Numerous soybean seeding rate studies have been conducted in the United States. These trials have shown that soybean plants have a tremendous ability to compensate for a wide range of planting populations. The yield per acre remains relatively constant across population. This is because the number of seeds produced per plant is inversely related to the number of plants per acre.
Given the above information, what is the optimum soybean seeding rate? In general, most agronomists agree that 100,000 relatively uniformly spaced plants at harvest will produce the maximum economic return under most conditions. This could justifiably be increased to 120,000 plants per acre to build in a buffer for adverse conditions and to increase peace of mind. In Northern Michigan where maturity group I varieties are planted, the target harvest populations should be between 125,000 and 140,000 plants per acre.
Higher planting populations are generally recommended under the following conditions:
- Early planting (before May 1)
- Untilled soil
- Late planting dates (after June 15)
- Low productivity soils
- Planting with a drill
Soybean seeding rates should be reduced to target a harvest population of 100,000 plants per acre in fields having a history of white mold or where excessive plant height or lodging have occurred in the past.
The next question is how many seeds per acre should be planted to end up with the desired harvest population? Consider factors such as seed quality, soil conditions, planting date, weather conditions, planting equipment and seed treatments as these can impact soybean germination and emergence.
The following formula is helpful for calculating soybean seeding rates.
Target Plant Population at Harvest
—————————————————————————————— = Seeding Rate
(Warm Germination Percent ÷ 100) x (Seed Emergence Percent ÷ 100)
For example, what seeding rate should be used if we want to end up with 120,000 plants per acre at harvest, the warm germination test is listed as 90 percent on the seed tag and soil conditions and weather conditions are adequate to achieve 90 percent emergence.
120,000 plants per acre at harvest
—————————————————————————- = 148,148 seeds per acre
(90 ÷ 100) x (90 ÷ 100)
Emerson Nafziger and Dennis Bowman at the University of Illinois have incorporated the formula into an excellent online Soybean Seeding Rate Calculator. Neither the online calculator nor the formula presented above adjust planting rates for the type of planting equipment used (planters versus drills and air seeders). To account for the variability in the seed metering, planting depth, and seed-to-soil contact associated with drills and air seeders, decrease the emergence estimate to 80 percent when using the formula listed above or the Illinois seeding rate calculator. The percent emergence should also be decreased by an additional 10 percent when planting early or when planting into untilled or poor soil conditions. Consider increasing the emergence estimate by 5 percent when using fungicide and insecticide seed treatments.
Comparing soybean seeding rates, harvest populations and yields in each of your fields every year will help determine the optimum planting populations for your fields over the long run.
This article was produced by the SMaRT project (Soybean Management and Research Technology). The SMaRT project was developed to help Michigan producers increase soybean yields and farm profitability. The SMaRT project is a partnership between Michigan State University Extension and the Michigan Soybean Checkoff program.