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PRODUCTION PAPER

Row-Spacing and Seeding Rate Effects on Glyphosate-Resistant Soybean for Mid-Atlantic Production Systems

Dep. of Nat. Resour. Sci. and Landscape Architecture, Univ. of Maryland, Room 1112-B, H.J. Patterson Hall, College Park, MD 20742-4452

^* Corresponding author (rk32{at}umail.umd.edu).

Received for publication July 3, 2003.

	ABSTRACT

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

 
Glyphosate [N-(phosphonomethyl)-glycine]-resistant soybean [Glycine max (L.) Merr.] production in the Mid-Atlantic occurs under full-season and double-crop production systems. Row-spacing and seeding rate manipulation affects the yield of soybean. This study evaluated the effects upon yield caused by reductions in seeding rate from current standards for these two systems. During 2000–2002, row spacing (19 and 38 cm) and seeding rates (current standard, 20 and 40% less, and 20% greater than the standard) were tested using four glyphosate-resistant cultivars representative of the maturity groups for the region. Tests were conducted for both full-season and double-crop systems at two Maryland locations for each per year on coastal plain soils. During the 3 yr, only 1 out of 48 total cultivar x row spacing comparisons for the two production systems had better yield with 38-cm rows. At best, 38-cm row spacing produced equivalent to the 19-cm rows. A 20% seeding rate reduction (345800 seeds ha^–1 for full-season and 444600 seeds ha^–1 for double-crop production) was generally found to not yield significantly different than the standard seeding rates for both systems. A 40% reduction consistently had yield significantly less than the standards in both systems. These results indicate a 20% seeding rate reduction can be a cost-saving practice for glyphosate-resistant soybean production in the Mid-Atlantic. With no yield loss at a 20% reduced seeding rate, additional profit ranging from $14.30 to $27.72 ha^–1 can be realized.

Abbreviations: LAI, leaf area index

	INTRODUCTION

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

 
GLYPHOSATE-RESISTANT soybean is produced in both full-season and double-crop (following small grain) production systems using both conventional tillage and no-till practices in the Mid-Atlantic. Shibles and Weber (1966) reported that to attain maximum yield potential for soybean, it is necessary to establish a leaf area indiex (LAI) of 3.2 or greater by the R2 reproductive stage as defined by Fehr and Caviness (1977). It has since been learned that a LAI of 3.5 to 4.0 by early reproduction is necessary to achieve 95% light interception and that this level of light interception is positively correlated with yield (Board and Harville, 1992; Westgate, 1999). A number of production practices (choice of relative maturity for cultivars at a location, suitable planting dates, row spacings, and plant populations) that can influence the attainment of the desired LAI have been investigated extensively.

Two commonly used row spacings in the Mid-Atlantic region are 19 and 38 cm, reflecting farmers' use of either grain drills or splitter-mounted planting units on row crop planters, respectively, for planting soybean. The adoption of narrower row planting has been based upon numerous, favorable reports concerning reduced row spacing for soybean production (Cooper, 1977; Costa et al., 1980; Ablett et al., 1984; Walker and Fioritto, 1984; Board et al., 1990; Lueschen et al., 1992; Board and Harville, 1994; Egli, 1994; Devlin et al., 1995; Bowers et al., 2000; Heatherly et al., 2002). The one limitation to these findings regarding narrow-row soybean is that all the studies were done with non-glyphosate-resistant cultivars.

Current seeding rate recommendations for soybean production provided by Cooperative Extension specialists in the Mid-Atlantic have been established with field studies conducted primarily with non-glyphosate-resistant cultivars. These recommendations can vary considerably from state to state with a range from approximately 247000 to 432000 live seeds ha^–1 for full-season production and from 445000 to 556000 live seeds ha^–1 for double-crop production. The recommendation standards in Maryland for these two production systems are on the high end of those ranges at approximately 432000 and 556000 live seeds ha^–1, respectively, for full-season and double-crop production (Kenworthy and Wood, 1999). The lower rate is recommended for full-season soybean because it is planted 4 to 6 wk earlier than double-crop production soybean. This early-season planting allows a longer growing season for development of the desired LAI. Another reason that the lower seeding rate recommendation exists for full-season soybean is because it is generally planted into tilled soil conditions that favor good seed–soil contact conducive for suitable germination and stand establishment. The higher seeding rate is recommended for double-crop production soybean because it typically is planted no-till into either wheat [Triticum aestivum (L.) Merr.] or barley [Hordeum vulgare (L.) Merr.] stubble. This higher seeding rate provides compensation for seeds that do not attain good soil contact due to the high level of small-grain residue that can be present. This higher rate has been found to improve the potential for attainment of a LAI of 3.5 to 4.0 within the shorter growing season that is available for double-crop production (Ball et al., 2000).

Glyphosate-resistant soybean has been widely accepted throughout the Mid-Atlantic region. The technology fee that accompanies purchase of glyphosate-resistant soybean causes this seed to be more expensive (on average $20 to $25 ha^–1) than seed for non-glyphosate-resistant soybean cultivars. Another factor that has increased the cost of production with the use of glyphosate-resistant soybean is the inability of producers to retain part of their harvested crop for seed, something that many producers had previously done as a cost-saving practice. It is argued that the increased seed costs are offset by a reduction in herbicide costs attained with the use of glyphosate for weed control. However, the recent confirmation of a glyphosate-resistant weed, marestail (Conyza canadensis), in the Mid-Atlantic region (Ritter, personal communication, 2003) has producers pondering the necessity for increased herbicide inputs that may result in even greater production costs.

One potential cost-reducing practice is seeding rate reductions for glyphosate-resistant soybean below currently recommended standards. However, little information is available about seeding rate recommendations for glyphosate-resistant soybean since current recommended standards have been established using non-glyphosate-resistant cultivars. In addition, seeding rate recommendations vary a great deal from state to state in the Mid-Atlantic region, making it difficult for a producer to determine a suitable seeding rate goal. Norsworthy and Frederick (2002) reported that a seeding rate 40% below the current recommended standard of 620000 seeds ha^–1 for glyphosate-resistant soybean grown in a full-season, narrow-row production system in South Carolina produced equal grain yield to the recommended standard. They concluded that this reduced seeding rate could be a management practice for lowering seeding costs without decreasing yields for southeastern coastal plains soils under growing conditions that did not include prolonged periods of drought stress. Holshouser and Whitaker (2002) investigated glyphosate-resistant soybean populations ranging from 103000 to 850000 seeds ha^–1 for early soybean production systems on coastal plains soils in Virginia. They concluded that a population of only 208 000 plants ha^–1 was necessary for maximum yield at sites that had only brief drought-stress periods. However, when drought stress limited LAI, plant populations of 600000 plants ha^–1 were necessary for maximizing yield.

These two studies concurred that a seeding rate reduction for glyphosate-resistant soybean may be a cost-saving practice for production of soybean on coastal plain soils when there is limited drought stress. However, they did not address the identification of a guideline rate that farmers could use. Our study evaluated a range of seeding rates within the two most commonly used row spacings for full-season and double-crop production systems in Maryland. The objectives of this study were (i) determine if seeding rate reductions from the current standards for the two production systems would produce equivalent grain yields to those standards and (ii) determine if the yield potential for soybean produced in the two commonly used narrower row spacings was affected by seeding rates.

	MATERIALS AND METHODS

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

 
During 2000 through 2002, glyphosate-resistant soybean cultivars were grown under both full-season and double-crop production systems at two locations each year (Table 1). A randomized complete block design (three blocks per location) with a split-split plot arrangement of experimental treatments was used. Whole plots were the two row-spacing treatments (19 and 38 cm). Both the 19- and 38-cm treatments were planted with a modified Great Plains grain drill (Great Plains Manufacturing, Inc., Salina, KS) that was equipped with 15 planter units that were spaced 19 cm apart. This drill was equipped with two cone-type seed distribution units (ALMACO, Nevada, IA) that allowed for simultaneous planting of two adjacent plots. The 19-cm plots consisted of seven rows, and the 38-cm plots had four rows each. Plot length was 7.6 m. The split plots were four glyphosate-resistant soybean cultivars (Table 2). These cultivars represented the range of relative maturity for soybean cultivars commonly grown in Maryland. The split-split plots were the four seeding rate treatments of (i) the recommended seeding rates for full-season (432250 seeds ha^–1) and double-crop (555750 seeds ha^–1) production systems and rates that were (ii) 40 and (iii) 20% less than and (iv) 20% greater than those two recommended rates. Seeding rate treatments were individually packaged for each plot by counting the appropriate number of live seeds for each cultivar.

A combined analysis of variance (ANOVA) for each respective cropping system was performed for each year using PROC MIXED procedure (SAS Inst., Cary, NC). Interactions that occurred were evaluated utilizing the SLICE option of the LSMEANS procedure. Because different cultivars (Table 2) were used each of the 3 yr and substantially different weather events occurred during the 3 yr, the data were not pooled over years. A primary objective of this research was to determine if seeding rate reductions from the currently recommended standards for full-season and double-crop production of glyphosate-resistant soybean were a feasible production strategy for reducing input costs. To reduce input costs, yield at a reduced seeding rate must be at least the same as the yield attained at the recommended seeding rate. For this reason, minimizing the occurrence of a type I experimental error was important since a change in the seeding rate recommendation for glyphosate-resistant soybean production may be made based upon this information. Fisher's Protected Least Significant Difference Test was utilized in this study for the planned comparisons between the recommended standard seeding rate and each of the three seeding rate treatments evaluated. In addition, orthogonal polynomial contrasts were conducted where appropriate to assist with elucidation of differences.

	RESULTS AND DISCUSSION

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

 
Location Effects
Overall, yield differences between locations during a year were not unexpected given the influence that different soil types (Table 1), variations in precipitation (Table 3), temperature extremes, and planting dates (Table 1) can have. For example, for full-season soybean production in 2000, soil type differences between the two locations (Table 1) resulted in nearly twofold better yield for WREC (silt loam soil) compared with CMREC (sandy loam soil), 3841 kg ha^–1 vs. 2137 kg ha^–1, respectively, even though rainfall was timely and sufficient at both locations throughout the growing season (Table 3). Contributing to the lower yield response for full-season production at CMREC was the nearly 2.5 wk later planting date (Table 1). Overall, yield for double-crop production in 2000 was similar at the two locations, 3545 kg ha^–1 (WREC) vs. 3285 kg ha^–1 (LESREC), respectively, where soil type (Table 1) and rainfall distribution (Table 3) were the same.

Drought conditions were prominent throughout 2002 (Table 3), impacting yield for full-season production (1936 kg ha^–1 at WREC and 1797 kg ha^–1 at CMREC). Double-crop production at WREC was also affected by this drought while the double-crop production at CMREC benefited from both more timely and more sufficient rainfall during July and August as well as late summer rains (early September) that resulted in better yield (3425 kg ha^–1) over that attained at WREC (1702 kg ha^–1).

Cultivar Effects
Significant cultivar and location x cultivar differences for yield were observed for both full-season and double-crop production systems (Table 4). Yield differences among cultivars were expected given the influence relative maturity has on a cultivar's performance. Interactions between locations and cultivars were also not an unexpected result as they emphasized the influence environment has on the performance of a cultivar. Though cultivar differences existed, they are only considered to be an important factor if they interacted with the row-spacing and seeding rate variables in this study and will be discussed where pertinent.

For 2001, the combined ANOVA for full-season production indicated a significant row spacing effect with no interactions between this factor and the other treatment variables. Soybean yield was 4316 kg ha^–1 for 19-cm rows and 3732 kg ha^–1 in 38-cm rows.

The ANOVA for 2002 full-season production indicated a significant row-spacing effect and no interactions with the other variables. Yield for 19-cm rows was 2211 kg ha^–1 and 1619 kg ha^–1 for 38-cm rows. The 2002 growing season was highlighted by severe drought (Table 3). The yield advantage observed for 19-cm rows during 2002 was clearly the result of the ability of the soybean produced in narrower rows to generate a closed vegetative canopy earlier than the wider-row treatment, enhancing moisture-holding capacity for the narrow-row treatment and ultimately resulting in better yield.

Row-Spacing Effects—Double-Crop Production
The ANOVA for double-crop production system for 2000 indicated no significant row-spacing effect and no interaction between row spacing and the other factors (Table 4). Average yield for soybean produced in 19-cm rows was 3466 kg ha^–1 compared with 3345 kg ha^–1 for 38-cm row spacing. Timely planting for double-crop production (Table 1) coupled with sufficient rainfall throughout the growing season provided suitable growing conditions that resulted in good double-crop yield in both row-spacing treatments for this year.

The ANOVA for 2001 double-crop system indicated that the row-spacing effect was confounded by both a row spacing x location and a row spacing x cultivar interaction (Table 4). The SLICE option was employed to identify interaction differences. For the location x row spacing interaction, the two row-spacing treatments averaged over the four cultivars at WREC had significantly different yields of 4284 and 3792 kg ha¹ for 19- and 38-cm rows, respectively. This was excellent double-crop yield that was the result of both timely planting (26 June) and sufficient and timely rainfall throughout the growing season (Table 3). At CMREC, no significant row-spacing yield difference was observed. Averaged over the four cultivars, yields of 1864 kg ha^–1 for 19-cm rows and 1954 kg ha^–1 for 38-cm row spacing were observed. The 2001 growing season was highlighted by timely and sufficient rainfall throughout, so double-crop yield at this location should have been better. However, planting date at CMREC was late (16 July), even for double-crop standards. This partially explains the lower yields at this location compared with those attained at WREC. Since this late-planted double-crop soybean crop was on a loamy sand soil at CMREC, significantly better production should have occurred with the 19-cm row spacing treatment, but it did not. An additional reason was an early killing frost (7 and 8 October). On two consecutive days, temperatures dropped to –3°C, causing considerable leaf mortality particularly for the two later-maturing cultivars that had not yet completed seed fill.

Since the combined ANOVA indicated a row spacing x cultivar interaction, but since the two locations performed so differently because of both planting date differences and an early killing frost at CMREC, mean separation analyses were conducted on the two row-spacing treatment means for each of the four cultivars at each location (Table 6). At WREC, three of four cultivars had significantly better production in the 19-cm row-spacing treatment (Table 6). Within the 19-cm row-spacing treatment, no significant yield differences among the cultivars were observed (Table 6). In 38-cm row spacing, yield response among the cultivars varied. Dekalb brand CX 383 had the lowest yield, as expected, but this cultivar was not different from either Asgrow brand ‘4101’ or UniSouth Genetics ‘7528’. These latter two cultivars did not yield differently in 38-cm rows from the top-producing cultivar, Mycogen ‘5472’. It did appear as though the yield performance in 38-cm rows improved with increasing maturity of the cultivars. At CMREC, within a cultivar, no yield differences occurred between the two row-spacing treatments. Within a row-spacing treatment at this location, the significant loss of yield for the two later-maturing cultivars is apparent in both row-spacing treatments (Table 6). The killing frost was particularly devastating to the Maturity Group V cultivar, UniSouth Genetics 7528.

Seeding Rate Effects on Emerged Stands
Actual emerged stand plant populations for 2000 are shown in Table 7. Though the seeding rate goals for the four seeding rate treatments were not attained, incremental increases for the emerged stands were found across the four rates (Table 7). Overall, the emerged stands ranged from 70 to 94% of the seeding rate goals. This range of stand densities was similar to that reported by Norsworthy and Frederick (2002).

Combined ANOVA for 2001 full-season soybean system indicated a significant seeding rate response (Table 4), with no interactions with other variables confounding this effect. The yield for the four seeding rate treatments was 3812 kg ha^–1 (–40%), 4029 kg ha^–1 (–20%), 4175 kg ha^–1 (standard seeding rate), and 4095 kg ha^–1 (+20%), respectively. The 40% reduced rate was the only seeding rate that produced significantly less yield compared with the other three seeding rates. This seed rate response was determined to have both a significant linear and quadratic response, indicating that yields increased across the three lowest seeding rates and then had begun to decline at the 20% increased seeding rate.

The combined ANOVA for 2002 full-season production system (Table 4) indicated a significant cultivar x seeding rate interaction. The SLICE option for LSMEANS procedure was employed, and it indicated that no significant yield differences were present across the four seeding rates for any of the cultivars. However, a great deal of variability among the within-cultivar seeding rate yields was observed (Table 9). This variability was attributed to the season-long drought that occurred during 2002. Within each of the four seeding rates, the late Maturity Group IV cultivar, Pioneer brand ‘9492’, consistently produced better compared with the other three cultivars. This was attributed to the ability of this latest-maturing cultivar to take advantage of the late summer rains that finally occurred following the season-long drought (Table 3).

For 2001 double-crop production, the combined ANOVA identified a significant effect for seeding rate (Table 4) and no interactions between this treatment factor and the others. The yield for each of the four seeding rate treatments averaged over the two row-spacing treatments, four cultivars, and two locations was 2838, 2891, 3062, and 3098 kg ha^–1 for the –40%, –20%, standard, and +20% seeding rates, respectively. Mean separation analysis (Fisher's Protected LSD_0.05) determined that the yield at the 40% reduced seeding rate was significantly less than at the standard seeding rate. However, the 20% reduced seeding rate had yield that was not different from the standard. The standard seeding rate was determined to be not different from the 20% increased seeding rate. Orthogonal polynomial contrast for linear and quadratic response across the four seeding rates was conducted, and it was determined to be not significant for either.

The 2002 ANOVA for double-crop soybean determined the presence of a significant seeding rate effect (Table 4), the same as occurred in 2001. Yields averaged over the two locations, two row-spacing treatments, and four cultivars were 2415 (–40%), 2522 (–20%), 2618 (standard seeding rate), and 2697 (+20%) kg ha^–1, respectively. Mean separation analysis was done using Fishers Protected LSD_0.05 and determined that the 40% reduced rate had significantly less yield compared with the standard seeding rate. The yields for three highest seeding rates were not significantly different. The response for 2002 appeared linear, so orthogonal polynomial contrasts were conducted to determine if this response across these four seeding rates was significant, and it was determined that it was not a significant linear response.

Economic Analysis for Seeding Rate Response
The true value to a reduction in seeding rate from a recommended standard can only occur if there is an economic benefit gained. In other words, either the soybean yield must be no different at the seeding rate reduction, or it must only be reduced by an amount that equals the cost savings that was experienced by the rate reduction. Table 12 indicates the amount of seed required to meet the full-season and double-crop seeding rate standards for three different seed sizes of soybean. The cost of this seed per hectare was determined assuming one price for glyphosate-resistant soybean seed. The amount of cost savings or increased expense was calculated for the three seeding rates evaluated in this study. Savings can range from $14.30 to $43.12 ha^–1, depending upon the rate reduction and seed size that is used for full-season production (Table 12). The savings and expense differences are also reported for double-crop production system (Table 12). Table 13 shows the yield changes required to result in either no profit lost or gained at three different selling prices for soybean for seeding rate reductions or an increase. For example, yield reductions could range from 56 to 118 kg ha^–1 for a full-season system and 72 to 151 kg ha^–1 for a double-crop production system with a 20% reduced seeding rate and not result in any loss of income. This research indicated that significant yield losses generally did not occur for either production system at the 20% reduced seeding rate compared with the standard. If yields do not significantly change as was indicated by these results, increased profit potential with a 20% seeding rate reduction can range from $14.30 to $21.56 ha^–1 for full-season production and an even better range of $18.48 to $27.72 ha^–1 for double-crop production (Table 12).

	CONCLUSIONS

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

During 2 of 3 yr of the full-season study (2000 and 2001), the yield response to seeding rates was relatively consistent. The reduced seeding rate of 40% compared with the standard rate resulted in significantly lower yield. The 20% reduced rate was found to yield comparably to the standard rate. During 2000, the earlier-maturing cultivars responded significantly to the 20% increased rate; however, in 2001, this response was not seen. The drought of 2002 confounded the effects that seeding rate had on yield for full-season production. Averaged over the two locations, all four cultivars were found to not differ at the three lowest seeding rates. However, at each of the locations, a different response occurred. At WREC, a significant yield response was observed for the two higher seeding rates. As reported by Holshouser and Whitaker (2002), this type response would be expected when drought conditions prevail. However, at CMREC, no differences among the four seeding rate treatments were observed. Since the prediction of weather conditions that will prevail for a particular growing season at the time the crop is planted is impossible and since none of the cultivars expressed significant yield differences between the 20% reduced rate and the standard, a seeding rate 20% less than the standard full-season rate of 432 250 seed ha^–1 would appear to be a viable option.

Seeding rate response for double-crop production was straightforward in 2 of 3 yr (2001 and 2002). During those 2 yr, very different weather conditions prevailed. However, soybean yield responded to the seeding rate treatments during both those years similarly, and they were not confounded by interactions with row spacing, cultivars, and location. A 40% reduced seeding rate had yield that was significantly less than the standard. During both those years, the 20% reduced rate produced yield equivalent to the standard seeding rate. And, during those 2 yr, there was no significant yield benefit for double-crop soybean to the 20% increased seeding rate. During 2000, location and cultivar both influenced the seeding rate response to some degree. During this year, which had timely and sufficient rainfall, yield differences among the seeding rates were less evident. Only one of the eight cultivar x location combinations had a significant yield difference between the 40% reduced rate and the standard seeding rate. And, for the comparison between the 20% reduced rate and the standard, there were no significant yield differences observed. A 20% reduction in seeding rate from the standard rate of 555750 seed ha^–1 for double-crop production did not cause significant yield loss.

Since yield differences between the 20% reduced rate and the currently recommended standard were generally not observed, a 20% seeding rate reduction to approximately 346 000 and 445000 seeds ha^–1 for glyphosate-resistant soybean grown under full-season and double-crop production systems, respectively, in the Mid-Atlantic can be an option for reducing input costs for both full-season and double-crop production systems. This result concurred with the results observed by Norsworthy and Frederick (2002) for southeastern coastal plains soils. A 40% reduced seeding rate to approximately 259000 and 333000 seeds ha^–1 for each respective production system is not recommended because this 40% reduced rate frequently had significantly lower yield than the recommended standard, even during years of either little or no drought stress such as occurred during 2000 and 2001. Thus, seeding rates that are similar to the 208 000 seeds ha^–1 reported by Holshouser and Whitaker (2002) to have allowed maximum yield under growing conditions that had minimal drought stress are considered too low to be used as a seeding rate guideline by farmers.

There was no occurrence of interactions between row spacing and seeding rate. These new seeding rate recommendations of 346000 and 445000 seeds ha^–1 for glyphosate-resistant soybean grown under full-season and double-crop production systems, respectively, in Maryland apply to both 19- and 38-cm row spacing.

	ACKNOWLEDGMENTS

 
The authors thank the Maryland Soybean Board for grant support and are grateful for the field assistance provided by the Maryland Agricultural Experiment Station's R&E Centers farm crews.

	NOTES

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

 
Mention of trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by University of Maryland and does not imply its approval to the exclusion of other products or vendors that may also be suitable.

	REFERENCES

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

 

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