extent grade factors, produced the lower revenue at both sites. In 2017, greatest economic value was for crops dug at 140 and 150 DAP, and this is agreement with yield and SMK observations.
Economic Analysis for Seeding Density
This study showed that the highest seeding density of 200 thousand seeds ha−1 produced the highest yield (Figure 1); and this is consistent with what is documented in the literature (Mozingo and Coffelt, 1984; Sorenson et al., 2004; Sconyers et al., 2007). In order to determine if the 200 thousand seeds/ha density was also the most economical, a cost analysis was performed. Seed weight (kg/ha) was calculated for each individual seeding density and, because there was no interaction between seeding density and cultivar, data were averaged across the three cultivars. Due to increased cost from seed purchase to achieve the highest seeding density, the 200 thousand seeds/ha density did not produce the highest economic return, even though it produced the highest yield (Figure 1). Instead, there was no statistical difference for economic return among the four seeding rates, even though the yields were significantly different (Table 9). The 143 thousand seeds/ha density had the greatest economic return ($2990/ha) numerically, even though it yielded significantly lower than the 200 thousand seeds/ha seeding density (Table 9). This agrees with the current recommendations for peanut seeding density in Virginia (Balota et al., 2018). In order to determine what combination of selling price and seed cost would have ensured the highest economic returned from seeding 200 thousand seeds/ha, a sensitivity analysis was further used. This analysis indicated that either 40% increase in the selling price; 30% decrease in seed cost; or a combination of 20% increase in selling price with a 20% decrease in seed cost could have resulted in the 200 thousand seeds/ha density to have the highest economic return. As such changes in the seed cost and selling price are less probable, the current recommendations for using less seed per hectare at planting, i.e., 143 thousand seeds/ha, seems to be justified for a balanced farm budget.
Summary and Conclusions
Our results indicated that increasing the seeding density from 109 thousand seeds/ha to 200 thousand seeds/ha increased the pod yield across all site-years, similarly for all cultivars. However, the seeding density producing the highest yield did not result in the highest economic return, as the increase in yield was not enough to compensate for the increased seed cost. Instead, the 143 thousand seeds/ha seeding density ensured the greatest economic return for the farmer; this agrees with the current recommendations for peanut production in Virginia. According to our results, the optimal time to dig the Virginia market type peanut cultivars currently grown in the VC region is at 140 DAP. This is because these cultivars appear to need at least 1400 GDD and no more than 1600 cumulative GDD to reach optimum maturity and, therefore, maximum pod yield. However, the decision on when to dig should be monitored on a field-to-field basis as not just temperature, but other factors may affect yield, such as the amount of precipitation at or right before digging.
This work was supported with funding from the National Peanut Board, North Carolina Peanut Growers Association, Virginia Peanut Board, and Virginia Crop Improvement Association. Appreciation is expressed to staff at both research stations for technical assistance with this research.
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