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How many soybeans per acre do you need?

The spotlight in soybeans shines on basics again. Higher input costs demand it. When it shines on soybean seeding rates, it reveals that most recommended seeding rates carry hefty “insurance policies” against significant plant losses. Maybe the cost is higher than you want to pay.

“We’re following the lead from neighboring states,” says Shaun Casteel, a Purdue University Extension agronomist. “Seeding rates aren’t as high as they once were. Our seeding-rate recommendation is 140,000 to 160,000 seeds per acre in 7.5-inch rows.” What counts is the number of plants at harvest. “We want at least 100,000 plants per acre,” he says. He hopes to establish on-farm trials to pin down how low seeding rates can go.

Seed costs skyrocket

Soybean seed prices were only 7 to 10 cents per 1,000 seeds before Roundup Ready soybeans. Now they’re 35 to 55 cents per 1,000 seeds. “That makes which seeding rate to use about seven to eight times more important than just 15 years ago,” says Vince Davis, an agronomist at the University of Illinois.

Davis says test data don’t support high seeding rates. “The yield curve is fairly flat, from 125,000 to 225,000 seeds per acre,” Davis says. “Most operations might benefit from modest decreases in seeding rates, provided the seed has good viability and is planted under normal conditions.”

Test the low side

Seed costs vary widely, depending upon whether you’re planting biotech, Roundup Ready seed or non-GMOS. Davis pegs the range at 15 to 45 cents per 1,000 seeds. Pairing those numbers up against yields from a recent Illinois study shows that economically, optimum seeding rates fluctuate by 26,000 seeds per acre. Based on the study, the optimum seeding rate, when seed cost is 25 to 45 cents per 1,000 seeds, falls between 100,000 and 129,000 seeds per acre.

“This is much lower than current recommendations,” Davis says. “It’s a little lower than I would be comfortable recommending.” At the same time, “buying” ample insurance through extra seed to compensate for things like major catastrophes that don’t occur very often may no longer be worth it, he concludes.

Why he chose 15-inch rows

A 15-inch row planter replaced a drill as the tool of choice for planting soybeans for Bill Warmoth, Eminence, Ind. “Seed cost was a big factor,” Warmoth says. “Better placement was also a plus. But we don’t use as much seed in 15-inch rows, and seed is expensive.”

The rate Warmoth selects depends upon the variety and germination percentage. He grows some non-GMO soybeans since an area processor pays a premium for non-GMO beans.

Warmoth isn’t alone in thinking 15-inch rows are a good choice. Vince Davis, a University of Illinois Extension specialist, says trials he’s reviewed indicate very little difference in yield between drilled and 15-inch rows in Illinois. Fewer dollars spent for the same yield equals more net profit.

Those same studies show a significant yield advantage for 15-inch rows over 30-inch rows, however.


Seed-conscious: The ability to buy less seed and produce good yields helped move Bill Warmoth back to 15-inch rows.

This article published in the February, 2010 edition of MID-SOUTH FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.