Full-grown corn plants should shade the ground Once corn has pollinated and grain fill starts, growers need to go into their fields and peer down the rows. Hopefully, they'll be mostly in the dark.

"It's a prime time of year to look at how well the canopy is absorbing the sunlight that is essential for high yields," points out University of Illinois agronomist Emerson Nafziger.

"To assess the canopy, go into fields in the late morning or early afternoon when the sun is high and bright," Nafziger recommends. "See how much sun is reaching the soil. A good canopy should be intercepting more than 95% of the sunlight. Any areas of sunlight hitting the soil should be small and few."

Nafziger says every patch of light that hits the soil surface during grain fill is lost to the crop. That means less yield.

Plant population, size and row width all affect canopy, he adds.

He conducted a three-year study with six hybrids, at populations of 10,000, 15,000, 20,000, 25,000, 30,000 and 35,000 plants per acre (30" rows). Sunlight interception increased with population, from about 70% at 10,000 plants/acre to more than 95% at 35,000. Nafziger calculated how the percentage of light interception related to yield.

"When averaged over those six hybrids, each percent increase in light interception meant 3.5 bu/acre more yield," he reports. The yields ranged from about 125 bu/acre at the 10,000 population to about 210 bu/acre at the 35,000 stand.

Independent crop consultant Dave Harms of Crop Pro-Tech, Bloomington, IL, wants his grower-clients to take full advantage of available sunlight. He and his staff, operating across Indiana, Illinois and Iowa, monitor midseason corn canopy as a regular part of their crop scouting program.

"For most growers, we advise a harvest stand of 28,000-32,000 plants/acre," says Harms. "At that range we find we are intercepting a very high percentage of sunlight."

But hybrids vary in plant size. Should this, in Nafziger's thinking, affect population recommendations?

"Hybrid size can be a factor," he replies. "However, the effect of hybrid within the common range of maturities a farmer might use would probably be much less than the effects of growing conditions, nutrient deficiencies or pests on effective canopy area. All things considered, I think that 30,000 emerged plants of adapted hybrids will in almost every case provide near-optimal light interception percentages."

Although Nafziger has been able to measure sunlight interception in research plots, he says it's not a simple thing to measure accurately in a farm field. He wants to remedy that.

"I really hope to work on this for 2001," he says. "It would include a color publication that has some low-tech ways to measure light interception, plus other helpful information on how to use this concept."