Farm Horizons, February 2018

Corn and soybean plant populations for optimum yield

By Dave Schwartz
Certified crop advisor, Gold Country Seed

Corn and soybean growers naturally apply the minimum amount of seed, fertilizer, and herbicides to grow crops, because they are in the business to make money. When profit margins are as tight as they are now, it becomes even more critical.

So, let’s take a look at optimum plant populations for corn and soybeans.

For high-yielding crops, plants need to canopy early and collect as much sunshine during the growing season as possible. So, more plants are normally better, but there is a limit. Too many plants cause plants to compete against each other for sunlight, water, and nutrients, and this can lower yields.

Corn plant populations have risen over the past 20 years, and soybean populations have dropped.

On good clay loam soils, the recommended seeding rate for corn is in that 35,000- to 36,000-seeds-per-acre range. If growers are planting corn on corn, we need to increase seeding rates 1,500 seeds per acre, because stands are reduced with more crop residue.

Another rule of thumb is to increase seeding rates 5 to 10 percent when planting corn in mid-April vs. mid-May.

Soil insects, frost, seedling diseases, etc. can take a toll on very early planted fields.

Lower plant populations are recommended for sandy loam soils – in that 26,000- to 28,000-seeds-per-acre range.

Today’s drills and planters are much more precise at planting soybean seed uniformly at the same depth, so growers have been able to lower seeding rates. Over the past few years, soybean growers have lowered seeding rates to 140,000 seeds per acre.

The research data I have seen shows optimum yields in our part of the state are achieved with final stands of 110,000 to 120,000 plants per acre.

Optimum seeding rates for soybeans are dependent on planting date, seed treatment, seed germs, and planting conditions. I usually recommend lowering seeding rates of tall, bushy lines to the 130,000- to 135,000-seeds-per-acre range to avoid lodging and potential white mold issues.

When grain markets are low like they are now, producers need to manage expenses carefully. University of Minnesota Extension Farm Management Specialist Dave Bau is projecting seed costs for the 2018 crop year to be $119 per acre for corn, and $57 per acre for soybeans.

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