What Would happen if you bumped up your average corn-seeding rate by 5,000 more seeds per acre? It might be worth testing, particularly if you have productive ground and are planting today's top hybrids.

Although the average plant population in Iowa has been around the 30,000-plants-per-acre mark for the last few years, Iowa State University (ISU) has been seeing good yield performance at 35,000 plants or more per acre.

Some soils and conditions might not be conducive to higher populations, says Roger Elmore, ISU agronomy professor, but he adds that, across the Corn Belt, the average plant population is increasing about 400 plants/acre/year.

Today's elite hybrids are more tolerant of higher plant populations, and as populations have increased, so has stress tolerance, Elmore says. ISU has conducted quite a bit of plant population research, and Elmore has found that, at 32 locations over the last three years throughout the state, populations of 35,000 to 37,000 seeds/acre have yielded better than lower or higher populations have.

37,000 max

Growers wanting to try higher seeding rates should do some strip testing in their fields first, Elmore says. He suggests increasing the seeding rate by 5,000 seeds/acre because increasing the rate by just 2,000 or 2,500 seeds/acre will be less likely to show significant yield improvement.

Higher yield potential begins to drop off above 37,000 seeds/acre, Elmore says. He has observed more stalk lodging and water-deficiency problems at these higher rates. Growers also need to consider the economics of higher seeding rates. Depending on the cost of the seed, growers may need to reduce their planting rates by 1,000 or 2,000 seeds/acre to 36,000 or 35,000 seeds/acre, Elmore says. Growers must consider whether additional seed will pay for itself.

Growers should plant replicated strips of the same hybrid, not just ones that are closely related, to get a true measure of increased seeding rate, Elmore says. He advises against testing changes in seeding rates and row spacing at the same time because this does not allow for true apples-to-apples comparisons.

Studies across the central Corn Belt have not shown that narrower row spacing (compared to the more traditional 30-in. rows) significantly increases yields, Elmore says. Most winners of the National Corn Yield Contest still plant in 30-in rows. But narrower row spacing has not resulted in yield reduction either. “Row spacing doesn't appear to be a yield-limiting factor,” Elmore says.

Research also has shown that hybrids with transgenic traits (such as resistance to the European corn borer or corn rootworm) perform similarly at the same population as hybrids without such traits in the absence of pest pressure or when an insecticide is used with a conventional hybrid, Elmore says. The trait should be considered separate from the hybrid in this case, he notes.

Calculating the economics

Like many universities, many seed companies have been conducting research on seeding rates. Pioneer, for example, has taken what is has learned over several years of research and developed a planting rate calculator. Pioneer sales professionals, such as Jerry Boeck, Exeter, NE, use this calculator to help customers determine the best seeding rate for optimal economic yield.

Boeck enters seed, expected grain prices, and yield environment information into the calculator to help customers determine optimal seeding rates. Last year and again this spring, Boeck worked with John Popisil, who farms 5,300 acres near Friend, NE, to calculate the best seeding rates for his particular fields.

At last year's seed and grain prices, they determined Popisil could plant 38,000 seeds/acre for the best return on investment. At this year's prices, they calculated that 36,500 seeds/acre would be the better choice. Before Boeck showed him the planting rate calculator, Popisil had been planting at 29,000 to 30,000 seeds/acre.

“We're planting at higher populations than we ever dreamed of,” Popisil says. “Some people are concerned about stalk lodging. But with the genetics and fungicides we use, there is no stalk quality issue.”

With the use of the planting rate calculator, Boeck hopes to help customers reach an average of 250 bu./acre.

No one size fits all when it comes to seeding rate recommendations. Growers need to take into account their soil productivity and historical yield performance when deciding whether to increase their seeding rate, says Marcus Jones, manager, Technology Development, Corn Systems and Corn Germplasm Technology Development, Monsanto.

Growers can generally plant at higher rates now because of elite hybrid germplasm and stress-mitigating insect protection traits that tolerate higher population densities, Jones says. He adds that more growers are getting the message that they can benefit from higher plant populations and are talking with seed representatives and crop consultants about strip trials.