If you are not increasing corn plant populations every year, you may not be reaping full yield benefits, says Roger Elmore, extension corn specialist, Iowa State University. Elmore has found that a population of about 35,000 plants/acre is optimal for yield in tests he conducted throughout Iowa.

Corn plant populations have increased by about 400 plants/acre/year in Iowa over the last 20 years. Plant population numbers also are going up in other parts of the Corn Belt due to development of hybrids that are more tolerant of high population stress. Peak populations of 35,000 to 40,000 plants/acre are about 2,000 plants/acre greater than just a decade ago.

Researchers and farmers in corn yield contests have planted as many as 60,000 to 65,000 plants/acre, but Elmore suggests that growers try testing 30,000, 35,000 and 40,000 seeding rates, with at least three replications within a field or in three different fields. The replications increase the chance that measured yield differences are due to the seeding rates and not the specific location within a field, Elmore explains.

Growers need to step up their level of management at higher plant populations, says Fred Below, University of Illinois crop physiologist. This includes managing row arrangement and fertilizer placement. Geography plays a large role in plant population, Below says. In shorter maturity areas with longer daylight, growers may plant higher populations to intercept more sunlight, he notes, adding that some growers have reduced row spacing from 30 to 20 in.

Iowa State University’s 2001 Corn Planting Guide indicates a relationship between seeds planted, distance between seeds and projected final plant stand. For a final plant population of 30,600 (assuming 15% mortality), for example, one would plant 36,000 seeds/acre if using a 30-in.-row system. In this scenario, there would be 5.8 in. between each seed in a row. To estimate plants per acre, one should count plants (not tillers) for one row and multiply by 1,000, taking into account the row width (30 in., for example) and row length (in feet equal to 1/1,000th acre).

Plant population refers to the number of plants per acre while seeding rate refers to the number of seeds planted per acre. Growers will need to plant more seed than the plant population they hope to achieve. The cost of seed, therefore, may be a limiting factor. Seed germination percentage, soil fertility and weather are also factors.

The development of more drought-tolerant hybrids may allow growers in dryer areas (western Iowa and eastern Nebraska, for example) to increase plant populations in the future.

With continual improvements in corn genetics and the growing demand for corn, plant populations will continue to rise, possibly from 400 to 500 plants/acre/year. “We’ll never see them go down again,” Below says.