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Select a plant stand you can live with

The recommended seeding rate issued by Purdue University Extension specialists has varied depending upon row width. It’s been common practice for decades. Shaun Casteel is about to revise his recommendations and vary from conventional thinking.

“We have enough data in the Midwest documenting that yield is maximized with uniform harvest stands near 100,000 plants per acre regardless of row width,” says Casteel, Purdue Extension agronomist.

Key Points

2012 Purdue crops guide will show soybean plant populations for all row widths.

Purdue agronomist confident that 100,000 to 120,000 plants per acre is sufficient.

Fine-tune seeding rate based upon seed quality, equipment, current field conditions.

Traditional recommendations were to seed 130,000, 160,000 and 200,000 seeds per acre in 30-, 15- and 7.5-inch rows, respectively. However, Casteel says the data doesn’t really back up this recommendation anymore. It’s especially high at narrower row widths.

Current strategy

“I like to start at 130,000 to 150,000 seeds per acre regardless of row width,” he says. Then he increases or decreases the rate based on seed quality, planting equipment and field conditions.

What he’s discovering is that approximately 100,000 to 120,000 plants per acre at harvest provide a significant savings in seed cost without sacrificing yield. That holds true even this year, when high projected soybean prices put an extra premium upon squeezing out more bushels.

Going above stands of 150,000 simply won’t kick out more yield, based upon university trials. Profit slips because of increased seed costs at higher seeding rate. The more expensive the seed, the more profits slip.

Vince Davis, while at the University of Illinois, confirmed in extensive trials and analysis of data that as seed prices have increased significantly, the most profitable range settles in at final populations lower than many people once thought, especially as soybean seed prices go higher.

To calculate your price per 1,000 seeds, simply multiply number of seeds per pound by weight of a unit of seed to get total seeds per unit. Divide by cost per unit. For example, if a 50-pound unit has 3,000 seeds per pound, that’s 150,000 seeds. At $50 per unit, you’re paying about 34 cents per 1,000 seeds.

If seeding rates of 130,000 seeds per acre and 150,000 seeds per acre produce the same yield, then you’re spending nearly $7 per acre more for seed without getting any extra return. That holds true regardless of the price of soybeans.

Test your knowledge

Most discussions about plant populations for soybeans have an important footnote. It’s assumed that stands are uniform. What if they’re not?

What if there are 2-foot skips in 50% of the row? Can you still expect near 100% yield? Maybe 95%? Or perhaps 90%?

Based on Purdue University trials, it’s 94% of yield. Soybeans near the skip compensate, but other plants can’t compensate as if they were spread out evenly.

According to the 2011 Purdue Corn and Soybean Field Guide, 3-foot skips drop yield potential to 87% of normal, and 4-foot skips on 50% of the row drop expectations to 85% of normal.


Big Gap:Two foot gaps repeated across 50% of the row area cut yield potential to 94% of normal.

This article published in the May, 2011 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.