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Numerous soybean seeding rate studies have been conducted across 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 early group I varieties are planted, the target harvest populations should be between 125,000 and 140,000 plants per acre.
Higher planting rates are generally recommended under the following conditions:
Reduce soybean seeding rates 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. Both the formula and the online calculator allow you to adjust the emergence percentage. When planting with a unit planter into ideal soil conditions, emergence can be estimated to be 100 percent. 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 by 10 percent. The emergence should also be decreased by an additional 10 percent when planting very early (before May 1) or when planting into untilled or poor soil conditions. Consider increasing the emergence estimate by 10 percent when using seed treatments containing a fungicide and an insecticide. If you estimate the emergence percentage to be 70 percent or lower, you should wait for better planting conditions to occur.
One of the best ways for producers to fine-tune their soybean planting rates in the long run is to conduct replicated planting rate trials on different soil types for several years. This is very easy to do if the planter or air seeder is equipped with hydraulic/electric drives on the metering system. If you are interested in conducting one or more soybean planting rate trials on your farm in 2016, please contact me by phone at 269-673-0370 ext.2562 or by email at firstname.lastname@example.org. You will learn how planting rate affects soybean yields and income on your farm and see how your final plant stands compare to your target planting rates.
Source: Mike Staton, Michigan State University Extension
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