- Mid-May or earlier planting dates for soybeans will usually maximize yield potential.
- Yield reductions of half a percentage point per day may occur each day planting is delayed after mid-May.
- Planting the fullest relative maturity (RM) possible for a geography will enhance yield potential.
- Seeding rates resulting in final stands greater than 100,000 will maximize yield potential and/or economic return potential.
Early soybean planting can help maximize photoperiodism. This impacts soybean development and helps the plant avoid excessive heat and moisture stress during critical flowering stages. Early season precipitation and temperatures can be beneficial for soybeans.1 Though planting too early may result in poor stands or delayed emergence from cool, wet soils or higher than average bean leaf beetle pressure, significant delays in planting often result in reduced yields.
How to Increase Soybean Yield
Optimizing soybean growth between vegetative and reproductive stages will result in increased yield potential. Balancing the time spent accumulating nodes during vegetative growth and the length of time in reproductive stages to fill pods is crucial. An ideal approach is planting fuller season varieties adapted for the region.2
Multi-year Planting Date and Seeding Rate Results
Golden Harvest® Agronomy In Action research trials were conducted in 2020 to demonstrate how planting date, relative maturity (RM) and seeding rate interact with each other and affect yield potential. Results from NE, IA and IL show that planting soybeans by mid to late May results in maximum light exposure and yield potential (Graph 1). Later planting results in average yield losses of 0.5% per day and risk of frost damage.
Final soybean population stand counts are usually lower than seeding rates, and in many cases, significantly less. Final stand population is more important than seeding rates in determining yield potential. Multi-year seeding rate trials have shown that final stands greater than 100,000 plants/A yielded similarly. Increasing seeding rates to achieve higher stands resulted in small inconsistent yield gains (Graph 2). When final stands were less than 100,000 plants/A, there was a 2% loss of maximum yield potential for every 10,000 fewer plants established.
2020 Planting Date Trials
Studies were conducted at Seward, NE, Slater, IA, and Clinton, IL, in 2020. 2 varieties of similar RM were selected per grouping of early, mid- or full-season varieties for each trial location. Early and mid-RM varieties were respectively 1.0 and 0.5 earlier than the fullest season variety normally planted in that location. Each of the 6 varieties were planted at 100,000, 140,000, 180,000 and 220,000 seeds/A.
Varied Responses Across Sites
Planting date responses behaved differently across the 3 locations, but overall followed general trends observed in multi-year planting date trials (Graph 3). Unlike Seward and Slater, the Clinton site did not see any advantages to planting in April. However, the most rapid yield loss resulting from late May to early June planting dates was consistent with multi-year trends.
The lack of response to the April 22, 2020 planting date at Clinton was a result of only establishing 68% of actual seeding rates, likely due to a period of cool, wet weather from April 23 through April 29 that slowed plant establishment. The spike in the June planting date yield at the Slater site is a good example of how planting date interaction with seasonal weather can result in variable yield response. Stands were roughly 37% below targeted seeding rates for the Slater May 22 planting date, resulting in a more severe yield penalty than expected. Additionally, after a relatively dry August at Slater, the June 5 planting date was likely able to take advantage of early September precipitation, whereas earlier planting dates were already nearing maturation.
Soybean RM Adjustments with Delayed Planting Considerations
Switching to an earlier RM is a common practice when planting is delayed. This enables an earlier harvest or avoids early fall frost. However, shortening RM too much can result in lost yield. In the 2020 Seward trial, yields declined with each subsequent planting date regardless of RM (Graph 4).
Generally, mid- and full-RM varieties outperformed early varieties at all planting dates, except for the final June planting date, when full-season RM was switched to a mid-season RM. However, it was never beneficial at any planting date to transition to any RM earlier than 2.9. Both the mid- and early varieties planted in early and late June were already yellowing and beginning to senesce leaves on September 17, whereas the full-season varieties were not (Figure 2).
Soybean Seeding Rate Adjustments
In general, increasing soybean seeding rates at normal planting dates to achieve final plant stands greater than 100,000 plants/A has shown minimal value. However, prior work has shown an advantage to increasing seeding rates when planting is delayed into late June or July. Late-planted soybeans are less able to maximize the number of nodes developed prior to flowering as compared to normal planting dates, capping the plants’ overall capacity to generate normal pod and seed numbers per plant. Due to plants being less elastic in their ability to increase per plant yield potential at late planting dates, increasing seeding rates may be beneficial. Although this has been observed in the past, there was not enough of an increase in yield in 2020 trials to justify increasing seeding rates (data not shown). Dry soil conditions with June or July planting dates can dramatically reduce emergence and should be taken into consideration to guarantee a minimum final stand of 100,000 or more plants per acre to maximize yield potential.
Impact of RM on Seeding Rate
In 2020 trials, there was no basis to adjust seeding rates if adjusting RM to an earlier or later variety. The relative performance of the 3 RM treatments was consistent across all 4 seeding rates (Graph 5).
Summary
This study shows the importance of soybean planting date and seeding rate. Data from this season and previous years suggest beginning soybean planting in the latter half of April and finishing by mid-May to avoid yield penalties in most Midwest geographies. Both April and mid-May planting dates have the ability to maximize crop canopy closure early in the season which helps improve photosynthesis efficiency as seen on July 27 at Seward (Figure 2). Soil temperature and weather forecast should be monitored closely when planting in April, as yield benefits from early planting are dependent upon date of emergence rather than actual planting date. Soil temperatures below 50° F and saturated soil conditions after planting can result in delayed emergence and consequently result in uneven and inconsistent stands that negate the value of planting early.
When planting is delayed, the balance between maximizing yield with a full-season RM and reaching maturity prior to frost with an earlier RM is important. These trials reinforced that when this happens, only small adjustments of 0.5-0.75 RM earlier than normal are necessary to reach maturity faster and still maximize yield potential (Figure 2).
Seeding rates from 2020 and prior years suggest that planting 140,000 seeds/A will typically result in final stands greater than 100,000 plants/A and maximize yield potential. There are years that increasing seeding rates higher than 140,000 have given slight yield benefits, but most often, increased rates don’t provide an economic return due to additional seed and seed treatment cost. If reducing seeding rates less than 140,000 seeds/A, it will be increasingly difficult to achieve the minimum final stand of 100,000 plants/A. Although yield penalties may not always be seen with reduced seeding rates, it will be more likely to occur as germination and stand establishment rates decrease.
For more information and recommendations on soybean planting dates, contact your local Golden Harvest Seed Advisor.
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References:
1 Hu, M. and P. Wiatrak. 2012. Effect of planting date on soybean growth, yield, and grain quality. Review. Agronomy Journal 1004:785-790. doi:10.2134/agronj2011.0382.
2 Boyer, C., M. Stefanini, J. Larson, S. Smith, A. Mengistu and N. Bellaloui. 2015. Profitability and risk analysis of soybean planting date by maturity group. Agronomy Journal. 107:2253-2262. doi:10.2134/agronj15.0148