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Published in Agron J 100:704-710 (2008)
DOI: 10.2134/agronj2007.0106
© 2008 American Society of Agronomy
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SOYBEAN

Effect of Row Spacing and Seeding Rate on Soybean Yield

Jason L. De Bruin* and Palle Pedersen

Dep. of Agronomy, Iowa State University, 2104 Agronomy Hall, Ames, Iowa 50011-1010

* Corresponding author (jsndbrn{at}iastate.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Soybean [Glycine max (L.) Merr.] yield response to narrow row spacing has been consistently positive in the upper Midwest and new split-row planters have made narrow row soybean production feasible, yet adoption has been slow in Iowa. Wide (76-cm) and narrow (38-cm) row spacing and four seeding rates (185,000; 309,000; 432 000; and 556,000 seeds ha–1) were evaluated at three locations during 2004, 2005, and 2006 to determine seed yield in wide and narrow row spacing and four seeding rates and evaluate economic advantages associated with changes in row spacing. Soybean planted in 38-cm row spacing yielded 248 kg ha–1 greater than soybean planted in 76-cm rows after adjustment for differences in final plant populations. Maximum yield at all locations was attained at a final harvest population of 462,200 plants ha–1 but >95% of the maximum yield was achieved with final populations as low as 258 600 plants ha–1. Increased production costs associated with greater seeding rates removed the yield benefit from greater harvest plant populations. Farm size of 144 ha with at least 50% of the land base dedicated to soybean production would benefit from conversion from wide to narrow rows. To break even on the investment in a split-row planter a yield increase of 124 kg ha–1 was necessary for farms with 30% of 288 ha dedicated to soybean production. These data indicate that yield and economic benefits are sufficient to support the production of soybean in narrow rows and at seeding rates below current seeding rate recommendations.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 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 March 25, 2007.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
SOYBEAN PRODUCTION in the 1960s and 1970s was conducted using row spacings ≥ 76 cm (Taylor, 1980; Weber et al., 1966). Since 1990, the trend has been toward production in rows planted at spacings <76 cm. Research conducted in the upper Midwest and southern Canada documents a consistent yield advantage of 134 to 604 kg ha–1 when yields for narrow row planting ( <76 cm) are compared to yields obtained from wide row planting (≥76 cm) (Ablett et al., 1991; Bullock et al., 1993, 1998; Cooper, 1977; Grau et al., 1994; Lueschen et al., 1992; Oplinger and Philbrook, 1992; Weber et al., 1966). However, instances occur when there was no yield response to narrow row spacing (Pedersen and Lauer, 2003). The magnitude of the response was location and year specific (Lueschen et al., 1992), and cultivar (Grau et al., 1994), or time of planting and tillage system dependent (Oplinger and Philbrook, 1992).

Average row spacing for soybean production in Iowa is 57 cm with the majority of acres planted using row spacings of 19- (14%), 38- (31%), and up to 76-cm (50%) row spacing (National Agriculture Statistics Service, 2007). Despite positive yield reports in soybean, lack of yield responses to narrow rows in corn (Zea mays L.) (Farnham, 2001; Hallman and Lowenberg-DeBoer, 1999; Westgate et al., 1997) and the potential for greater incidence and severity of Sclerotinia stem rot (Sclerotinia sclerotiorum) in narrow rows (Grau and Radke, 1984) has limited wide-spread adoption. However, other research indicates that cultivar selection (Buzzell et al., 1993) and plant populations (Lee et al., 2005) are more important factors for Sclerotinia stem rot development.

Lee (2006) indicated row spacing <76 cm produced a positive yield response in northern environments but a less consistent response south of 43°N latitude. Based on this, producers in Iowa, whose northern border is located at 43°N may not benefit from narrow-row soybean production as much as producers further north. Split-row planting equipment, planters that have additional row units between traditional 76-cm spaced row units that can be raised or lowered depending on the crop, may allow for production of corn and soybean at the optimal row spacings for each crop. Increased equipment costs and the potential for no yield increases may prevent producers from investing in this planting technology.

An advantage of narrow row spacing is more equidistant plant spacing that leads to increased canopy leaf area development and greater light interception earlier in the season (Shibles and Weber, 1966; Weber et al., 1966). These changes in canopy formation increase crop growth rate, dry matter accumulation, and seed yield (Andrade et al., 2002; Bullock et al., 1998).

Abiotic and biotic stresses can mitigate the yield response of soybean to narrow-row spacing production. Moisture stress has been documented to reduce the yield benefit from narrow row spacing in Kansas (Devlin et al., 1995), Nebraska (Elmore, 1998), Texas (Heitholt et al., 2005), North Dakota (Alessi and Power, 1982), and Iowa (Taylor, 1980). Nitrogen stress (Cooper and Jeffers, 1984) and increased seeding rates in dry, low-yield potential environments reduced yield in narrow rows (Devlin et al., 1995; Elmore, 1998). Presence of brown stem rot caused by Cadophora gregata (Phialophora gregata) reduced yield benefit from narrow row spacing and early planting for a susceptible cultivar (Corsoy 79) (Grau et al., 1994) and Pedersen and Lauer (2003) speculated that the presence of soybean cyst nematode (Heterodera glycines Ichinohe; SCN) removed any benefit from planting soybean in narrow row spacing. Weber et al. (1966) speculated the benefit from narrow row spacing would be minimized in more productive environments and maximized in stressful production environments.

Increased seeding rates potentially could be used in a narrow row system to maximize space utilization. Prior research indicates that optimal seeding rates increase in a narrow row spacing system (Devlin et al., 1995; Oplinger and Philbrook, 1992; Weber et al., 1966). A concern, though, is that as seeding rate increases plant competition increases, generating stress on the canopy, minimizing the benefit to narrow row spacing, especially when environmental conditions limit plant growth (Devlin et al., 1995; Elmore, 1998). Target plant populations at harvest have been 272,000 and 321,000 plants ha–1 (Whigham, 1998; Whigham and Lundvall, 1996). Seeding rates used in Iowa differ depending on row spacing and common seeding rates used to ensure sufficient harvest population are 494,200; 444,800; and 370,000 seeds ha–1 for 19-, 38-, and 76-cm row spacing, respectively.

Our hypothesis is that narrow row spacing will produce greater yields than wide row spacing and that economic advantages exist for narrow row soybean production. The objectives were (i) to determine seed yield response to changes in row spacing and seeding rates and (ii) to measure changes in costs and revenues associated with a change in row spacing using a split-row planter compared with a traditional planter.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Studies were conducted at three locations during 2004 to 2006. Locations were in eastern Iowa near De Witt (Klinger silt loam, fine-silty, mixed mesic, Typic Hapludolls), central Iowa near Nevada (Canisteo clay loam, fine-loamy, mixed mesic, Typic Hapludolls), and western Iowa near Whiting (fine-silty, mixed mesic, Typic Hapludolls). The experiment was a randomized complete block in a split-plot arrangement with four replications. Main plot was 38- and 76-cm row spacing and subplot was seeding rates of 185,000; 309,000; 432,000; and 556,000 viable seeds ha–1 of the glyphosate [N-(phosphonomethyl) glycine] resistant cv. AG2801 (Monsanto Co., St. Louis, MO).

Plots were planted the last week of April using an Almaco grain drill (Almaco, Nevada, Iowa) for all locations with the exception of Nevada in 2004 when plots had to be replanted the first week of June due to flooding and Nevada in 2005 when plots were planted on 9 May. Plot size was 2.7 m wide by 6.1 m long. Before planting, all seed was inoculated with Bradyrhizobium japonicum (EMD Crop BioScience, Brookfield, WI). Glyphosate was applied twice during the season at a rate of 1.1 kg a.i. ha–1 for weed control. Lorsban 4E (Dow AgroScience, Indianapolis, IN) [Chlorpyrifos 0,0-diethyl-0-(3,5,6-trichloro-2-pyridinyl) phosphorothioate] was applied at a rate of 0.84 kg a.i. ha–1 twice at De Witt during 2005 to control soybean aphid (Aphis glycines) and spider mites (Tetranychus urticae) and at Nevada in 2006 to control bean leaf beetles [Cerotoma trifurcate (Forster)]. Baythroid (Bayer CropScience, Research Triangle Park, NC), cyfluthrin [cyano (4-fluoro-3-phenoxyphenyl)methyl-3-(2,2-dichloroethenyl)-2,2-dimethyl-syclopropanecarboxylate] was applied at 0.49 kg a.i. ha–1 at Whiting in 2005 to control bean leaf beetles.

Yield was determined by harvesting the center four 38-cm rows and two 76-cm rows with an Almaco plot combine and yield was adjusted to a moisture content of 130 g kg–1. Other measurements taken at harvest were final plant population, plant height, and seed mass based on a sample of 300 seeds.

Partial budget analysis compared three farm sizes and three corn and soybean rotations. Farm size ranged from the state average of 144 (National Agricultural Statistics Service, 2007) to 1294 ha. Corn–soybean rotations were 50/50, 60/40, or 70/30. The standard practice of both corn and soybean planted in 76-cm row spacing was compared with corn planted in 76-cm row spacing and soybean planted in 38-cm row spacing using a split-row planter. Corn was not considered in this analysis because expenses and revenues associated with corn production would not change. Soybean price was determined following the method of Stanger and Lauer (2006) where 50% of the crop was sold in November and 25% of the crop forward marketed to both March and July. An average 5-yr cash price of $0.203 kg–1 was determined from production years 2001 to 2005 (National Agricultural Statistics Service, 2007). Future prices for March ($0.239 kg–1) and July ($0.246 kg–1) were based on the Chicago Board of Trade on 5 Dec. 2006 and were adjusted for basis (futures price–cash price). Final sale price was $0.213 kg–1. Seed cost was set at $1.2 kg–1 and handling and hauling costs were $0.00054 kg–1 (Duffy and Smith, 2006).

Cost of the different planters was estimated by gathering price information from the three most commonly used brands in Iowa (John Deere, Moline, IL; Kinze Manufacturing, Williamsburg, IA; Case IH, Racine, WI). Cost differences between planter types were consistent among manufactures. The purchase price of a 12-row split-row planter was $18,000 more than the purchase price of a traditional 12-row planter. Useful life of the planter was estimated at 10 yr and a salvage value of 0.40 (Edwards, 2005). An interest rate of 0.08 was used to determine interest costs with the split-row planter. Insurance was increased based on a rate of 0.005% for the increase in equipment cost (Edwards, 2005). Additional ownership costs (principal, depreciation, interest, and insurance) were estimated at $2275 yr–1 to purchase a 12-row split-row planter compared with the purchase of a 12-row planter without split rows. Additional cost and ownership costs for a 16-row split-row planter were $22,000 and $2781 yr–1, respectively. Only the purchase of 12-row planters was considered in this analysis, however, because larger farms would consider a 16-row planter due to the increase of 1.22 ha h–1 planting capacity (Hanna, 2002) it is included. Repair cost differences between a conventional and split-row planter were estimated using the program Ag Decision Maker (Iowa State University Agricultural Economics, www.econ.iastate.edu/default.asp; verified 7 Feb. 2008). Equations used in this decision program are based on values reported by American Society of Agricultural and Biological Engineers (ASABE standards, 2007). Repair costs were estimated based on planter purchase price and were $0.52 and $0.69 ha–1 for a conventional 12-row and split-row planter, respectively.

Revenue changes for the transition from 76-cm row spacing to 38-cm row spacing can be calculated with the following equation:

Formula

In this equation kg ha–1 refers to yield difference between 38- and 76-cm row spacing, ha refers to hectares of soybean production, ownership costs are either $2275 or $2781 for 12- and 16-row, respectively, hauling and handling charges are $0.00054 kg–1, and repair cost increase of $0.17 ha–1.

Data were analyzed using Proc Mixed in SAS (SAS Institute, 2003). Boxplots and residual plots were evaluated to determine variance assumptions (Oehlert, 2000). When variance was determined to be homogenous data were analyzed in a combined analysis treating years and blocks as random effects and location and row spacing as fixed effects. Plant establishment rates (i.e., number of plants at harvest for each seeding rate) were not influenced by location but did differ for 38- and 76-cm row spacing. Establishment rates were assumed to be linear and slopes and intercepts were calculated for each row spacing treatment using individual plot data in Proc Reg. Plants ha–1 at harvest for individual plots was used as a covariate in the analysis. The final model for each variable was determined using a backward stepwise selection process where the full model is considered first and factors and their interactions are sequentially deleted until all factors that remain are significant at P ≤ 0.05. Least square means of the fixed effects were computed and differences among least square means were compared using the PDIFF option. Coefficient of determination (R2) was calculated using the predicted values generated by Proc Mixed using the formula R2 = 1 – [(yij – ypred)2/(yij – ygrand mean)2]. Harvest plant population for maximum yield was calculated by taking the first derivative of the yield equation present in Table 4, and solving for the variable (P), harvest plant population. This population was used to calculate the maximum yield. Harvest populations at 95% of maximum yield were calculated for each location by row spacing combination. When calculated maximum yields and 95% of maximum yield harvest populations were outside of the actual yield or harvest population tested, the experimental means were presented rather than predicted values.


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  		Table 4. Regression equations, predicted maximum soybean yields and harvest plant population, and predicted yields for average harvest plant populations at three Iowa locations, 2004 to 2006.

 

   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSION
 REFERENCES
 
Growing conditions, especially rainfall patterns, were variable among locations and years. Heavy single rainfall events in May at Nevada (2005), De Witt (2006), and Whiting (2006) caused soil erosion, crusting, and contributed to lower than expected emergence. In 2004, rain fall was above average in May, below average in June, and average in July except at De Witt, and below average in August at all locations (Table 1 ). In 2005, rainfall was below average all season long, but was above average during June through August at other locations. In 2006, rainfall during May and June was 58 and 97 mm 47 and 51 mm below normal at Nevada and Whiting, respectively (Table 1). Plots were irrigated at Whiting each year and irrigation amounts are listed in Table 1.


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  		Table 1. Rainfall and air temperature data recorded at the three experimental locations during 2004 to 2006. Deviations from the 30-yr average reported in parentheses.

 
Harvest Plant Population
Implementing harvest plant population as a covariate, a continuous variable rather than a discrete variable, allowed for comparisons among locations and row spacing to be made on an equal harvest plant population. Plant establishment rates for 38- and 76-cm row spacing indicated that establishment was greater in narrow rows compared with wide rows (P < 0.05). On average 67 plants survived until harvest for every 100 seeds planted in 38-cm row spacing compared with a rate of 49 plants in 76-cm row spacing (data not shown). Greater establishment in narrow rows has been observed by others in the upper Midwest (Elmore, 1998; Oplinger and Philbrook, 1992) and has also been reported by Ethredge et al. (1989) in southern environments. Increased intra-row competition in 76-cm row spacing as seeding rates increase potentially explains that greater establishment rate in 38-cm row spacing. Improved plant establishment using narrow row spacing presents an opportunity for producers to minimize the risk of a poor stand at lower seeding rates. Differences in plant establishment between wide and narrow rows could potentially confound yield responses attributed to row spacing.

Seed Yield
Locations varied in yield potential (Table 2 ) and yields were significantly greater at both Whiting and De Witt compared with Nevada (Table 3 ). Whiting and De Witt were high-yield locations that produced yields more than 1500 kg ha–1 greater than the lower-yielding location of Nevada (Table 3), mainly due to soil characteristics and high pathogen pressure from SCN, sudden death syndrome, caused by Fusarium virguliforme O'Donnell & Aoki [synonym F. solani (Mart.) Sacc. f. sp. glycines] (Akoi et al., 2005), and seedling diseases (De Bruin, 2007; Murillo-Williams, 2007). This range in environmental yield potential presented the opportunity to investigate the theory posed by Weber et al. (1966) that high-yielding environments do not benefit as much from reduced row spacing. There was no location by row spacing interaction with mean differences among 38- and 76-cm row spacing treatments averaging 275 kg ha–1 at De Witt and Nevada and 194 kg ha–1 at Whiting (data not shown). Lack of a significant location by row spacing interaction (Table 2) and an average yield increase of 248 kg ha–1 between 38- and 76-cm row spacings (Table 3) supports the benefit of narrow rows for all production environments and is consistent with other reports from the upper Midwest (Bullock et al., 1998; Lueschen et al., 1992; Oplinger and Philbrook, 1992; Taylor 1980), but does not support the hypothesis of Weber et al. (1966).


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  		Table 2. Significance of F values from analysis of covariance of seed yield, seed mass, plant height, and grower return, 2004 to 2006.

 

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  		Table 3. Main effect means for seed yield, seed mass, and plant height at three locations for two row spacing treatments adjusted for harvest plant population in Iowa, 2004 to 2006.

 
There were no interactions between the main effects of location, row spacing, and the covariate variable (harvest plant population; Table 2), indicating that a similar response existed between yield and harvest plant population for both 38- and 76-cm row spacing. The square of the covariate harvest plant population (P2) was significant and indicated that the yield response to harvest plant population was not strictly linear, and that a maximum yield and harvest plant population existed (Table 2). Quadratic regression equations were determined and separate intercepts existed for each location and row spacing with a common slope and quadratic component among models. Equations did not always produce high R2 values, however, this is not uncommon (Pedersen and Lauer, 2002), and equations presented do account for all sources of variation. Practically poor fits occurred at Whiting (Table 4 ). Residuals were plotted against the predicted values and were normally distributed (data not shown). Normal distribution of the residuals indicates that while the model did not explain a high percentage of the variation the yield response would not be biased more by high or low harvest plant populations. The harvest population required to achieve maximum yield was 462,200 plants ha–1 (Table 4). Even though our seeding rates were well above this value the final harvest populations attained in our study averaged 402,700 plants ha–1 at harvest for the highest seeding rate. At this final population, yields were >99% of the predicted maximum yield, however, we present the maximum yield attained in each location by row spacing combination if the predicted maximum yield was greater (Table 4). Yields remained very stable even with large declines in harvest plant population. For all locations and row spacing combinations >95% of the maximum yield was attained at harvest populations between 118,800 and 213,800 plants ha–1 depending on location and row spacing. (Table 4). Seeding rates of 185,000 and 309,000 seeds ha–1 were sufficient to achieve this final population for either row spacing (Table 4). These seeding rates are 30 and 17% less than the current seeding rates used by producers in Iowa in 38- and 76-cm row spacing, respectively.

Large changes in harvest population were required to cause significant yield reductions indicating that the majority of populations tested reached phase III yields when increasing harvest populations do not significantly increase yield (Duncan, 1986). This type of response is similar to previous observations by Carpenter and Board (1997), Egli (1988), and Weber et al. (1966) that soybean does have the ability to compensate for space in the canopy and maintain yield.

Seed Mass
An interaction was identified between location and row spacing (Table 2). Seed mass was not influenced by row spacing at Whiting and De Witt, but was reduced by 0.5 g 100 seeds–1 with 38-cm row spacing compared with 76-cm row spacing at Nevada (Table 5 ). This finding is in agreement with other studies (Egli, 1994; Elmore, 1998; Ethredge et al., 1989). Seed mass increased 4% as final plant populations increased from 166,900 to 402,700 plants ha–1. This response in seed mass to seeding rate has been documented in another study using the same soybean cultivar (De Bruin and Pedersen, 2008) and is in agreement with Elmore (1991) who also documented increased seed mass at seeding rates >111,000 seeds ha–1. Our study does differ from other reports (Egli, 1988; Elmore, 1998; Ethredge et al., 1989) that seed mass decreased as seeding rates increased. Previous work has shown that seeds m–2 is a more important determinate of yield than seed mass (Board et al., 1999; De Bruin and Pedersen, 2008; Egli and Zhen-wen, 1991). Increased seed mass without a concurrent increase in yield, as documented in this study, indicates the seed mass increase was compensated for by fewer seeds m–2 at higher seeding rates and would be consistent with the yield response to harvest plant population reported in this study.


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  		Table 5. Location x row spacing interaction for seed mass.

 
Plant Height
Plant height was 17 and 9 cm greater at both Whiting and De Witt, respectively, compared with Nevada (Table 3). Row spacing had no influence on plant height (Table 2) but final populations of 316,000 and 402,700 plants ha–1 increased plant height 2 and 6 cm, respectively, compared with final populations of 166,900 and 258,6000 plants ha–1. Elmore (1991) documented increased plant height was associated with plant population, but only as final stands were >346,000 plants ha–1. Lodging was not a significant problem with greater seeding rates and was not influenced by changes in row spacing (data not shown).

Grower Return
Locations differed in the gross profit that remained to pay land rent, chemicals, depreciation on equipment other than the planter, and labor (Table 6 ). As expected, grower return followed closely with yield and both De Witt and Whiting had greater economic returns compared with Nevada. Production in 38-cm row spacing did not (P < 0.05) produce greater return for 144 ha farms with <50% of the land dedicated to soybean production (Table 6). For farms of 575 and 1294 ha the conversion to 38-cm row spacing was always cost-effective for all three corn–soybean rotations (Table 6) and is in agreement with Lambert and Lowenberg-DeBoer (2003) that conversion to narrow-row production systems is profitable. The covariate harvest plant population was not significant and number of plants at harvest, or seeding rate, did not influence grower return (Table 2).


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  		Table 6. Soybean production grower return differences among three farm sizes, with three corn–soybean rotations, based on yield differences among three locations and 38- and 76-cm row spacing, in Iowa from 2004 to 2006. Grower return differences account for yield change, ownership and repair costs, and hauling and handling charges associated with conversion from 76- to 38-cm row spacing as well as seed cost differences for each seeding rate.

 
Most of the cost increase for 38-cm row spacing production with a split-row planter was a fixed cost. Larger farms and farms with a greater percentage of land in soybean production benefit more from the change to narrow rows due to economics of scale.

Farms that are 288 ha or greater, with a land base >30% dedicated to soybean production, would benefit financially by changing to a narrow row system as long as a yield increase of 124 kg ha–1 can be attainted. Yield increases of 17 to 248 kg ha–1 yield were necessary, depending on farm size and percentage of land in soybean production, to achieve an economic benefit for the conversion to reduced row spacing (Table 7 ). The risk associated with purchasing a narrow row planter decreases dramatically as farm size increases, although small farms could expect a return on this investment because the overall yield benefit from this study and other reports support that narrow row spacing will increase yield by an amount >248 kg ha–1 (Table 7). Only the costs of new equipment were determined and the purchase of used equipment would reduce one or more of the following requirements: farm size, soybean production acres, and yield gain necessary to achieve economic benefit from narrow row soybean production.


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  		Table 7. Soybean yield increase required to cover the economic cost for the conversion from conventional 76-cm row spacing production to 38-cm row spacing production with a split-row-planter for nine farm sizes and three corn–soybean rotations.

 

   CONCLUSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
REFERENCES
 
The majority of farmers in large soybean producing states such as Illinois and Indiana use row spacing <76 cm while Iowa still predominately uses row spacing 76 cm or greater. Both yield and economic benefits were documented in this extensive study to exist for narrow row spacing soybean production in Iowa. Additional cost of a split-row planter was less than the economic gain for most farm sizes and corn–soybean rotation. Smaller farms may not benefit as much from converting to narrow rows given the large fixed costs associated with the planter, but farms >144 ha with levels of soybean production >30% of the land base would benefit economically from narrow row soybean production. Changes in seeding rates contributed to significant yield changes but not to changes in profitability. Adoption of narrow row spacing and seeding rates less than current production recommendations could be used to reduce production costs and increase yield and profitability.


   ACKNOWLEDGMENTS
 
The authors thank Jodee Stuart and Adriana Murillo-Williams for their technical assistance and Joseph Lauer for his help conducting the analysis of covariance with plant population. This research was funded by the Iowa Soybean Association.

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 AND DISCUSSION
 CONCLUSION
 REFERENCES
 




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