Soybean seeding rate studies show final plant populations around 80,000 to 120,000 plants per acre is likely sufficient for an economic return.

April 24, 2016

Numerous soybean seeding rate studies have indicated that a final plant population of about 80,000 to 120,000 plants per acre is likely sufficient for ensuring an economic return, with the latter being based on weighing the slightly higher yield potential with higher final plant populations against the cost of planting more seed/acre to get to higher plant populations than this range.

For example, if your goal is to have 100,000 plants per acre at harvest, you will need to adjust your seeding rate accordingly for two factors:

• One adjustment accounts for the fact that it is rare to have a bag of seed with 100% seed viability. Check the seed bag tag for the official germination percentage. It may be 98%, 95% or perhaps even 90%.

• The second adjustment accounts for the fact that not every germinated seed will result in an emerged seedling, and not every emerged seedling will survive the season to become a final mature plant. A review of the literature indicates that, on average, the seedling and plant survival percentage is about 85%. You can choose a different percentage to suit your experience and the field condition the day you plant.

To adjust for these factors, multiply the decimal equivalents of the two percentages (for example, 95% germ and 85% survival). In this case, 0.95 x 0.85 = 0.8075. Now, divide 100,000 by that decimal number: 100,000/0.8075 = 123,839 seeds per acre. Rounding that number to 125,000 seeds per acre gives a seeding rate that is likely to get you to your goal of 100,000 plants per acre.

Laura Thompson, University of Nebraska Extension educator and Nebraska On-Farm Research Network coordinator, notes on-farm research studies since 2006 have shown many growers have room to decrease their soybean planting populations. "A lot of growers are in the 150,000 to 180,000 up to 200,000 seeds per acre range," Thompson says.

From 2006 to 2010, 13 sites evaluated four different populations, including 90,000, 120,000, 150,000 and 180,000 seeds per acre. All sites were irrigated, and most soybeans were planted on 30-inch rows.

"There was a continuous increase, but going from 90,000 seeds per acre up to 180,000 seeds per acre only increased yield 1.4 bushels per acre on average across the 13 sites," says Thompson. "The 1.4 bushel per acre increase was not enough to cover that additional seed cost. If you are planting 180,000 seeds per acre, decreasing population down to 120,000 seeds per acre only resulted in a 0.6 bushel per acre yield decrease."

What works for one farm doesn't work everywhere else – it depends on the situation. "For example, if hail is a big issue in your area, you might not want to push it down that far," Thompson says. "But if you're in the 160,000 to 180,000 range, it might be good to try to come down to 120,000 to 130,000."

Source: UNL CropWatch

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