Introduction
Controlling weeds is critical in agricultural systems, given their potential to reduce yields, crop quality, and harvesting efficiencies (Jones et al. Reference Jones, Walker and Wehtje1997; Smith et al. Reference Smith, Baker and Steele2000; Ward et al. Reference Ward, Webster and Steckel2013). Herbicides are an essential tool for weed control, as they increase farming efficiencies and profits and have also allowed for soil conservation practices (Kudsk and Streibig Reference Kudsk and Streibig2003; Price et al. Reference Price, Balkcom, Culpepper, Kelton, Nichols and Schomberg2011). However, overreliance on herbicides for weed control has led to the evolution of herbicide-resistant weed populations and has threatened some of these beneficial practices (CAST 2012; Kudsk and Streibig Reference Kudsk and Streibig2003; Price et al. Reference Price, Balkcom, Culpepper, Kelton, Nichols and Schomberg2011). Integrated approaches recommended to reduce selection pressure from herbicides for weed control include, but are not limited to, proper herbicide application timing, optimum crop row spacing, cover cropping, crop rotation, and manipulation of seeding rates (Bell et al. Reference Bell, Norsworthy, Scott and Popp2015; Harker Reference Harker2013; Johnson and Hoverstad Reference Johnson and Hoverstad2002; Mischler et al. Reference Mischler, Curran, Duiker and Hyde2010; Vann et al. Reference Vann, Reberg-Horton and Brinton2016).
Increasing crop population has been proven to increase weed control but raises concerns regarding seed costs as a result of technology fees associated with herbicide-resistant crops (Bell et al. Reference Bell, Norsworthy, Scott and Popp2015; Harder et al. Reference Harder, Sprague and Renner2007; Hoffner et al. Reference Hoffner, Jordan, York, Dunphy and Everman2012). In soybean, prior to resistant weed problems, a second application of glyphosate increased the profit margin over increasing seeding rates (Norsworthy and Oliver Reference Norsworthy and Oliver2001). At that time, an investment in a subsequent glyphosate application was more profitable than increasing the soybean population to control weeds. Other researchers reported increased economic returns with glyphosate- and glufosinate-resistant soybean planted at high (342,000 to 560,000 plants ha–1) vs low (122,000 to 221,000 plants ha–1) densities (Hoffner et al. Reference Hoffner, Jordan, York, Dunphy and Everman2012). Harder et al. (Reference Harder, Sprague and Renner2007) reported a similar trend when glyphosate-resistant soybean populations ranged from 124,000 to 445,000 plants ha–1. Yet, the previous studies may not be representative of the situation today as a result of the widespread incidences of glyphosate-resistant Palmer amaranth, which continues to be one of the most troublesome and damaging weeds in soybean in North Carolina (Poirier et al. Reference Poirier, York, Jordan, Chandi, Everman and Whitaker2014) and in several other regions of the United States (Heap Reference Heap2019).
Although PRE herbicides are generally recommended for weed management programs, resource constraints or climatic conditions may prevent farmers from applying PRE herbicides or may compromise the efficacy of such herbicides (Everman et al. Reference Everman, Rana, Schrage, Stowe and York2018). Herbicide application timing is critical when relying on POST herbicide programs to ensure that adequate control is achieved. Gower et al. (Reference Gower, Loux, Cardina and Harrison2002) applied glyphosate to glyphosate-resistant corn (Zea mays L.) when height of giant foxtail (Setaria faberi Herm.) was 5 or 10 cm, or 15 or 23 cm, or 30 cm. When single applications were made to giant foxtail 23 or 30 cm tall, weed control was generally better compared to a single, early (5-cm height) application. Less control with the early treatment was attributed to weed re-infestation; however, greater weed interference occurred with late applications and resulted in reduced corn yields. In another study, a mixed, natural weed population in corn was treated with a single application of glyphosate when average weed heights were 5, 10, 15, 30, or 60 cm (Nelson Reference Nelson2007). Weed density (plants m–2) and biomass (g m–2) were greater when glyphosate was applied to 5-cm-tall weeds in 1 of 2 yr. The same result was obtained in soybean across 2 yr of study. Therefore, applying glyphosate too early is not advisable, as it would allow for higher weed re-infestation. Weed re-infestation after an early herbicide application, from a late-season flush of weeds, may adversely affect the soil weed seedbank, as the late flush may still produce seed. Palmer amaranth can continually emerge throughout the growing season and has the ability to produce seed even when emerging many weeks after the crop (Jha and Norsworthy Reference Jha and Norsworthy2009; Webster and Grey Reference Webster and Grey2015). Additionally, control may be adversely affected if contact herbicides are applied late when the weeds are large, allowing for regrowth and eventual seed production (Bellinder et al. Reference Bellinder, Arsenovic, Shah and Rauch2003; Corbett et al. Reference Corbett, Askew, Thomas and Wilcut2004; Tharp et al. Reference Tharp, Schabenberger and Kells1999).
Much of the previous research with soybean plant populations and weed control interactions has focused on glyphosate-based programs. Currently in North Carolina, the most troublesome broadleaf weed in soybean is Palmer amaranth, which commonly possesses multiple resistance to glyphosate and acetolactate synthase (ALS) inhibitor herbicides (Poirier et al. Reference Poirier, York, Jordan, Chandi, Everman and Whitaker2014; Van Wychen Reference Van Wychen2016). Growers in this region have been heavily relying on contact herbicides such as glufosinate or fomesafen for broadleaf weed control in soybean. Herbicide application timing is critical when considering contact herbicides for Palmer amaranth control. Coupling chemical and cultural weed control is needed to effectively manage Palmer amaranth presently and when considering future management challenges. Data are needed on how soybean population and POST herbicide application timing influence in-season Palmer amaranth control and soybean yield. Additionally, few research studies have investigated how management practices in one season affect weed populations in a subsequent crop. Thus, the objectives of this study were to (1) determine how soybean population and POST herbicide application timing affect in-season Palmer amaranth control and soybean yield, and (2) determine how those variables influence Palmer amaranth densities and cotton yields the following season.
Materials and Methods
In-Season Soybean
Five field experiments were conducted at the Upper Coastal Plain Research Station (35.897°N, 77.675°W) near Rocky Mount, NC, in 2016, 2017, and 2018. Soils were an Aycock very fine sandy loam (fine-silty, siliceous, subactive, thermic Typic Paleudult) and a Goldsboro fine sandy loam (fine-loamy, siliceous, subactive, thermic Aquic Paleudult). The selected fields were dominated by naturally occurring Palmer amaranth populations that largely possessed resistance to glyphosate and ALS-inhibiting herbicides. Soybean (‘AG 69X6’; Monsanto Co., St. Louis, MO) was planted in mid- to late June on 19-cm rows using a grain drill following disking and field cultivation. Similar to grower practices observed in North Carolina, dicamba-resistant soybean was planted to protect the crop from potential drift from nearby fields. Seeding rate was altered to provide low-, medium-, and high-density soybean populations. Stowe et al. (Reference Stowe, Crozier, Bullen, Dunphy, Everman, Hardy, Osmond, Piggot, Randa, Reisig, Roberson, Schrage, Thiessen and Washburn2018) recommends approximately 415,000 plants ha–1 for June-planted soybean on 19-cm rows in North Carolina. Soybean stand counts recorded 2 wk after planting (WAP) showed that populations averaged approximately 268,000, 546,000, and 778,000 plants ha–1 for the low-, medium-, and high-density populations, respectively. Plot sizes were 3.7 m by 10.7 to 13.7 m.
The trials were arranged in a randomized complete block design with four replications. In addition to the effect of plant population, herbicide application timing was also evaluated. Fomesafen (Reflex; Syngenta Crop Protection, LLC, Greensboro, NC) and clethodim (Select 2EC; Valent USA Co., Walnut Creek, CA) were applied at 280 and 210 g ai ha–1, respectively, at 7 to 8 d after planting (DAP; EPOST), 14 to 16 DAP (MPOST), or 21 to 25 DAP (LPOST). Soybean plants were at the VE, V1, or V2 to V3 growth stages at these timings, respectively (Fehr et al. Reference Fehr, Caviness, Burmood and Pennington1971). Palmer amaranth heights ranged from 2 to 3 cm, 4 to 6 cm, and 9 to 14 cm at each application timing, respectively. Nontreated plots were included to evaluate the effect of soybean population alone. Density of Palmer amaranth from the nontreated control at the LPOST timing ranged from 40 to 70 plants m–2 and depended on the year of the experiment was initiated and the history of weed management in each field.
Herbicides were applied with a hand-held, CO2-pressurized backpack sprayer calibrated to deliver 140 L ha–1 at 131 kPa fitted with AIXR 11002 nozzles (TeeJet® flat-fan nozzles; Spraying Systems Co., Wheaton, IL). Other production practices, such as management of fertility, insects, and diseases, were conducted in accordance with Cooperative Extension Service recommendations for North Carolina (Stowe et al. Reference Stowe, Crozier, Bullen, Dunphy, Everman, Hardy, Osmond, Piggot, Randa, Reisig, Roberson, Schrage, Thiessen and Washburn2018). Palmer amaranth control was visually estimated on a scale of 0 to 100 (0 = no control and 100 = complete control) approximately 18 WAP. Foliar chlorosis, necrosis, plant stunting, and reduction in weed population and size were considered when making the visual ratings. Soybean was harvested using a combine, and yield was adjusted to 13% moisture content.
A soybean production budget created by the North Carolina State University Agriculture and Resource Economics Department was used to estimate net economic return (Bullen et al. Reference Bullen, Dunphy and Washburn2019). The conventional-till soybean production system in the Coastal Plain region of North Carolina was customized for the seeding rates, hauling cost, and herbicide treatments. Seeding rates were approximately 268,000, 546,000, and 778,000 seeds ha–1 for the low-, medium-, and high-density populations, respectively. Seed costs for Xtend and Roundup Ready soybean were both used in calculating net returns and were priced at $0.36 and $0.32 1,000 seeds–1, respectively. A custom herbicide application cost of $12.36 ha–1 was added to the $55.18 ha–1 herbicide cost. Average soybean prices from 2010 to 2018 ($0.41 kg–1) were estimated from the U.S. Department of Agriculture Nations Agriculture Statistics Service (USDA-NASS 2019).
Subsequent-Season Cotton
The following season, cotton ‘DP 1646 B2XF’ (Monsanto Co., St. Louis, MO) was planted behind the soybean crop into seed beds (91-cm spacing) following tillage (10- to 12-cm depth) at a seeding rate of approximately 143,500 seeds ha–1. A PRE herbicide was not applied following planting, allowing for Palmer amaranth counts to be determined subsequent to each soybean treatment from the previous year. The plots were large enough so that when the soybean was harvested, the chaff remained in its original plot. Tillage the following season occurred in the same direction as the plots to alleviate movement as much as possible. Though some movement may have occurred, the Palmer amaranth counts were taken from the center two rows toward the center of the plot to further avoid moved seed. In-row Palmer amaranth densities were quantified at 3 WAP by averaging counts from three 0.5-m2 samples taken randomly from the two center rows of the four-row plots. After counting, dicamba (Engenia; BASF Co., Research Triangle Park, NC) plus glyphosate (Roundup PowerMax II; Monsanto Co., St. Louis, MO) was applied at 561 and 947 g ai ha–1, respectively. Application parameters were similar to those used in soybean except that TTI 10002 nozzles (TeeJet flat-fan nozzles; Spraying Systems Co., Wheaton, IL) were used in accordance with label recommendations. Palmer amaranth height ranged from 8 to 14 cm at application. At 7 WAP (4 wk after POST), the weeds were counted again; then the plots were sprayed with glyphosate (947 g ha–1) 1 wk later. A late-season count was done 19 WAP (15 wk after POST). Cotton was harvested in late October using a small-plot spindle picker.
Data Analysis
All data collected in soybean and cotton were analyzed using PROC GLIMMIX in SAS (SAS 9.4, SAS Institute Inc., Cary, NC). Palmer amaranth counts taken in cotton were square-root transformed so as to meet normality. Soybean population, herbicide application timing, and their interaction were considered fixed effects, whereas replication and environment (year-by-field combination) were considered random. Means were separated according to Fisher’s protected LSD at α = 0.05. Relationships among variables were determined using Pearson Correlation coefficients with the correlation procedure (PROC CORR) in SAS.
Results and Discussion
In-Season Soybean
A significant positive relationship between soybean population and Palmer amaranth control was observed when pooled over all environments and herbicide application timings (P < 0.0001, r = 0.40), suggesting that weed control increases as soybean population increases regardless of herbicide application (Table 1). When examining the relationship of soybean population and Palmer amaranth control by herbicide treatment timing, a significant positive relationship was also present and was further exacerbated as herbicides were applied later in the season (P < 0.0001 to 0.0012, r = 0.37 to 0.70). These trends are also evident when examining the ANOVA analysis. The interaction of soybean population and herbicide application timing influenced Palmer amaranth control (P = 0.0017; F = 5.4). When herbicides were not applied, Palmer amaranth control increased as soybean population increased (17% to 38% increase); however, the control in the nontreated control plots was 14% to 81% lower when compared to any soybean population that received an herbicide application, regardless of timing (Figure 1). When herbicides were applied EPOST, control was similar (86% to 95%) across soybean populations. When soybean population was low, the EPOST herbicide application timing was more effective than LPOST application (20% increase). Within the LPOST application timing, Palmer amaranth control was improved with the high-density soybean population compared to the low-density population (23% increase). Additionally, within the LPOST application timing, the medium-density population provided increased control (18% increase) compared to the low-density population. Comparing across populations and timings, when the medium- and high-density soybean populations were treated EPOST, control was increased compared to the low-density population treated either MPOST (22% to 24% increase, respectively) or LPOST (27% to 29% increase, respectively). Collectively, these results suggest that an earlier application timing is critical for adequate weed control and becomes more critical as soybean population decreases. If applications are inadvertently delayed until a later timing, higher populations will aid in control; however, this should be avoided and is not recommended for weed control and resistance management strategies.
Pearson correlation coefficients quantifying the relationships between in-season soybean plant population, Palmer amaranth control, and soybean yield when pooled or sorted by herbicide application timings.a,b
a Abbreviations: EPOST, early POST; MPOST, mid-POST; LPOST, late POST.
b Herbicide application timings for the EPOST, MPOST, and LPOST corresponded to the VE, V1, and V2 to V3 soybean growth stages, respectively.
The influence of soybean plant population and herbicide application timing on late-season (approximately 18 wk after planting) Palmer amaranth control. Soybean populations averaged 268,000, 546,000, and 778,000 plants ha–1 for the low-, medium-, and high-density populations, respectively. Early POST (EPOST), mid POST (MPOST), and late POST (LPOST) timings corresponded to VE, V1, and V2 to V3 soybean growth stages, respectively. Bars with the same letters are not significantly different according to Fisher’s protected LSD at α = 0.05.
Norsworthy and Oliver (Reference Norsworthy and Oliver2001) reported that an average of two glyphosate applications (1.12 kg ai ha–1 each) was required to maintain 90% weed control or greater when soybean seeding rates ranged from 185,000 to 741,000 seeds ha–1. This dropped to an average of 1.3 applications at seeding rates above 800,000 seeds ha–1. Higher soybean seeding rates (676,000 plants ha–1) has been shown to reduce the density and seed production of sicklepod [Senna obtusifolia (L.) H.S. Irwin & Barneby] compared to a lower population of 245,000 plants ha–1 (Nice et al. Reference Nice, Buehring and Shaw2001). Likewise, in another study, the overall weed biomass was less at high-density soybean populations of 302,000 to 445,000 plants ha–1 compared to that with 192,000 plants ha–1 in (Harder et al. Reference Harder, Sprague and Renner2007). Herbicide application timing is critical to achieve acceptable control, and herbicide efficacy can be improved with increased soybean populations. High-density soybean populations alleviate the pressure on herbicide efficacy, thus mitigating the evolution of herbicide-resistant populations.
Plant populations and yield were correlated overall (P = 0.0005, r = 0.22), but when examining relationships by herbicide timing, only with nontreated soybean (P = 0.0151, r = 0.35). These results suggest that soybean population did not influence yield when an herbicide application was made. When the data were analyzed with ANOVA, the main effects of soybean population (P = 0.0017; F = 13.9) and herbicide application timing (P = 0.0038; F = 7.9) influenced soybean yield, with only herbicide application timing affecting net economic returns (P < 0.0081; F = 6.4). When averaged over herbicide applications, the low-density population yielded less (2,880 kg ha–1) than the medium- (3,250 kg ha–1) and high-density populations (3,420 kg ha–1), which were similar (Figure 2 A). When pooled over soybean populations, the EPOST (3,520 kg ha–1), MPOST (3,290 kg ha–1), and LPOST (3,420 kg ha–1) timings resulted in similar yields and were greater than the nontreated yield (2,490 kg ha–1; Figure 2B). Similar results were obtained with respect to net economic return. Differences in net return were not detected between soybean cultivars (Xtend and Roundup Ready soybean), and an average seed cost ($0.34 1,000 seeds–1) was used in the analysis. When soybean was not treated, net returns were lower ($402.82 ha–1) than the treated soybeans, which produced $693.78 to $783.61 ha–1 net returns.
The influence of soybean population (A) or herbicide application timing (B) on soybean yield. Early POST (EPOST), mid POST (MPOST), and late POST (LPOST) herbicide timings corresponded to VE, V1, and V2 to V3 soybean growth stages, respectively. Bars with the same letters are not significantly different according to Fisher’s protected LSD at α = 0.05.
These results suggest that regardless of herbicide application timing, a higher soybean population will reduce weed interference and provide higher yields. Additionally, regardless of the plant population, herbicides remain critical for providing high yields. Higher density soybean populations did not reduce net economic return, yet improved Palmer amaranth control. In the long term, the producer can utilize higher populations in tandem with an herbicide program to mitigate weed seed returning to the soil seed bank while not adversely affecting their annual income. Removing pressure from herbicides for weed control is critical to mitigate resistance evolution. Increasing plant populations to improve weed control may not always be profitable in the short term. A study by Norsworthy and Oliver (Reference Norsworthy and Oliver2001) showed increasing soybean yield with population density from 185,000 to 988,000 seeds ha–1, but with reduced net economic returns at the higher density plant populations. This difference may be due to the primary weed being barnyardgrass [Echinochloa crus-galli (L.) Beauv], which is less competitive with soybean and easier to control than Amaranthus species (Hock et al. Reference Hock, Knezevic, Martin and Lindquist2006). Harder et al. (Reference Harder, Sprague and Renner2007) observed increased soybean yield when herbicides were used compared to no herbicide, regardless of soybean population size. Within treatments, the medium- and high-density populations had greater yields than did the low-density population. The medium-density soybean population slightly increased net returns compared to the low-density population, but high- and low-density soybean populations provided similar returns.
Subsequent-Season Cotton
Significant negative correlations between Palmer amaranth control in soybean the previous year or soybean plant population with Palmer amaranth density in cotton were noted. At 3 WAP cotton, correlation of Palmer amaranth density in cotton with soybean Palmer amaranth control or soybean plant population were both significant (r = –0.52, P < 0.0001; Table 2). By 7 WAP cotton, the correlations were weaker with soybean Palmer amaranth control (r = –0.31, P < 0.0001) and soybean plant population (r = 0.15, P = 0.0122), but still evident. By 15 WAP cotton, the correlations were not significant (P > 0.16). These data suggest that as soybean plant population and Palmer amaranth control in soybean increases, Palmer amaranth densities the following season decrease up until 7 WAP cotton. Reducing future weed populations is an important strategy for mitigating evolution of herbicide resistance in weeds, allowing existing technologies greater longevity (Bagavathiannan and Davis Reference Bagavathiannan and Davis2018; Norsworthy et al. Reference Norsworthy, Korres and Bagavathiannan2018). Whereas most farmers will apply PRE herbicides in cotton, either inclement weather can prevent the application or inadequate rainfall after application may reduce the efficacy. Minimizing deposits to the weed seedbank in soybean thorough greater crop competition provides high long-term weed management value.
Pearson correlation coefficients quantifying the relationships between Palmer amaranth densities (plants m–2) in cotton at 3, 7, and 15 wk after planting (WAP) and cotton yield following various soybean plant population densities and Palmer amaranth control in soybean the previous season.
When considering the density count timings individually, only the main effect of herbicide application timing in soybean influenced Palmer amaranth densities in cotton 3 WAP (P < 0.0001, F = 7.7). Pooled over soybean planting populations, when no herbicide was applied, densities were greatest at 88 plants m–2 (Figure 3). Comparing herbicide application timings, Palmer amaranth densities in cotton were lower following the EPOST timing (54 plants m–2) in soybean compared to the LPOST timing (66 plants m–2), with the MPOST being intermediate (60 plants m–2). By 7 WAP cotton, Palmer amaranth densities were not influenced by soybean planting population (P = 0.8100), herbicide application timing (P = 0.3564), or their interaction (P = 0.1363). Palmer amaranth infestation ranged from 12 to 29 plants m–2 (data not shown). At 15 WAP cotton, Palmer amaranth density ranged from 36 to 57 plants m–2 and was not influenced by any treatment in the previous season (P > 0.30).
The influence of herbicide application timing in soybean on Palmer amaranth densities 3 wk after planting cotton the following season. Early POST (EPOST), mid POST (MPOST), and late POST (LPOST) timings corresponded to VE, V1, and V2 to V3 soybean growth stages, respectively. Bars with the same letters are not significantly different according to Fisher’s protected LSD at α = 0.05.
The reduction of Palmer amaranth density 3 WAP cotton is an important aspect for weed seedbank and resistance management. In soybean, Palmer amaranth control was numerically greatest in the EPOST treatment in general, followed by MPOST and LPOST (Figure 1), which probably contributed to the reduced Palmer amaranth infestation in cotton the following season (Figure 3). Palmer amaranth counts taken 3 WAP represent the initial flush of weeds that will need to be controlled by a PRE application; thus, better control in soybean may reduce the number of Palmer amaranth plants present that need to be controlled by a PRE application, placing less pressure on the herbicide utilized for control. Although the chances of spontaneous mutation rates are low (Mortimer et al. Reference Mortimer, Ulf-Hansen, Putwain, Denholm, Devonshire and Hollomons1992), the fact remains that the chances of selecting herbicide-resistant weeds increase as the number of individuals placed under the selection pressure increases. Reducing the number of seeds returned to the soil seedbank and the number of individuals exposed to selection pressure with herbicides will, in turn, minimize the rate of resistance evolution (Bagavathiannan and Davis Reference Bagavathiannan and Davis2018; Neve et al. Reference Neve, Norsworthy, Smith and Zelaya2011; Norsworthy et al. Reference Norsworthy, Korres and Bagavathiannan2018). Achieving better PRE control may also reduce the number of large weeds present when growers make their POST applications, thus improving control and resistance management.
Cotton lint yield was negatively related to Palmer amaranth densities at 3 and 15 WAP (Table 2). The data show that as Palmer amaranth densities increased, cotton yield decreased. Unlike Palmer amaranth densities in cotton following the soybean crop, neither relationships of cotton lint yield with the previous soybean plant population (P = 0.2959) nor Palmer amaranth control in soybean (P = 0.7114) were significant. This is further supported by the ANOVA results, which showed that cotton lint yield was not influenced by soybean plant population (P = 0.8934), herbicide application timing in soybean (P = 0.3201), or their interaction (P = 0.2709) the previous season. Cotton lint yield ranged from 1,070 to 1,240 kg lint ha–1 with no apparent treatment effects (data not shown). This result is probably due to the relatively high Palmer amaranth densities in the study. Average Palmer amaranth densities never fell below 10 plants m–2. Previous research by Webster and Grey (Reference Webster and Grey2015) illustrated that even at densities of 0.5 plants m–2, cotton yields can be severely reduced whether the weed emerged with the crop (67%), or 3 wk after (30%).
Increasing the number of herbicides and/or the diversity of herbicide mechanisms of action in a single season did not decrease economic return in glyphosate-tolerant cropping systems (Edwards et al. Reference Edwards, Jordan, Owen, Dixon, Young, Wilson, Weller and Shaw2014). In this study, increasing the soybean population did not cause a decrease in economic return, even though there was a greater expense due to higher seed cost. Greater weed control with no reduction in short-term economic gain also results in fewer weed seed entering the soil seedbank. Using data from Edwards et al. (Reference Edwards, Jordan, Owen, Dixon, Young, Wilson, Weller and Shaw2014), Gibson et al. (Reference Gibson, Young, Owen, Gage, Matthews, Jordan, Shaw, Weller and Wilson2015) determined the best management practices that resulted in similar short-term economic returns also resulted in fewer weeds in the soil seedbank. Increasing the soybean plant population can be seen in the same light as increasing herbicide diversity and/or the number of herbicide applications when considering integrated management strategies. Both approaches (increased plant population and additional herbicides) make positive impacts on long-term weed management without negatively affecting short-term economics.
Acknowledgments
This research was supported financially by the North Carolina Agricultural Foundation. Additionally, the authors are grateful for the staff at the Upper Coastal Plain Research Station for their assistance.
No conflicts of interest have been declared.
