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Published in Agron J 100:971-976 (2008)
DOI: 10.2134/agronj2007.0210
© 2008 American Society of Agronomy
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SOYBEAN

Soybean Response to Plant Population at Early and Late Planting Dates in the Mid-South

Chad D. Lee*, Dennis B. Egli and Dennis M. TeKrony

Dep. of Plant and Soil Sci., Univ. of Kentucky, 105 Plant Science Bldg., 1405 Veterans Dr., Lexington, KY 40546-0312

* Corresponding author (cdlee2{at}uky.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The widespread adoption of glyphosate [N-(phosphonylmethyl)-glycine]-resistant soybean [Glycine max (L.) Merr.] and the increased cost of soybean seed have generated interest in determining the minimum plant population needed for maximum yield. The objective of this study was to determine yield and economic return responses to plant population for normal and late planting dates. Cultivars with relative maturities of 2.8 to 4.9 were planted at five seeding rates (43,000 to 560,000 seeds ha–1) in May and/or June in 38-cm rows during 2003 to 2005. The effect of plant population on both yield and economic return was explained with a variation of a Mitscherlich equation. Optimum plant population (OPP) and economically optimum plant population (EOPP) were defined as those resulting in 95% of the estimated yield or estimated economic return, respectively, at the maximum plant population. Optimum plant population ranged from 108,000 to 232,000 plants ha–1 for May planting dates and 238,000 to 282,000 plants ha–1 for June planting dates. Economically optimum plant populations were 7 to 33% less than OPPs. Complete canopy cover at R1 produced maximum yield in 8 of 10 comparisons. These results suggest that seeding rates below those that are currently recommended could lower seed costs without reducing yield.

Abbreviations: EOPP, economically optimum plant population • OPP, optimum plant population


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Received for publication June 17, 2007.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOYBEAN SEED costs increased dramatically over the past decade with the introduction of glyphosate-resistant (Roundup Ready) cultivars, increasing from an estimated $27 ha–1 in 1996 to $80 ha–1 in 2005 (USDA-NASS, 2007). Glyphosate [N-(phosphonylmethyl)-glycine)] herbicide kills a wide range of weed species, typically without causing injury or reducing yields of glyphosate-resistant cultivars (Nelson and Renner, 1999). Glyphosate-resistant cultivars are grown on about 90% of the soybean area in the United States (USDA-NASS, 2006). This increased seed cost has generated new interest in the optimum seeding rate needed to attain the plant population for maximizing yield.

Soybean yield is relatively insensitive to plant population with a wide range in seeding rates usually producing the same yield. Heatherly and Elmore (2004) suggested that 300,000 to 370,000 viable seeds ha–1 were adequate but with poor soil conditions, the rates should be increased up to 50%. Extension recommendations for soybean seeding rates in the Mid-South United States are from 343,000 to 516,000 seeds ha–1 for row widths at or near 38 cm (Beuerlein and Dorrance, 2005; Christmas, 1993; Flinchum, 2001; Herbek and Bitzer, 1988).

Soybean yield responses to plant population can vary in specific combinations of environment, cultivar maturity, and seeding date. In Arkansas, highest yields with short-season cultivars in 56-cm rows occurred at populations of 210,000 plants ha–1 in 1997 and 540,000 plants ha–1 in 1998 (Ball et al., 2000). In Virginia, a Maturity Group III cultivar required twice the plant population (741,000 plants ha–1) of a Maturity Group V cultivar (370,500 plants ha–1) to reach maximum yield when planted in late June/early July (Holshouser and Jones, 2003). In an Ontario study using indeterminate cultivars, seeding rates of 395,000 and 790,000 seeds ha–1 in three row widths (25, 51, and 76 cm) produced similar yields; however, determinate cultivars yielded more at a seeding rate of 395,000 seeds ha–1 compared to 790,000 seeds ha–1 in 2 of 3 yr (Ablett et al., 1991). Under drought conditions in Virginia, nearly a threefold increase in plant populations was required for maximum yield compared to when adequate moisture was present (Holshouser and Whittaker, 2002).

Part of the yield response to plant population (and seeding rate) is based on canopy development and light interception. Shibles and Weber (1965) and Wells (1991) showed clearly that crop growth rate and canopy photosynthesis reached a maximum as light interception approached 100%. Consequently, maximum yield of soybean has been shown to require canopy closure at R1 (Kane and Grabau, 1992); or a Leaf Area Index (LAI) of 3.5 at R5 (Holshouser and Whittaker, 2002); or 95% light interception at R5 (Board and Harville, 1994).

The objective of this study was to determine both yield and economic return responses to plant population at normal and late planting dates. Several cultivars with maturity appropriate to the region were included in the experiment.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The experiment was conducted during 2003, 2004, and 2005 at Spindletop Farm near Lexington, KY (38° N, 84° W). Soil types were Loradale silt loam (fine-silty, mixed, mesic Typic Argiudoll) in 2003, Maury silt loam (fine, mixed, semiactive, mesic Typic Paleudalf) in 2004, and Lanton silty clay loam (fine-silty, mixed, superactive, thermic Cumulic Epiaquoll) in 2005. The previous crop was wheat (Triticum aestivum L.), with the field in fallow until soybean seeding the following year. The seeds were planted no-till in 2003 and after two diskings (approximately 10-cm deep) in 2004 and 2005. Each field was fertilized according to soil test results and the University of Kentucky soil fertility recommendations.

Soybean cultivars with relative Maturity Group ratings from 2.9 to 4.9 were planted on dates in May considered normal for full-season planting or in June (a late planting date) (Table 1 ) using a small-plot drill with cone delivery (Hege, Colwich, KS). Seed lots of each cultivar (standard germination above 83% for all seed lots) were compared in 2003, 2004, and 2005. Seeding rates were 43,000; 172,000; 300,000; 430,000; and 560,000 viable seeds ha–1 in 2003 and 43,000; 86,000; 172,000; 300,000; and 474,000 viable seeds ha–1 in 2004 and 2005. Neither pesticide seed treatments nor at-planting treatments were used. Inoculant was not used, because soybean had been grown within 3 yr at each study site. Each plot was six rows (38-cm row spacing) wide by approximately 6-m long when planted and trimmed to 4.5 m for harvest.


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  		Table 1. Seeding dates and cultivars included in the population experiment (2003–2005).

 
Careful attention to controlling weeds without hindering canopy development was taken when nonglyphosate-resistant cultivars were used. In 2003, weeds were controlled with a pre-emergence (burndown) application of metolachlor (Dual) plus chlorimuron + sulfentrazone (Canopy XL) plus glyphosate (Roundup WeatherMax). Yellow nutsedge (Cyprus esculentus L.) was removed with a combination of hand-hoeing and bentazon (Basagran) applied with a hooded, single-nozzle, sprayer that avoided herbicide contact with the soybean plants. In 2004, weeds were controlled with a pre-emergence application of chlorimuron + sulfentrazone (Canopy XL) plus glyphosate (Roundup WeatherMax). Grass weeds were controlled with sethoxydim (Poast) in the May plantings only. Broadleaf weeds and yellow nutsedge were controlled by hand-hoeing plus imazethapyr (Pursuit) and bentazon (Basagran) sprayed with the hooded, single-nozzle sprayer described previously.

Weed control for glyphosate-resistant cultivars was accomplished with the same pre-emergence herbicide applications described earlier, plus a single, post-emergence application of glyphosate (Roundup WeatherMax) in 2004 and two post-emergence glyphosate applications in 2005. No foliar insecticides or fungicides were used in this study.

Plant populations were measured near growth stage R1 (Fehr and Caviness, 1977) by counting all plants in the two center rows of each plot. Soybean canopy closure was estimated at growth stages R1 and R5 by observing each plot from the same position at the end of the plot (looking down the rows) and visually estimating the proportion of soil surface visible between the rows (100% representing complete ground cover, i.e., no soil visible between the rows and 0% representing all soil visible, i.e., no plants) (Kane and Grabau, 1992). Seed yield was determined by harvesting all plants from the three center rows with a small-plot harvester (Hege, Colwich, KS) at harvest maturity. Harvested seeds were dried to a constant weight, weighed, and yield was adjusted to 130 g kg–1 moisture.

Treatments were arranged in a split-block design to facilitate harvest with cultivars as whole plots and seeding rates as split-plots within each cultivar. There were three replications of each treatment. After investigating different statistical models, a variation of the Mitscherlich equation was determined to be the most appropriate. This model, y = y0 + a(1 – e(–bx)), related yield (Y) to plant population (X) for each cultivar–year combination using Sigma Plot (Systat Software, Inc., San Jose, CA). This model produces the maximum yield at the highest plant population. The linear-plateau model was investigated, but too few data points occurred in the linear portion of the curve to provide much confidence in the estimate of the optimum population (i.e., the breakpoint between the linear and plateau portions of the curve). A second-order polynomial also was not appropriate since this model results in a yield decline as plant population increases, an outcome that did not occur in this study. The model chosen provided an excellent fit to the data. The OPP was defined as the population producing 95% of the predicted yield at the highest observed plant population per Edwards et al. (2005).

For purpose of this study, the EOPP was calculated as the population that produced 95% of the predicted partial economic return at the highest observed plant population. Partial economic return was the product of commodity price ($.265 kg–1) and seed yield minus the sum of seed cost ($1.41 kg–1 x seeding rate) and hauling cost ($0.0044 kg–1 x seed yield). Soybean commodity price was determined using a marketing strategy of selling 50% of the crop in November and 25% forward contracted (minus basis) to March and July, respectively (Stanger and Lauer, 2006). The November average cash price was derived from Kentucky Ag Statistics for November prices from 2001 through 2006, and the March and July future prices were derived from the Chicago Board of Trade futures price on 29 Aug. 2007. Basis was determined by averaging five Kentucky grain elevator prices for November soybean and comparing that average to the Chicago Board of Trade futures for November as checked on 29 Aug. 2007. Seed cost was based on $32 bag–1 of seed (USDA-NASS, 2007), with the assumption that one bag weighs 22.7 kg (50 lb). Hauling charges were determined by averaging prices from several sources in Kentucky and neighboring states (G. Halich, personal communication, 2007).

Yields were related to estimated canopy cover at growth stages R1 and R5 with simple linear or linear-plateau models (SAS Institute, Cary, NC). The linear and linear-plateau models were compared with an F test (P ≤ 0.05) to determine if the linear-plateau model provided a significantly better fit than the linear model.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Rainfall was timely and adequate in 2003 and 2004, while 2005 was dry through May, June, and early July (Fig. 1 ). The low rainfall in 2005 greatly reduced the yield of cultivars planted in May (average yield of 1753 kg ha–1 compared with an average of 3018 kg ha–1 for the May plantings in 2003 and 2004).


Figure 1
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  		Fig. 1. Daily maximum and minimum temperature and precipitation at Spindletop Farm. Measurements taken within 1.6 km of research plots each year. Source: University of Kentucky Ag Weather Center: http://www.agwx.ca.uky.edu/ (verified 31 Mar. 2008).

 
Plant populations averaged about 63% of seeding rates in 2003 and above 80% for 2004 and 2005 plantings with little variation among cultivars (data not shown).

Yield increased rapidly as population increased and reached maximum levels at relatively low populations (Fig. 2 ). When yield levels were low for the May 2005 plantings (1700 and 2533 kg ha–1), the regressions were not significant (data not shown); however, the 10 other comparisons had significant regressions relating plant population to yield (Table 2 ).


Figure 2
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  		Fig. 2. Response of yield for selected cultivars to plant population in May and June plantings, 2003 and 2004.

 

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  		Table 2. Yield response to soybean plant population.

 
When there was a significant relationship between yield and plant population, the OPPs were below 260,000 plants ha–1 for 9 of the 10 comparisons (Table 2). There were differences among years (lowest populations for OPP were observed in 2003) and between planting dates in 2004 (higher populations for OPP were required at the later planting date). There also was some indication in 2004 that earlier-maturing cultivars required higher OPP. Economically optimum plant populations ranged from 76,000 to 241,000 plants ha–1 (Table 3 ) and were 7 to 33% lower than OPPs shown in Table 2.


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  		Table 3. Economic response to soybean plant population. Partial return was the product of commodity price ($.265 kg–1) by seed yield minus the sum of seed cost ($1.41 kg–1) by seeding rate and hauling cost ($0.0044 kg–1) by seed yield.

 
There was a significant relationship between yield and estimated canopy cover at R1 and R5 for all comparisons, except the two cultivars planted in May 2005 (Table 4 , Fig. 3 and 4 ). Eight of the relationships between yield and estimated canopy cover at R1 were explained by linear regression, while the linear-plateau model provided a better fit in two comparisons (Stressland planted in May 2003, P < 0.001 and B336 planted in May 2004, P < 0.001) (Fig. 3). All comparisons at R5 were linear (Fig. 4).


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  		Table 4. Results of regression analysis of the relationship between estimated canopy cover and yield.

 

Figure 3
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  		Fig. 3. Response of yield to estimated canopy cover at growth stage R1. There was no significant relationship (P = 0.28 and 0.62) for either cultivar in the May 2005 planting. Significant (P ≤ 0.05) equations are: May 2003, Stressland (linear-plateau): If X ≤ 35 then Y = 496 + 90X, if X ≥ 35 then Y = 3625 and b = 0; May 2003, CF461: Y = 1269 + 22, r2 = 0.87; May 2003, CF492: Y = 173 + 32X, r2 = 0.97; May 2004, B283: Y = 1447 + 29X, r2 = 0.78; May 2004, B336 (linear-plateau): if X ≤ 58.5 then Y = 1560 + 36x, if X ≥ then Y = 3720 and b = 0; June 2004, B283: Y = 882 + 24X, r2 = 0.98; June 2004, B336: Y = 906 + 27X, r2 = 0.98; June 2005, B283: Y = 1309 + 30X, r2 = 0.72; June 2005, B336: Y = 1986 + 18X, r2 = 0.95; where X = estimated canopy cover (%) at R1 and Y = seed yield (kg ha–1).

 

Figure 4
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  		Fig. 4. Yield response to estimated canopy cover at growth stage R5. There was no significant relationship (P = 0.49 and 0.42) for either cultivar in the May 2005 planting (data not shown). Significant (P ≤ 0.05) equations are: May 2003, Stressland: Y = –112 + 38X, r2 = 0.93; May 2003, CF461: Y = –583 + 36X, r2 = 0.82; May 2003, CF492: Y = 54 + 32X, r2 = 0.94; May 2004, B283: Y = –491 + 38X, r2 = 0.70; May 2004, B336 Y = 466 + 32X, r2 = 0.93; May 2004, CF461: Y = –5212 + 95X, r2 = 0.88; June 2004, B283: Y = 266 + 27X, r2 = 0.87; June 2004, B336: Y = –3133 + 61X, r2 = 0.70; June 2005, B283: Y = –2371 + 61X, r2 = 0.96; June 2005, B336: Y = –1895 + 52X, r2 = 0.95; where X = estimated canopy cover (%) at R5 and Y = seed yield (kg ha–1).

 
Most cultivars required complete canopy cover at R1 for maximum yield (Fig. 3) with Stressland (May 2003) and B336 (May 2004) the only exceptions and they required only 35 and 59% canopy cover, respectively, to produce maximum yield. All cultivars required complete cover at R5 for maximum yield (Fig. 4).


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Seeding rate affects plant population which ultimately may affect grain yield. Many previous studies (Pedersen and Lauer, 2002; Bertram and Pedersen, 2004; Norsworthy and Frederick, 2002) reported a constant yield from a wide range in seeding rates and/or plant populations, but they did not identify OPPs or EOPPs. Our objective was to identify these populations for cultivars with a range in maturity and grown in normal and late plantings.

The OPPs for May planting were 108,000 to 232,000 plants ha–1 (Table 2) and required seeding rates of 171,000 to 264,000 seeds ha–1 based on emergence levels (data not shown). These seeding rates are well below the region's recommended seeding rates of 300,000 to 516,000 seeds ha–1 (Beuerlein and Dorrance, 2005; Christmas, 1993; Flinchum, 2001; Heatherly and Elmore, 2004; Herbek and Bitzer, 1988) but only slightly below the plant populations that produced maximum yield for Kratochvil et al. (2004).

June plantings required OPPs of 238,000 to 282,000 plants ha–1 which were obtained with seeding rates of 266,000 to 307,000 seeds ha–1 (data not shown). The June seeding rates are near or below current recommendations for the region. Optimum plant populations were higher in the June plantings (Table 2) because more plants are required to counteract the smaller plants often associated with later plantings (Egli and Bruening, 2000; Heatherly and Elmore, 2004). Kratochvil et al. (2004) also found that double-crop (i.e., late) plantings required higher seeding rates (333,450 and 444,600 seeds ha–1), rates that were about 10 to 40% higher than those reported here. Kratochvil et al. (2004) planted into wheat stubble, where the soils would likely be drier than in this study that was planted into soil that lay fallow for 11 mo after wheat harvest (Pearce et al., 1993; Sanford and Hairston, 1984). Drier soils in Kentucky could have caused less seedling emergence, slower crop growth and canopy development (Pearce et al., 1993), thus requiring more plants (higher seeding rates) to reach full canopy during reproductive development.

Early maturing cultivars required higher OPPs than later maturing cultivars in 2004, the only year when cultivars with a broad range of maturity were compared. Higher seeding rates for early maturing cultivars have been reported in other environments (Holshouser and Jones, 2003; Edwards and Purcell, 2005). These higher seeding rates (and populations) are needed because early cultivars reach R1 sooner than later cultivars (Kane and Grabau, 1992; Egli and Bruening, 2000) resulting in smaller plants and less canopy development at R1. This causes the need for more plants to maximize light interception (Wells, 1991; Kane and Grabau, 1992).

Economically optimum plant populations were always slightly less than OPPs for each cultivar and planting date (Tables 2 and 3) but followed the same general trends as OPPs. Since OPPs resulted from seeding rates less than those currently recommended, the EOPPs provide additional confirmation to the validity of lower plant populations.

Soybean crop growth rates (Shibles and Weber, 1965) and canopy photosynthesis (Wells, 1991) are dependent on light interception, so it is not surprising that much of the yield response to population can be related to ground cover during reproductive growth. Since soybean yield is determined by canopy photosynthesis during reproductive growth (growth stages R1 through R7) (Jiang and Egli, 1995), maximum levels of light interception and canopy photosynthesis are needed for maximum yield (Wells, 1991). Seed number is related to average crop growth rate during flowering and pod set (R1 through R5) (Ramseur et al., 1985; Egli and Zhen-wen, 1991; Jiang and Egli, 1995) and reducing photosynthesis for part of this period always reduced seed number (Schou et al., 1978; Kokubun and Watanabe, 1983; Jiang and Egli, 1995). Significant pod production, however, does not begin until 10 to 15 d after R1 (Egli and Bruening, 2006) suggesting that complete ground cover and maximum canopy photosynthesis may not be needed until sometime after R1. It is not entirely clear from the literature if complete ground cover has to occur by growth stage R1 to maximize pod set, seed number per unit area, and yield. Complete ground cover at growth stage R1 was needed in this experiment to produce maximum yield in four of six comparisons in the May planting (Fig. 3). One of the exceptions was Stressland, a cultivar selected for extensive vegetative growth (Cooper et al., 1999), and the other was B336, but without data from more environments it is not possible to determine if these are cultivar differences or just random environmental effects. Yield was always reduced if complete ground cover was not achieved by growth stage R5. These data suggest that complete ground cover by R1 or shortly thereafter is usually required for maximum yield even though vegetative growth continues until R5 (Egli et al., 1985). The picture is clearer for the later planting date (June) where maximum yield always required complete ground cover at R1. Our findings are generally consistent with those of Board (2004) and Board and Harville (1994) who found maximum seed yield occurred when light interception approached 90% at R1 and 95% during seed filling and they support the concept that maximum photosynthesis is needed during most of the R1 to R5 period to maximize yield.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Optimum plant populations (and the seeding rates required to produce those populations) in this experiment were consistently much lower than current recommendations. The later planting date (June) and earlier maturing cultivars required higher populations (and seeding rates) than the earlier (May) planting date and later maturing cultivars. There was some variation in our population estimates, but the data suggest that seeding rates can be reduced well below current recommendations without losing yield. The higher seeding rates in current recommendations provide protection against low seedling emergence caused by poor seed quality and/or stress after planting, factors that could reduce yield if seeding rates are lower. The use of high-vigor seed could reduce this risk (Egli and TeKrony, 1995; Egli and TeKrony, 1996; Hamman et al., 2002) when seeding rates are lowered to reduce production costs. Field emergence averaged 70% for plantings in May, June, and July in experiments that included 272 seed lots of commercial cultivars over 10 yr (Egli and TeKrony, 1996). Emergence was lower (60%) for plantings in April. These values provide only a rough indication of seeding rates needed to obtain a specific population because emergence levels could be above or below these averages depending on seedbed conditions. While the data in this study suggest that seeding rates much lower than current recommendations will produce maximum yield, producers should be cautious of seedbed conditions or seed lots that may result in reduced germination and unacceptable final plant populations.


   ACKNOWLEDGMENTS
 
We would like to acknowledge that the seed lots used in this experiment were provided by Beck's Hybrids, 6767 E. 276th Street, Atlanta, IN 46031; and Caverndale Farms, 1921 Bluegrass Pike, Danville, KY 40422.

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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