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SOYBEAN

Soybean Seed Yield Response to Planting Date and Seeding Rate in the Upper Midwest

Iowa State University, 2104 Agronomy Hall, Ames, IA 50011

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

	ABSTRACT

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

 
Planting date and seeding rate are agronomic decisions that producers can use to maximize soybean [Glycine max (L) Merr.] seed yield and economic return. Current information on the response to planting date and seeding rate may underestimate the yield penalty for delayed planting in northern climates and overestimate the seeding rate required to optimize yield. The objective was to determine the effect of planting date and seeding rate on soybean seed yield at environments with different yield potentials in Iowa. Field experiments were conducted in six locations between 2003 and 2006 for a total of 13 environments. Four seeding rates were planted at four dates between late April and the middle of June. No yield difference existed between the late April and early May planting dates and the yield decline rate between each date was consistent among the six locations. Harvest plant populations were not influenced by planting date indicating that plant establishment does not pose a limitation to late April planting. Harvest plant populations required to reach 95% of the maximum yield were as great as 290,800 or as low as 194,000 plants ha^–1, for locations seeded in 38-cm row spacing and 157,300 to 211,800 plants ha^–1 for locations seeded in 76-cm row spacing. Planting in late April or early May increased economic return ha^–1 but there was no difference between seeding rates of 185,300 and 556,000 seeds ha^–1, accounting for additional seed costs. In the cool spring climate of the upper Midwest soybean producers can increase yield by planting soybean 1 to 2 wk earlier than current planting dates. These data also suggest that the optimal seeding rate for most locations is less than current seeding rates.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND 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 April 1, 2007.

	INTRODUCTION

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

 
PLANTING SOYBEAN 1 May compared with 15 May has increased yield by 0.2 Mg ha^–1 in Minnesota (Lueschen et al., 1992) and 0.3 Mg ha^–1 in Wisconsin (Grau et al., 1994), but has shown no benefit in either northern or southern Iowa locations between 1994 and 1996 (Whigham, 1998a). Yield benefit from earlier planting increased rapidly as planting was delayed to late May or early June compared with early May planting in states of the upper Midwest (Anderson and Vasilas, 1985; Elmore, 1990; Lueschen et al., 1992; Oplinger and Philbrook, 1992; Pedersen and Lauer, 2004; Whigham et al., 2000; Wilcox and Frankenberger, 1987). Other reports document where no yield response occurs to delayed planting (Elmore, 1990; Oplinger and Philbrook, 1992; Lueschen et al., 1992), and in one study, there was a negative response at one location in 1 yr (Lueschen et al., 1992). Magnitude of the response to early planting can be year (Pedersen and Lauer, 2003), location (Lueschen et al., 1992), and cultivar specific (Elmore, 1990; Grau et al., 1994).

Soybean compensates for space in the canopy by adding branches, resulting in no yield response from increased seeding rates (Carpenter and Board, 1997). Final plant stands as low as 70,000 plant ha^–1 in equidistant spacing (Egli, 1988) or as high as 388,000 plants ha^–1 (Oplinger and Philbrook, 1992) have produced optimum yields. Other studies indicate that final plant populations of 250,000 to 350,000 plants ha^–1 are sufficient to maximize yield (Beuerlein, 1988; Elmore, 1991; 1998; Weber et al., 1966; Wiggans, 1939).

Major soybean yield components are the number of seeds produced m^–2 and individual seed mass (Board et al., 1999; Egli, 1998). Past research has documented compensatory effects between these two yield components (Board et al., 1999; Sadras, 2007). Little information exists to document yield component changes that occur with changes in time of planting and seeding rate. Elmore (1990) reported that pod number plant^–1 decreased as planting was delayed and was in agreement with Anderson and Vasilas (1985). Seed mass response varied between the studies as seed mass declined (Elmore, 1990) and increased (Anderson and Vasilas, 1985) from delayed planting. Yield component changes from different seeding rates for indeterminate cultivars indicated that both seed mass (Egli, 1988; Weber et al., 1966) and seeds m^–2 decreased as seeding rate was reduced (Egli, 1988). Response of primary soybean yield components to planting dates and seeding rates for the Upper Midwest would be useful for identifying primary yield limitations at late planting dates.

Soybean planting typically begins when corn (Zea mays L.) planting is complete but can be further delayed based on the recommendation that planting should begin after soil temperature reaches 10°C (Hoeft et al., 2000). Soil temperature data for the past 5 yr indicates that in the north, central, and southern zones of Iowa, the soil temperature at 10-cm soil depth was >10°C and was adequate for planting by the last week of April (Table 1 ). Probability of a killing frost event (<–1.6°C) (Meyer and Badaruddin, 2001) in Iowa during spring is less than 25% after 25 April for areas south of 42° latitude (south and central) and after 1 May for the area north of that latitude (Table 1). Time to emergence for planting dates in late April has been estimated at 7 to 14 d (Andales et al., 2000) and by that time the potential of late spring frost would be reduced to zero. Based on the 5-yr average (1999–2003), <10% of the soybean acres are planted by 2 May, with 80% planted by 30 May (National Agriculture Statistics Service, 2007) even though soil temperatures are >10°C before 2 May.

Current recommendations in Iowa advise producers to begin planting by the last week of April but indicate that equivalent yields can still be attained by planting in the middle of May (Whigham, 1998a; Whigham et al., 2000). Seeding rates used in Iowa differ depending on row spacing. Target plant populations at harvest have been 272,000 and 321,000 plants ha^–1 (Whigham, 1998b). Common seeding rates used in the state to ensure this harvest population are 494,200; 444,800; and 370,000 seeds ha^–1 for 19-, 38-, and 76-cm row spacing, respectively. Interactions between planting date and seeding rates have led to the recommendation to increase seeding rates at late planting dates (Beuerlein, 1988; Whigham et al., 2000). Increased seeding rates can be part of a weed management strategy (Nice et al., 2001), but seed costs have increased rapidly within the last decade and low-cost postemergent weed control programs are available.

Specific knowledge of planting date responses at different locations is necessary to determine whether early planting should be a general recommendation. Interactions between cultivar, poor early season growing conditions that limit seedling emergence and growth, and soilborne pathogens may reduce any benefit to planting earlier in the season (Grau et al., 1994). It is therefore hypothesized that the yield response from planting date varies by location, depending on abiotic and biotic stress and yield potential, and that seeding rate is not planting date specific if weeds are controlled. The objective of this study was to determine the effects of planting date and seeding rate on soybean yield, primary yield components, and harvest plant population at locations with different yield potentials.

	MATERIALS AND METHODS

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

 
Studies were conducted at Ames, Crawfordsville, and Nashua between 2003 and 2005 and at De Witt, Nevada, and Whiting, IA between 2004 and 2006. Specific location positions, soil characteristics, and fertility measurements are listed in Table 2 . Experimental design was a randomized complete block in a split-plot arrangement with four blocks. Main plots were four planting dates and subplots were seeding rates of 185,300; 308,900; 432,400; and 556,000 viable seeds ha^–1. Target planting dates were the fourth week of April, second week of May, fourth week of May, and the second week of June. Actual planting dates are listed in Table 3 and did not always match target dates due to field conditions that limited timely planting. Fields were chiseled (Ames, Crawfordsville, Nashua) or plowed (De Witt, Nevada, Whiting) in the fall and cultivated in the spring. Cultivars were NK S24-K4 (Syngenta Seed, Minneapolis, MN) at Ames and Nashua, DKB 36–51RR (Monsanto Company, St. Louis, MO) at Crawfordsville, and AG 2801 (Monsanto Company, St. Louis, MO) at De Witt, Nevada, and Whiting. All cultivars were glyphosate [N-(phosphonomethyl) glycine] resistant. Seeds were inoculated with Bradyrhizobium japonicum (EMD Crop BioScience, Brookfield, WI). Plot size was 4.6 by 15.2 m at Nashua and 2.7 by 6.1 m at Ames, Crawfordsville, De Witt, Nevada, and Whiting. Row spacing was 76-cm and planted with a 7000 John Deere planter (John Deere Corp. Moline, IL) at Crawfordsville and Nashua. An Almaco grain drill (Almaco, Nevada, IA) with 38-cm row spacing was used at Ames, De Witt, Nevada, and Whiting.

Yield was determined by harvesting the center four and two rows of the 38- and 76-cm row spacing, respectively, with an Almaco plot combine and adjusted to moisture content of 130 g kg^–1. Other data acquired at harvest were final plant population and seed mass based on a sample weight of 300 seeds. Seeds m^–2 was calculated based on seed mass and yield (Board and Modali, 2005).

Economic return for the four seeding rates was evaluated by using a partial budget analysis that included only the factors that changed between treatments. Gross revenue from the sale of the soybean seed was determined and the economic return was adjusted for the additional seed costs for higher seeding rates. Soybean sale price was determined based on the method of Stanger and Lauer (2006). Fifty percent of the crop was sold in November and 25% of the crop was forward marketed to both March and July. Between 2001 and 2005 the average 5-yr cash price was $0.203 kg^–1 (National Agriculture 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. Final sale price was $0.213 kg^–1. Seed cost was set at $1.2 kg^–1, hauling and handling charges were $0.00054 kg^–1 (Duffy and Smith, 2006) and seeds kg^–1 was set at 6600 seeds kg^–1. Equipment and drying costs were assumed to remain the same or have little influence on the economic return.

Data were analyzed using Proc Mixed in SAS (SAS Institute, 2003). Years and blocks were treated as random effects and all other effects were considered fixed. Box plots and residuals were used to evaluate variance assumptions. Variance was determined to be homogenous for seed yield, seeds m^–2, seed mass, and harvest plant populations. A combined analysis was conducted for all variables. Seed yield reductions due to delayed planting were assessed by mean separation among planting dates using least significant difference. Rate of seed yield decline between planting dates was determined for each block based on the difference between late April-early May, early May-late May, and late May-early June planting dates. Differences between planting dates were divided by the number of days between planting and converted to seed yield decline wk^–1. Comparison of rates among locations for each planting delay was determined by least significant difference. Differences among rates for the three planting delays, within a location, were determined by testing between the three delays using a 2 degree-of-freedom contrast. Linear models were fit between seeding rate and harvest plant population for each of the six locations using Proc Reg in SAS to determine plant establishment rates. Slope and intercept from each model was used to predict the required seeding rate necessary to achieve desired harvest population for maximum and 95% of the maximum yield. Fitted curves, between the harvest plant population and yield were selected based on model significance and coefficient of determination (R²) values using Proc NLIN in SAS. Theses curves were used to predict the plant stand that gave yields 95% of the maximum.

	RESULTS AND DISCUSSION

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

 
Rainfall varied considerably among years and locations (Table 4). Below normal rainfall occurred at Crawfordsville in 2003, De Witt and Nashua in 2004, and Nevada and Whiting in 2006. At Nevada in 2006 rainfall totals were 58, 97, and 31 mm below average in May, June, and July, respectively. Rainfall was below normal at Whiting in June, July, and August each year of the study even though plots were irrigated at various times in the season. Air temperature in May and August tended to be slightly cooler than normal at all locations for all years. At Nevada and Whiting in 2006 June air temperature was 2.7 and 2.1°C above normal, respectively but was normal at other locations.

Locations in this study represent the wide range of growing conditions that exist in Iowa. Average soybean yield for the state between 2002 and 2006 was 3118 kg ha^–1 (National Agriculture Statistics Service, 2007). Locations close to this average were considered average-yield potential environments and locations above this average were considered high-yield potential environments. Whiting and De Witt were considered high-yield environments as both have well-drained soils and high fertility levels (Table 2). Other locations were average-yield potential environments. Nevada and Ames are located in the Des Moines lobe (Steinwand and Fenton, 1995), an area with high pH, low P availability, and poorly-drained soils. Phosphorus levels were well below test levels in the Nevada, 2005 location (Table 2). Nashua, a northern environment is on highly erodible land with a thin A-horizon, making it rainfall dependent. Crawfordsville is a southern environment with well-drained soil. However, during this study it received below normal rainfall (Table 4).

Harvest Plant Population
An interaction between location and seeding rate indicated that plant establishment was not equal at all locations (Table 5 ). This interaction was separated using the slice statement in SAS but results were inconclusive. Populations were different from one another at all locations but for each seeding rate there was no difference among locations. Based on the two-way means the interaction occurred at the 556,000 seeds ha^–1 seeding rate. Plant establishment was lower at Whiting and at Crawfordsville compared with establishment at Nashua (Table 6 ). Harvest plant populations averaged 273,600 plants ha^–1 for late April planting and were not increased by delayed planting. No interactions existed with planting date. Oplinger and Philbrook (1992) reported that plant establishment increased at later planting dates as a result of increased soil temperature. Our study indicated that soil temperature and soil moisture do not pose a limitation to plant establishment at late April planting, regardless of location, or seeding rate, and is more consistent with data reported by Lueschen et al. (1992).

Among 13 environments planted from late April to early June there was no planting date by seeding rate interaction (Table 5). Our data do not support increased seeding rates at later planting dates. This study did not test extremely late planting dates (later than 15 June) that may occur if replanting is necessary. Our research was similar to past studies that indicated a yield response to more plants ha^–1 but only to the point where yield becomes independent of plant density (Duncan, 1986). Seeding rate did influence yield but did not interact with location or planting date (Table 5).

Optimal seeding rate varied among locations (Table 5). Separate equations were used at each location to predict the harvest plant population required to achieve yields that were 95% of the maximum yield of a location (Table 9 ). Ninety-five percent of the maximum yield was close to the LSD based on P = 0.05 and was representative of the point where nonsignificant yield differences occur. Establishment rates for seeding rates and harvest plant populations ranged from 0.57 to 0.84 and averaged 0.67. Four of the locations were seeded in 38-cm row spacing. Ames and Nevada are located in central Iowa and required final stands of 290,800 plants ha^–1 with seeding rates of 345,700 seeds ha^–1 for 95% of the maximum yield (Table 9). Final populations of 194,000 plants ha^–1 and seeding rates of 199,000 seeds ha^–1 were necessary to achieve 95% of the maximum yield at the high-yield potential locations, Whiting and De Witt. Nashua and Crawfordsville were seeded in 76-cm row spacing and produced yields 95% of the maximum with harvest stands of 157,300 and 211,800 plants ha^–1 with seeding rates of 173,200 and 253,900 seeds ha^–1, respectively.

Accounting for additional seed costs ($22.40 ha^–1 123,500 seeds^–1), the economic return from the four seeding rate treatments was not significantly different (Table 7). Among locations the seeding rate of 185,300 seeds ha^–1 had an economic return equal to other seeding rates because the reduction in seed cost offset the reduced seed yield. Harvest plant populations from the 185,300 seeding rate averaged 171,200 plants ha^–1 averaged across locations. These populations would not be sufficient to achieve the highest yield, but due to increased seeds costs, represent the economic seeding rate. This economic seeding rate is less than current seeding rates regardless of row spacing. Previous studies have not reported economic seeding rates for the upper Midwest. As seed costs continue to increase, greater emphasis should be placed on economic seeding rates compared with seeding rates that produce the greatest yields.

Primary Yield Components
Number of seeds m^–2 was similar among locations and correlated with yield even though differences among locations were >1000 seeds m^–2. Seeds m^–2 was the yield component that showed a continual reduction to delayed planting and accounted for the yield decline from late April to late May planting. Similar to yield no reduction in seeds m^–2 occurred at Nevada between early and late May planting. Seeding rates did not change seeds m^–2 (Table 7).

Seed mass response to planting date varied by location and seeding rate (Table 5), but separating the location x planting date x seeding rate interaction by the slice statement in SAS, showed no conclusive trend to be evident. Seed mass was equal between the first two planting dates at Whiting and the first three at Nevada and De Witt (data not shown). By the June planting date, seed mass had declined 10% at Nevada, 8% at Whiting, and 3% at De Witt from the earlier planting dates. At Nashua and at Ames seed mass increased for the last two planting dates compared with the first two planting dates. Our data indicate that location conditions determine whether seed mass responds positively or negatively to delayed planting and is in agreement with previous research (Elmore, 1990; Anderson and Vasilas, 1985).

Range in seeds m^–2 for locations and treatments was 1717 to 3463 seeds m^–2 (102% difference) and average seed mass varied between 120 and 164 mg seed^–1 (37% difference). These changes resulted in yield differences as great as 190%. A strong linear relationship (R² = 0.89) existed between seeds m^–2 and yield (Fig. 1 ) but not between seed mass and yield (data not shown). Based on these data, seeds m^–2 was the component that drove yield responses between planting dates. Regression analysis indicated that the response slope between seeds m^–2 and yield was 1.93x and did run parallel to the reference lines of known seed mass (seed mass x seeds m^–2 = yield). As seeds m^–2 increased, seed mass also increased to further improve seed yield. Soybean in high-yield locations produced more seeds m^–2 and a greater number of seeds did not limit seed fill. At both De Witt and Whiting the greatest number of seeds m^–2 and the largest seeds occurred at the late April planting date (data not shown). One location (Nashua 2004) produced very large seeds ( >160 mg seed^–1) at the two later planting dates, but yields were <3500 kg ha^–1 due to low seeds m^–2. Greater seed mass has limited ability to increase seed yield and full yield potential is not attained due to a seeds m^–2 limitation. As planting was delayed to late May or early June seeds m^–2 was the yield component most notably affected and posed the greatest limitation to achieving high yields. Changes in seed mass were variable and location specific and are not the major yield limitation at later planting dates in the upper Midwest.

View larger version (20K): [in this window] [in a new window]   Fig. 1. Seed yield response to changes in seeds m–2 and seed mass at Ames, Crawfordsville, De Witt, Nashua, Nevada, and Whiting, IA, from 2003 to 2006. Solid lines are reference lines for three seed masses (mg seed–1 x seeds m–2). The dashed line is the regression line for all data points.

	CONCLUSIONS

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

	ACKNOWLEDGMENTS

 
The authors thank Adriana Murillo-Williams and Jodee Stuart for their technical assistance. 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 CONCLUSIONS REFERENCES

 

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