There’s an art to determining the optimum plant population for a particular field — the sweet spot where there are enough ears per acre without being too many for the plants to fill. Though researchers and their computers are chewing through mountains of data to try to create automated tools to help match populations to patches of soil, it’s still a decision based largely on experience and instinct.

Over the past couple of decades, breeders have focused on selecting shorter hybrids with fewer, more upright leaves; smaller tassels; better synchrony between silk emergence and pollination; and improved endurance. The combination of those traits allows today’s hybrids to flourish in higher populations.

The key is planting higher populations where you expect to harvest high yields.

The magic number

According to a Dekalb review of field trials in four states, the sweet spot for most Midwestern fields with high yield potential and good management practices is between 32,000 and 39,000 plants per acre, or ppa. Plant more than optimum populations and you may just be burying money that won’t deliver more yield. Plant too few seeds and you could be cutting your crop short.

In all, it’s better to err on the high side, notes Peter Thomison, Extension corn specialist at Ohio State University.

“In the absence of major environmental stresses, most research suggests that planting a hybrid at suboptimal seeding rates is more likely to cause yield loss than planting above recommended rates, unless lodging becomes more severe at higher population levels,” Thomison says.

Think through your plan

Consider these variables when determining how many seeds to plant per acre:

  • Yield potential of the field. Invest in seed for good ground. On fields with long-term yield averages of 190 bushels per acre or better, aim for 33,000 ppa or more, suggests Thomison. For 150-bushel fields, 30,000 ppa may do the trick, and if 120 bushels or less is all you can expect on a field, back off to 20,000 to 22,000 ppa, he advises.
  • Seed germination. Check each bag’s seed tag for germination percentage of that lot, suggest breeders at Dekalb. Most seed has a germination percentage greater than 95%, but some can be as low as 85%.
  • Historic attrition rate. A certain percentage of seeds won’t germinate due to a wide variety of challenges in a field. Normal attrition rates in Iowa State University studies range from 3% to 7%, notes Roger Elmore, Iowa State University Extension corn specialist. Figure out what your attrition rate typically is — and if it’s much greater than local averages, find out why.
  • Planting conditions. The cool soils of no-till fields in early to mid-April can stress seedlings and limit populations, Thomison points out. He suggests increasing seeding rates 10% to 15% higher than your desired population in those circumstances. On the flip side, if conditions are dry at planting and show no signs of improving, be conservative with your populations, advises Elmore.
  • Maturity. Early-maturing hybrids typically require higher populations than later-maturing ones, according to Dekalb.

30-inch corn rules

Of course, hitting your target population requires fewer seeds per foot in narrower rows. In corn, the question of optimum row spacing seems to be mostly a matter of your equipment and your comfort level.

In Iowa, modern hybrids in 30-inch rows intercept 95% of the available sunlight at silking — the prime target for corn, says Elmore. That’s what it takes to produce optimum yields, he notes, so rows any tighter than 30 inches rarely improve Iowa harvests unless early-season stress, like drought, slows canopy closure in the middles.

As with most seed-related issues, your seed dealer will be an invaluable source of information
on optimum populations in your conditions. Ask.

 

Maximizing population needed for higher yields

One of the most popular stops at the 2013 Farm Progress Show was to see a 30-row, 12-inch planter from Marion Calmer. Long an advocate of narrower row spacing for corn, Calmer strives to meet the harvest needs of the most forward-thinking producers. And yes, there are experiments on 12-inch rows. In fact, 11-inch rows are being studied, too.

It’s all about trying to achieve equidistant spacing to maximize the solar energy the corn crop receives, and to obtain that magic 300-bushel-per-acre yield, plant population has to go up, too. While the 12-inch row won’t be everyone’s choice, chances are that narrower rows — to 20 or 15 inches — may be in the cards. But you have to run the numbers to see why you need narrower rows.

Fred Below, plant physiologist at the University of Illinois, runs the numbers as part of his Seven Wonders of the Corn World. Below has been trying to consistently hit 300 bushels per acre in his field-size plots, and he’s getting closer, but what must happen to achieve those yields?

He lays it out pretty simply on a per-acre basis:

For 200 bushels per acre, the average stand would have to be 32,000 plants per acre, and each ear would need 550 kernels. Dry weight for each kernel should be 250 milligrams.

For 250 bushels per acre, you’ll need a stand of 36,000 plants per acre, on average, and every ear should have 600 kernels. You get a break on the dry weight, at 255 milligrams per kernel. If farmers are hitting those levels now with lower stand counts, they’re getting bigger ears with heavier kernels.

As for the mythical 300 bushels per acre? You’ll want 45,000 plants per acre, and each ear should have 565 kernels with a weight of 260 milligrams per kernel.

If higher plant population is your ticket to higher yields, pushing those rows closer together is key to the high-yield math you’ll perform for your farm.

 

Seeking variable rates

The technology exists on many farms to significantly adjust your planting rate on the go. It’s an idea that makes perfect sense, and many growers are experimenting with it. Exciting new tools that crunch the numbers on yield potential, environment and seed populations are emerging in the marketplace, including Monsanto’s FieldScripts and a new prescription tool from South Dakota State University.

But right now, it’s hard to pin profits to the practice.

“Predicting where and by how much you should vary seeding rates is challenging and may not be repeatable from one year to the next,” notes Pat Reeg, technology manager for the Iowa Soybean Association On-Farm Network. He points out that 146 fixed-rate corn population trials conducted between 2009 and 2011 showed low correlation between optimum population and the variables that are typically assessed in variable rate seeding programs, like yield history, corn suitability ratings, soil conductivity or bare soil reflectance. That makes it difficult to accurately set optimum seeding rates.

On the other hand, agronomists like Norman Mieth of Central Valley Ag Cooperative’s Advanced Cropping Systems swear by the results of variable rate seeding. Mieth says reducing seeding rates from a typical 34,000 ppa to 24,000 on sandy pockets around Wayne, Neb., cut seed costs by $30/a and increased yields by 70 bu./a because plants weren’t fighting for water in droughty soils.

Stay tuned – and expect further fine tuning.