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Factors Influencing Soybean Planting Date Response

  • 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).