Thesis: Double crop soybean yield response to seeding rate and maturity


Introduction     3

Materials and Methods    8

Results and Discussion    16

Conclusions    26

References    27

Appendix    29


Soybean [Glycine max (L.)Merr.]producerscan boost output through doubling the cropping of soybeans on the millions of acres of wheat(Triticumaestivum L.) every year. Even though production prospective for double-crop of soyabean is decreased through delayed planting, doubling the cropping of soybean following wheat turns into economically feasible while contributing the worth of 20 to 30 bushels for every acre of soybean to the worth of the wheat plant. Doubling the planting of soybean and wheat present chances to augment soybean acreage and boost productivity for the farmer.
Nevertheless, double cropping outputs are often restricted because of the truth that farmers do not spend in good supervision processes or latest technologies. The date of planting and ripeness are agronomic choices that growers can employ to capitalize on soybean seed produce and financial return. Modern information on the reaction to date of planting and maturity might undervalue the yield punishment for belated sowing of double-crop soybean and overvalue the rate of seeding needed to capitalize on output. Elevated crop populations can encompass several benefits such as faster canopy conclusion, superior light interruption, and lesser weed rivalry. Nevertheless, grain output does not at all times augment as crop population augments. While the quantity of plants for every acre augments, every crop attracts less light that confines every plant’s development and development. Elevated crop populations may as well augment intraspecific rivalry for water and nutrients, support plant lodging, and contribute seed expenses.
Soybean crops are very adaptable. Once crop populations are small, independent soybean crops augment their leaf area that permits every crop to attract extra sunlight and develop additional branches that in turn permits every plant to create extra pods. This feature implies that small soybean crop populations may present competitive outcomes to high soybean plant populations.
Soybean plants that are planted later than the normal planting timearetypically shorter in height because of the early start and conclusion of the flowering procedure in connection to vegetative phases; to dry climate following materialization that frequently restricts development; and since elevated temperatures, particularly at nighttime, fast growth of the plant. These effects are frequently larger as the soybean seeds are sowed into dehydrated earth and have to anticipate rain for germination to occur. The maturing wheat plant takes out water from the upper earth contour and might leave it extremely dehydrated at the occasion of wheat produce and double-crop soybean sowing. An untimely fall frost can also damage the soybean harvest that naturally requires all of a standard budding period to attain ripeness. Output prospective of double-planted soybeans is normally 40% to 60% of that got with complete-season soybean sowed during May, although double-planted soybean yields differ broadly.
Results from other soybean planting date studies show the variability in grain produce over years and surroundings in soybean reaction to sowing date in Figure 1.  Planting in the first half of May typically generates the finest produces. Sowing in April can decrease yield, planting delayedat the end of Mayregularly carries moderate penalties, whileplanting in June results in larger yield penalties.

Figure 1.  Soybean planting date response
Illinois Agronomy Handbook: Chapter 3; Emerson Nafziger,
University of Illinois Department of Crop Sciences

Depending on the truth that delayed sowing makes the accessible growing period so small, several consider that employing shorter ripeness choices creates sense. That may not be the situation: If a diversity that is early for a position is sowed extremely belatedly, vegetative growth before flowering is very restricted, and crops will frequently finish up extremely short, having partial canopies and short pod and node amounts yet in narrow rows. Rather, the finest choices for double-cropping produces are those that are categorized as mid-season to complete-season for a region. If wheat yield is untimely and soybean sowing can be completed by June 15th to 20th, then use varieties at slightest as overdue as those sowed at the usual period. If soybean sowing is late into July or is into dehydrated ground so the plant will not come out fast, it may be a little less dangerous, from the point of view of evading frost at the conclusion of the growing period, to sow a selection that is around half a ripeness group lesser (MG 4.0 instead of 4.5) (Illinois Agronomy Handbook, University of Illinois ).
With complete-season soybean, crop population can differ broadly without influencing outcome. Nevertheless, within double-crop soybean, an above-standard crop population might boost yield. Furthermore, advanced seeding levels are suggested where soybean are sowed in no-till areas where it is more difficultto control seeding depth and/or achieve adequateplanting slot closure.
The Illinois Agronomy Handbook offers directions for delayed sowing (University of Illinois at Urbana-Champaign, College of Agriculture, Cooperative Extension Service, 2009 – Field Crops).Sowing during the initial half of May normally lead to the greatest outcome.  Planting delayed past the middle of May leads to reduced produces and at an ever-growing level because the calendar time gets afterward. Through the conclusion of June, yield prospective decreases to approximately 50 to 60 percent of what may have been attained with well-timed May sowing. As late planting decreases soybean yield prospective; the produce punishment for late-sowed soybean is not as immense like that for corn (University of Illinois Extension, Extension Ag Update, Ellen Phillips, Extension Educator, and Countryside Extension Center). For late-sowed soybeans, consider increasing the seeding rate. Ellen Phillips, Extension Educator, Countryside Extension Centersuggests that if planting is delayed to June 10, increase seeding rate by 10 to 15 percent, planting is overdue to June 20, boost seeding level by 20 to 30 percent, and if sowing is overdue to July 1, raise seeding level by 50 percent.
Research data has shown that when double cropping wheat-soybeans, a 3.8 maturity variety to a 4.8 variety is well adapted for Southern Illinois (Illinois Grower’s Guide to Superior Soybean Production, University of Illinois College of Agriculture Cooperative Extension, Circular 1200).

Figure 2.  Soybean maturity map

Soybean compensate for room within the canopy through adding branches, leading to no yield reaction from augmented rates of seeding (Carpenter and Board, 1997).  ultimate crop stands as short as 70,000 crops ha–1(28,340 plants/acre) within equidistant distancing (Egli, 1988) or as high as 388,000 plants ha–1(157,000plants/acre)(Oplinger and Philbrook, 1992) have generated optimal produces. Other researches point out that ultimate crop populaces of 250,000 to 350,000 plants ha–1(100-140,000 plants/acre) are adequate to capitalize on produce (Beuerlein, 1988; Elmore, 1991; Elmore, 1998; Weber et al., 1966; Wiggans, 1939).
Soybean varieties for double cropping utilize as greatly of the spell as likely, but they must mature quickly enough to evade yield losses because of frost. Normally, they are medium-spell selections for the usual sowing era in a region. Adapted selections should generate sufficient vegetative development to create a clogged canopy required to shade out wild plants.
Soybean selections that mature early are small, set pods near the earth plane, yield lower, and become problematic to harvest.  Later maturing varieties develop favorable growth,but may remain green at the occasion of the initial killing frost (Figure 2). Determinate semi-dwarf varieties contain low stature and a small flowering era, therefore, should not be planted for double cropping.

Figure 3. Average First Fall Frost Dates for Illinois Based on 1971-2000 Averages.
State Climatologist Jim Angel of the Illinois State Water Survey.

The intention of this research was to determine the effect of seeding rateon soybean seed yield in a no-till wheat-soybean double crop system. The economicvalue of seeding rate of soybean in a no-till wheat-soybean double cropping will also be evaluated.

A field experiment was establishedusing a randomized complete block design (RCBD) in a wheat field in the southwest corner of Washington County in Southern Illinois (Picture 1).  The experiment consisted of three soybeanvarieties (Asgrow 3936, Asgrow 4232, and Asgrow 4632) treated withAcceleron®fungicide and insecticide seed treatment (pyraclostrobin,metalaxyl, fluxapryoxad, imidacloprid) andplanted at three seeding rates(140,000, 180,000, and 220,000plants per acre) at a plot length of 200 ft.Each individual treatment was replicated three times and the trial layout is presented in Table 1.

Table 1.Double-crop soybean trial layout.

Rep 1 Variety Population x 1000
AG3936 140, 220, 180
AG4232 180, 140, 220
AG4632 140, 180, 220
Rep 2 Variety Population x 1000
AG4232 220, 140,180
AG3936 140, 220, 180
AG4632 180, 220, 140
Rep 3 Variety Population x 1000
AG3936 180, 140, 220
AG4632 180, 220, 140
AG4232 220, 180, 140

Wheat was harvested on July 1, 2015 and GramoxoneInteon®(1,1’-dimethyl-4,4’-bipyridinium dichloride)herbicide applied pre-plant on July 2, 2015 at a rate of 48 fl oz/acre.Soybeans were planted no-till at a depth of 1 inch into the wheat stubble using a 7-row, 15-inch row spacing Kinze 2000 planter on July 6, 2015 (Picture 2).  Roundup PowerMAX® herbicide (glyphosate, N-(phosphonomethyl)glycine) was applied post-emergent at a rate of 28 fl oz/acre on July 25, 2015 for in-season weed control.
The field consists of Oconee-Darmstadt-Coulterville silt loams [Oconee: Fine, smectitic, mesicUdollicEndoaqualfs; Darmstadt: Fine-silty, mixed, superactive, mesicAquicNatrudalfs; Coulterville:Fine-silty, mixed, superactive, mesicAericEpiaqualfs] that has phosphorous and potassium levels at optimum levels with a soil pH of 6.8.

Picture 1. Aerial picture of field location.

Picture 2. Kinze 2000 planter.

When choosing a variety to plant for double–crop soybeans, there needs to be a complimentary mixof defensive disease traits, agronomic traits, and yield potential within a maturity group that is adapted for a geography.  The agronomic and disease characteristics for the three soybean varieties used in this study are shown in Table 2.

Table 2.  Soybean trial variety agronomic characteristics and disease tolerance.

AG3936 AG4232 AG4632
Maturity Group 3 4 4
Relative Maturity 3.9 4.2 4.6
Herbicide Tolerant Trait GENRR2Y RR2Y/STS RR2Y/STS
Growth Habit Indeterminate Indeterminate Indeterminate
Flower Color Purple Purple Purple
Pubescence Color Gray Light Tawny Light Tawny
Hilum Color Imperfect Black Black Black
Pod Wall Color Brown Tan Brown
Plant Height Category Medium-Tall Medium-Tall Tall
Emergence 1 2 2
Standability 2 3 3
No-till Adaptability 1 2 2
Iron Chlorosis 5 5 5
Soybean Cyst Nematode MR3 R3 MR3
PRR Resistance RSP1c RSP1a RSP1a
PRR Field Tolerance 4 5 4
White Mold
Brown Stem Rot 1
Sudden Death Syndrome 5 5 5
Southern Stem Canker 3 3 3
Frogeye Leaf Spot 6 5 3
Aerial Blight
Southern Root Knot S S S

*Agronomic and disease rating scale:  1-2 Excellent, 3-4 Good, 5-6 Average, and 7-9 Poor.

Early observations
There were three early plant observations measured on July 16, 2015for this trial.Early stand count, seedling vigor, and seedling emergence were determined three times per treatment replication on randomly selected areas of the center two rows.  Plants on the end of the treatment plots (four plants from the end of each row) were excluded to minimize any end-row effects.
Early stand count – The total number of plants emergedwithin a 36-inch diameter hoop.
Seedling emergence -How even the center four rows emerged from the soil based on a 1-9 scale, 1 being excellent even emergence and 9 having no plants emerged. Picture 3represents a ‘3’ in the rating scale.

Picture 3. Soybean seedling emergence representing a ‘3’ on rating scale.

Seedling vigor- The functioning description selected upon by the Association of Official Seed Analysts in 1979, MEASUREMENT OF VIGOR IN SEEDS OR SEEDLINGS, Margaret S. Gibson, Ph.D., Plant Physiologist, American Crystal Research Center, declared “Seed vigor consists of those seed qualities that establish the prospective for fast even emergence and growth of usual seedlings in a broad variety of field circumstances.”  Varieties were rated on a 1- 9 scale, a ‘1’ rating being where all plants  emerged and were at the same growth stage, and were healthy while a ‘9’ rating would be where no plants emerged.Ratings were taken between V2- V4 growth stages. Picture 4 represents soybeans that would be considered a ‘3’ vigor scale.
Picture 4. Soybean seedling vigor representing ‘3’ on the scale.

In-season observations
Rainfall- Measured from the Climate Corporation, a web-founded dashboard that trails modern climate and forecasts. Weather collects precipitation information from radar mechanisms that are sustained by the National Oceanic and Atmospheric Administration (NOAA) and regional heat from National Weather Service checks. 
Temperature-Measured from the Climate Corporation.The Climate Corporation uses the temperature values reported from temperature stations.
Disease pressure-Evaluate disease presence typically observed during the growing season.
Insect pressure-Amount and species present from emergence to harvest.
Weed pressure- Amount of weeds present that could prevent optimal growth and inhibit growth.

Picture 5.Weed-free soybean stand.

Trial harvest
Yield data was collected on November 8, 2015with a Gleaner R65 Combine (Picture 6). The combine has a yield monitor that collects seed moisture, test weight, and calculates grain yield.

Picture 6. Gleaner R65 combine used to harvest soybean plots.

Harvest grain weight – The total weight of soybeans harvested from each individual plot was taken from a Par-Kan GW-150 weigh wagon(Picture 7) and used to determine grain yield.

Picture 7. Par-Kan weigh wagon used to determine grain yield.

Grain moisture – The percent moisture remaining in the harvested seeds was determined by a calibrated moisture tester (DICKEY-john GAC 2100, DICKEY-john’s parent company is Churchill Industries, Minneapolis, MN).

Harvest Observations
Plant height- Height of the plant from the soil to top pod on the plant at R8 (Full Maturity).
Lodging- Number of plants that lean over and do not stand upright. Scored on a scale of 1 – 9 with ‘9’ representing plants completely flat on the ground and ‘1’ representing standing completely straight. Picture 8 represents a ‘6’ lodging rating.

Picture 8. Lodged soybeans representing a ‘6’ on the rating scale.

Statistical analysis
The data in this planting experiment weredesigned to evaluate the effects of seeding rate on soybean yield (bu/acre) and effect of soybean variety on grainyieldgenerated by SAS (‘Local’, X64_7PRO) (SAS Institute Inc., Cary, NC).Graphical analyses were preformed to identify correlations and any anomalies in the data.  Dependent variables were yield, plant height, stand count, and lodging scores.Treatment means were compared to determine significant differences between observations at a 0.05 alpha interval.


Soybean crops should spend six weeks of excellent growing climate from appearance to initial flower. Normally, Initial flower appears sometime in the initial three weeks of July, based on site, soybean range, and temperatures. Sowing by early June in Southern Illinois generally offers adequate development for superior yield prospective. Later-sowed soybeans, like double-cropped grow quicker during the warmer climate, but they frequently do not create the complete canopy and leafy development (height and node number) required for top produces if not flowering and vegetative development are expanded by favorable climate. Soybean range choice is also significant. You should sow as belatedly of a ripeness group as likely that will ripen prior to the initial frost. This will permit extra time for additional development. Desirable varieties for Southern Illinois range from a 4.0 – 4.8 maturity range.
Climate offers the key share of the ecological impact on soybean growth and yield. Soybeans are finest accustomed to humid summers having abundance of sunshine and adequate precipitation. They will produce well while temperatures are within the higher 80’s if humidity is adequate and have a daily mean temperature of 75-77oF in the summer.
The 2015 climate was cooler and wetter when compared to the 30-year average (Table 3). Rainfall for the month prior to planting was 4.7 inches greater than the 30-year average while receiving 4.06 inches more of rain from planting to harvest compared to the 30-year average. Temperature was cooler for the average low and average high being 5° F and 4.7° F less, respectively. Typically growers can plant their double-crop soybeans the first week in June in Southern Illinois.  Due to excessive moisture, planting was delayed until July 6th.

Table 3.  Rainfall and temperature values for 2015 and 30-year average for location.

Rainfall (inches) 2015 30-year average
June 6th – July 6th (preplant rainfall) 9.09 4.37
Planting date to harvest 12.80 8.74
First killing frost (30° F) October 17 October 15
Average high temperature: June 6-July 6 83° F 88° F
Average low temperature: June 6-July 6 65.4° F 70.1° F
Average high temperature: July 6-harvest 78° F 82° F

Grain Yield

For grain yield, main treatments of variety and plant population were significant; however, there was a significant variety x plant population interaction for grain yield.

Table 4.  Soybean yield for the three soybean varieties at the three planting rates in the
wheat-soybean double cropping experiment.

Plants/acre AG3936 AG4232 AG4632 Average
140 K 44.1 b 34.7 c 43.1 b 40.6
180 K 50.3 a 51.7 a 52.7 a 51.6
220 K 44.3 b 42.9 b 50.8 a 46.0
Average 46.2 43.1 48.9

*Numbers containing the same letter are not significantly different at the alpha = 0.05% level.

When looking at the yield results shown in Table 4 and Graph 1, there were no significant yield differences across soybean varieties at the 180,000 population and at the 180,000 plant population, the yield was the highest for all three varieties. The 140,000 population yields for all three soybean varieties were significantly lower than the 180,000 population yields. When the plant population was increased to 220,000, there was a significant decrease in grain yield with two of the soybean varieties (AG3936 and AG4232) compared to AG4632 which showed a similar yield at the 180,000 and 220,000 planting population.  At the 140,000 plant population, AG4232 showed significant yield reduction compared to the AG3936 and AG4632 varieties.  In regards to the soybean maturity and grain yield relationship, there was a significant difference between the AG4632 (48.9 bu/a) and AG4232 (43.1 bu/a) primarily resulting from a higher lodging score (Table 9).

Graph 1.  Grain yield of three soybean varieties at three planting populations
in a wheat-soybean double cropping system.

Plant Height
Plant height showed a significant variety and plant population response but there was no variety x plant population interaction.  AG4632 (34.0 in.) was significantly taller than the other two soybean varieties (AG3936 at 30.7 in. and AG4232 at 31.7 in.) (Table 5). Soybean plant height significantly increased with each incremental increase in plant population (Table 6).  The shortest soybean plants were observed with the 140,000 plant population (27.7 in.) compared to the tallest plants observed at the 220,000 plant population (36.3 in.) while the 180,000 plant population showed an intermediate plant height (32.3 in.).  The plant height means for all of the treatments across soybean varieties and plant populations are shown in Graph 2.

Table 5.  Plant height (inches) response to soybean variety
for wheat-soybean double cropping experiment.

Variety Mean Plant Height
AG3936 30.7 b
AG4232 31.7 b
AG4632 34.0 a

*Numbers containing the same letter are not
significantly different at the alpha = 0.05% level.

Table 6.  Plant height (inches) response to plant population
for wheat-soybean double cropping experiment.

Plant per acre Mean Plant Height
140,000 27.7 c
180,000 32.3 b
220,000 36.3 a

*Numbers containing the same letter are not
significantly different at the alpha = 0.05% level.

Graph 2.  Mean plant height (inches) for soybean variety and plant
population in a wheat-soybean double cropping system.

Stand Count
With Stand Count, only plant population (Table 7) showed a significant increase in number as the planting population increased, as would be expected.  There was no variety (Table 8) or variety x plant population interaction (Graph 3) for stand count.  The percent stand loss calculated by dividing the final mean stand count by the planting population is shown in Table 7.The 140,000 final stand count was 5.7% lower than the planting population while the 180,000 and 220,000 stand counts were 4.5% lower than the initial planting population.

Table 7.  Mean stand count across planting population in a wheat-soybean
double cropping system.

Plants/acre Mean Stand Count Percent Stand Loss
140,000 132,111 c 5.7
180,000 171,944 b 4.5
220,000 210,222 a 4.5

*Numbers containing the same letter are not significantly different at the
alpha = 0.05% level.

Table 8.  Mean stand count across soybean varieties in
a wheat-soybean double cropping system.

Variety Mean Stand Count
AG3936 170,722 a
AG4232 171,333 a
AG4632 172,222 a

*Numbers containing the same letter are not
significantly different at the alpha = 0.05% level.

Graph 3. Mean stand count across soybean varieties in a wheat-soybean double cropping system.

Plant Lodging
The soybean varieties in this study had no significant effect on the lodging score at the end of the season just prior to harvest (Graph 4 and Table 9).  There were significant differences in the mean lodging scores across planting populations (Table 10) at the end of the season.

Graph 4.  Mean lodging score across planting soybeanvarieties in a wheat-soybean
double cropping system.

Table 9.  Mean lodging score across planting soybean
Varieties in awheat-soybean double cropping system.

Variety Mean Lodging Score
AG3936 2.3 a
AG4232 2.7 a
AG4632 2.3 a

* Numbers containing the same letter are not
significantly different at the alpha = 0.05% level.

Table 10.  Mean lodging score across planting population
in a wheat-soybean double cropping system.

Plants/acre Mean Lodging Score
140,000 2.1 b
180,000 2.4 b
220,000 2.9 a

*Numbers containing the same letter are not
significantly different at the alpha = 0.05% level.

There was a higher mean lodging score with the 220,000 plants per acre (Table 10), the plants at the 220,000 planting rate were taller than the 140,000 and 180,000 rates which may account for the increase in lodging.

Graph 5.  Mean lodging score across planting population and soybean
Varieties in a wheat-soybean double cropping system.

In summary, only yield and plant height differ significantly for the varieties while there were no significant differences for stand count and lodging score. When it comes to significant variety x plant population interactions, only mean yield showed a significant difference. Lodging and plant height increased as population increased while grain yield increased from 140,000 to 180,000 but decreased at 220,000 plants per acre.

Even though yield prospective for double-plant soybeans was decreased by delayed sowing, double-planting soybeans following wheat turns into economically feasible while adding in the worth of 20 to 30 bushels for every soybeans acreage to the worth of the first wheat harvest.  In this study, soybean yields ranged from 35 bu/acre to 50 bu/acre which provided significant benefits to the double-cropping of soybeans following wheat harvest even with the late planting in 2015.
It is important to understand which planting rate will provide the best economic return.  The highest planting rate is expected to have the highest yield, but may not be cost effective because of seed costs.  Increased soybean populations brings increased seed costs that may or may not outweigh the benefits when yields are calculated.  The average revenue less the seed,herbicide, planting, and harvesting costs were most efficient with the 180,000 planting rate at $451.72 per acre, it provided a $95.72 advantage over the 140,000 planting rate and $70.29 advantage compared with the 220,000 rate (Table 11).

Table 11. Economic of different soybean planting populations
Planting Population (seeds/acre) Seed Cost ($) Average Yield (bu/acre) Soybean Price ($/bu) Average Revenue ($) Average Revenue Less Seed Cost
140,000 50.00 40.6 10 406 $356.00
180,000 64.28 51.6 10 516 $451.72
220,000 78.57 46.0 10 460 $381.43


Production costs and revenue vary from year to year depending on crop price, direct, indirect, and land costs (Table 12).  The last three columns show the double-crop return for this study was highest for the 180,000 planting rate at $362/acre while the 140,000 and 220,000 planting rates were $266.00 and $291.43/acre, respectively.
Table 12. Double-Crop Soybean Revenues and Costs, Southern Illinois, Actual for 2009 through 2014
2009 2010 2011 2012 2013 2014 2015/140K 2015/180K 2015/220K
Yield per acre 32 30 36 17 32 35 40.6 51.6 46
Price per bu $10.03 $11.70 $12.76 $14.46 $13.21 $10.20 $10.00 $10 $10
Crop revenue $321 $351 $459 $246 $423 $357 $406 $516 $460
Herbicides 19 22 29 30 37 37 35 35 35
Seed 40 39 42 43 47 49 50 64.28 78.57
Total direct costs $59 $61 $71 $73 $84 $86 $85 $99 $113
Planting hire/lease 15 15 16 16 17 17 20 20 20
Harvesting 30 30 31 32 32 33 35 35 35
Total non-land costs $45 $45 $47 $48 $49 $50 $55 $55 $55
Farmer return $217 $245 $341 $125 $290 $221 $266 $362 $291
Results for 2009 through 2014 are summarized from grain farms enrolled in Illinois Farm Business Farm Management

In conclusion, this study found that with one year of data, increasing plant populations to 180,000 seeds per acre in double-crop soybeans did increase yields in comparison to the lower plant population (140,000 seeds per acre) for all three soybean varieties. Increasing the seeding rate to 220,000 seeds per acre did not increase grain yields and even for two varieties (AG3936 and AG4232) resulted in a significant yield decrease from the 180,000 seeds per acre rate.  Increasing plant populations to 180,000 plants per acre improved profitability in this trial while yields were primarily due to the favorable environmental conditions.
When comparing to other research data, it was expected to see the highest yield at the 220,000 plants per acre rate would exceed the 180,000 and 140,000 plants per acre.  The 180,000 plants per acre rate maximized yield and profitability. The 140,000 plants per acre seeding rate had lower yields that did not maximize profitability.
These results will likely vary in subsequent trials and years, depending on environmental conditions.This trial should be repeated in time and across locations to be able to recommend the optimum seeding rate for optimum stands across a broad geography with a wheat-soybean double cropping system.


Association of Official Seed Analysts.MEASUREMENT OF VIGOR IN SEEDS OR SEEDLINGS, Margaret S. Gibson, Ph.D., Plant Physiologist, American Crystal Research Center (NEED A REFERENCE CITATION)

Beuerlein, J.E.  1988.  Yield of indeterminate and determinate semidwarf soybean for several planting dates, row spacings, and seeding rates.  J. Prod. Agric. 1:300-303.

Board, J.E., M.S. Kang, and B.G. Harville.1999. Path analyses of the yield formation process for late-planted soybean. Agron. J. 91:128–135.

Carpenter, A.C., and J.E. Board. 1997.  Branch yield components controlling soybean yield stability across plant populations.  Crop Sci. 37:885-891.

Cooper, R.L.  1977.   Response of soybean cultivars to narrow rows and planting rates under weed-free conditions.  Agron. J. 69:89-92.

De Bruin, J.L., and P. Pedersen. 2008.  Effect of row spacing and seeding rate on soybean yield. – Agron. J. 100:704-710.

De Bruin, J.L., and P. Pedersen.  2008.  Soybean seed yield response to planting date and seeding rate in the Upper Midwest.  Agron. J. 100:696-703.

Egli, D.B. 1988.  Plant density and soybean yield. Crop Sci. 28:977-981.

Ellen Phillips, Extension Educator, Countryside Extension Center, 708-352-0109, [email protected] (personal communication)

Elmore, R.W.  1991.  Soybean cultivar response to planting rate and tillage.  Agron. J. 83:829-832.

Elmore, R.W.  1998.  Soybean cultivar responses to row spacing and seeding rates in rainfed and irrigated environments.  J. Prod. Agric. 11:326-331.

Illinois Grower’s Guide to Superior Soybean Production, University of Illinois College of Agriculture Cooperative Extension, Circular 1200)

Oplinger, E.S., and B.D. Philbrook.  1992.  Soybean planting date, row width, and seeding rate response in three tillage systems.  J. Prod. Agric. 5:94-99.

Sadras, V.O. 2007. Evolutionary aspects of the trade-off between seed size and number in crops. Field Crops Res. 100:125–138.

Southern Illinois. Illinois Grower’s Guide to Superior Soybean Production University of Illinois College of Agriculture Cooperative Extension Circular 1200.
Illinois Agronomy Handbook: Chapter 3; Emerson Nafziger,University of Illinois Department of Crop Sciences.

University of Illinois Extension; First Frost Date Map, University of Illinois at Urbana-Champaign

Weber, C.R., R.M. Shibles, and D.E. Byth.  1966.  Effect of plant population and row spacing on soybean development and production.  Agron. J. 58:99-102.

Wells, 1991; Carpenter and Board, 1997; Board, 2000.

Wiggans, R.G.  1939. The influence of space and arrangement on the production of soybean plants. J. Am. Soc. Agron. 31:314–321.

Appendix Tables

Appendix 1.  ANOVA for grain yield for soybean variety and plant population for wheat-soybean double cropping study.

Source DF Type III SS Mean Square F Value Pr > F
Rep 2 62.9632277 31.4816138 3.27 0.0645
Variety 2 151.3482165 75.6741082 7.86 0.0042
Population 2 542.7423771 271.3711886 28.18 0.0000
Variety*Population 4 126.2852514 31.5713129 3.28 0.0383

Appendix 2.  ANOVA for plant height for soybean variety and plant population for wheat-soybean double cropping study.

Source DF Type III SS Mean Square F Value Pr > F
Rep 2 20.6666667 10.3333333 3.65 0.0495
Variety 2 52.6666667 26.3333333 9.29 0.0021
Population 2 338.6666667 169.3333333 59.76 0.0000
Variety*Population 4 1.3333333 0.3333333 0.12 0.9743

Appendix 3.  ANOVA for stand count for soybean variety and plant population for wheat-soybean double cropping study.

Source DF Type III SS Mean Square F Value Pr > F
Rep 2 8685185 4342593 1.07 0.3674
Variety 2 10240741 5120370 1.26 0.3110
Population 2 27459685185 13729842593 3371.97 0.0000
Variety*Population 4 9592593 2398148 0.59 0.6754

Appendix 4.  ANOVA for plant lodging for soybean variety and plant population for wheat-soybean double cropping study.

Source DF Type III SS Mean Square F Value Pr > F
Rep 2 1.40740741 0.70370370 1.55 0.2423
Variety 2 1.18518519 0.59259259 1.31 0.2982
Population 2 2.74074074 1.37037037 3.02 0.0771
Variety*Population 4 0.14814815 0.03703704 0.08 0.9869