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Corn+Soybean Digest

Don't Underplant

For growers who haven't increased their corn population rates over the past five years, now might be a good time to reassess. And for two important reasons, experts say.

First, corn genetics have improved, increasing yields from higher plant populations. And second, corn prices have spiraled upwards, providing financial rewards for higher plant populations.

“It's hard to generalize with so many different hybrids and soil conditions, but most growers are probably planting below their optimum economic planting rate,” says Steve Paszkiewicz, research scientist with Pioneer Hi-Bred International's agronomy research department.

He gives the following example. At a grain price of $3.10/bu., seed cost of $1.81 per 1,000 seeds and an economic optimum planting rate of 31,845 seeds/acre, a grower is leaving more than $9/acre on the table by planting 27,000 seeds/acre instead of 32,000.

Paszkiewicz says the optimum economic planting rate — factoring in seed costs and corn prices — is about 2,000-3,000 seeds/acre less than what's possible agronomically.

Not all agree, however. Bob Starke, corn technical manager for Monsanto's Dekalb and Asgrow Seeds, says that “maximizing yield is what pays the bills,” and planting for the optimal agronomic yield will result in the greatest income potential per acre.

The right corn population is dependent on a grower's expected yield potential, Starke says. For example, in central Illinois a farmer who is fertilizing for 220 bu./acre corn will likely need about 34,000-35,000 seeds/acre to maximize yields. However, under western Kansas dryland conditions, a farmer who is fertilizing for 100 bu./acre might only need 20,000 seeds/acre to maximize yield potential.

Planting equipment has become far more precise over the past five years, Starke says, and growers in general “are doing a great job setting planting equipment. I see a lot fewer doubles or skips than four to five years ago.” He also sees more use of yield monitors and on-farm testing. However, it's important to plant at a slow enough speed to allow for the right corn population to get in the ground, he says.

That said, are growers planting too thin, too thick or about right? Some are planting too thin, but far more are planting too many seeds per acre, Starke says.

Iowa State University agronomist Roger Elmore says that growers are increasing their plantings by about 400 more seeds/acre/year across the Corn Belt, and that's where the yield increases of 2 bu./acre/year are coming from.

The reason why growers are able to do that, he says, is that breeders are selecting hybrids tolerant of being planted closer together. Perhaps in a decade or two, hybrids will be planted as close together as they can to achieve optimal yields in 30-in. rows.

At that point, one of two things will have to occur: either growers move to narrower rows so more corn can be planted per acre, or breeders focus on more prolific hybrids that produce more than one ear per plant.

In Elmore's view, most growers are planting the right number of seeds per acre. For Iowa, he says a “good average” is 32,000 seeds/acre, “and as you talk to people, most are within 2,000-3,000 of that.” He says that in general, populations should be lower in lighter soils, while somewhat higher in heavy or loamy soils.

To find the right population for an individual grower's field, Elmore advises using test strips of levels as high as 40,000 seeds/acre to see if higher levels than the average are economically justified.

One change growers will need to pay attention to this year, Elmore says, is that there is going to be more corn following corn, with corn acres increasing. That means more planting will be done in cooler and wetter soils with higher levels of residue.

“However, the best data available suggests that responses to planting rates do not change with different crop sequences. That means we don't need to worry much about changing plant populations as we go to more corn following corn.”

The important element in any cropping system, he adds, is to provide a good environment for seed germination and seedling establishment.

Kevin Turnblad, head of corn marketing for Syngenta, says that planter settings and the level of seed that gets in the ground are not necessarily the same thing. For example, farmers aiming to plant a population 32,000 or 33,000 in many cases are coming in closer to 28,500-31,300. So to hit 32,000, he suggests planting 3-5% higher to make sure growers plant the population they are shooting for. “That's where I would set my planter,” he adds.

What allows growers to continue to plant higher population levels, he says, are improved plant breeding combined with biotech traits that allow for tighter spacing.

One issue is whether increasing plant population will cause standability issues in the fall. Pioneer's Paszkiewicz says current hybrids are more tolerant of increasing plant populations because through selection, breeders have made them more “defensive” to pest and disease problems that in the past didn't allow growers to plant at higher populations.

In Minnesota, a corn population of 29,000-31,000 makes the most economic sense statewide, says University of Minnesota agronomist Dale Hicks.

He adds, however, that studies conducted in southern Minnesota at Waseca show that levels of 34,000-36,000 seeds/acre “aren't too high.” Hicks says that if corn is planted prior to May 1, the seed drop probably needs to be increased by 15% to achieve a final population of 30,000, however.

Turnblad says that 10 years ago, the germplasm for northern varieties couldn't take a plant population of 32,000, but that's no longer true.

Hicks says that the 2 bu./acre/year increase in plant yields has been about half management and half breeding.“We've gotten to the end of where we can go with management. Further increases will have to come from breeding,” he adds.

Part of that, Hicks says, will be the ability to boost plant populations. With corn following corn, growers may need to increase plantings by as much as 15% to obtain their desired yields.

He adds, however, that growers have to determine economic yield vs. total possible yield.

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