Soybean Seeding Rates, Plant Population Density and Seed Treatments

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Soybean seed costs are about 40% of the variable costs in soybean production, and optimizing seeding rate will help to produce high yields without overspending on variable costs. But determining the optimum seeding rate can be a challenge.

From my observations in the county, seeding rates can range from approximately 100,000 seeds per acre to more than 150,000 seeds per acre. More frequently, I see the lower seeding rates with earlier planting dates and as the planting date gets later, it increases.

From information generated in North Carolina, the following has been reported with regards to soybean seeding rates and plant populations.

  • Historical recommendations for North Carolina:

A tremendous amount of seeding rate work was carried out under Dr. Jim Dunphy. This work was largely conducted with determinate varieties and indicated that growers should aim for final stands as follows:

Month of Planting Seeding Rate            (PPD divided by 0.8)* Plant Population Density (PPD)
May 93,750 75,000
June 112,500 90,000
July 125,000 100,000

* Plant population density (PPD) is simply the number of plants per acre. Seeding rates must be higher than the expected final stand of soybeans, but how much higher can be a difficult question. With good seed (germination of 90% or so) and good planting conditions, dividing desired PPD by  0.8 (multiplying by 1.25) should give a reasonable seeding rate.

  • Seeding Rate Impact in the North Carolina Soybean Yield Contest:
    • In the statewide analysis of all 877 yield contest entries from 2002-2019, when planting later than May 12 and using an MG≤4 variety, using a seeding rate <130,000 seeds/A resulted in a 7 bu/A yield advantage over using higher seeding rates.
    • In high yield environments , there were yield advantages to using lower seeding rates.
    • Planting earlier maturing varieties (MG≤4), planting before mid-May, and the use of a foliar fungicide were the strongest predictors of yield
  • Recent plant population work by the NC State Extension Soybean Program reported the following:
    • Preliminary results indicate that across most planting dates, yield plateaued around moderate seeding rates (125,000 seeds/A) and yield tended to be more sensitive to higher seeding rates as planting date was delayed.

As for seed treatments in soybeans, this is what has been found with field tests in North Carolina:

  • Fungicidal seed treatment can have value for protecting yield at planting dates earlier than mid-May in environments where disease pressure and varieties used are conducive for disease development.
  • Insecticide seed treatments do not provide a yield benefit for soybean in North Carolina
  • Nematicide seed treatments are chemical or biological compounds with nematode-killing properties applied to the seed before planting. These seed treatments are designed to provide early season protection, guarding young plants during an early growth period. Nematicide seed treatments are most beneficial under low to moderate nematode pressure. Fields with high or heavy levels of nematode pressure are unlikely to be successfully managed through seed treatment alone.

For additional information about seed rates, and plant populations and for soybean seed treatments.