Should you change your corn plant population depending on when you plant? The answer is no, according to recent population trials from the University of Minnesota (U of M). “The optimum plant population does not change with planting date,” says Extension Corn Agronomist Jeff Coulter. His research, funded by the Minnesota Corn Growers Association, confirms findings from Illinois and other Midwest states.

Comparing plant populations in 2008 and 2009 in southern Minnesota, corn yields were highest at 36,800 plants/acre, regardless of whether planting was done in late April, mid-May or late May, Coulter found. The most profitable plant population — which varies with corn value and seed price — was a bit lower, ranging from 32,000 to 34,000 plants/acre.

The main reason that higher populations raise grain yields is because of increased light interception by the crop canopy during grain fill, Coulter says. As plant population increased from 15,000 plants/acre to 32,000, canopy light interception rose from 82% to 92%, and grain yield rose from 157 bu./acre to 190 bu. But bumping up the plant population from 32,000 plants/acre to 44,000 increased light interception by only 3 percentage points and grain yield by just 1 bu./acre.

That means “growers are on a bit of a teeter totter when it comes to population,” Coulter says. “If you overseed, you waste money. But if you underseed, you lose the potential to capture higher yields under good weather conditions at grain fill.”

EVEN THOUGH YOUR target population will not change with the planting date, you might want to adjust the number of seeds you put in the ground based on expected emergence, Coulter adds. With later-planted corn and warmer soils, you may be able to back off 2% on seeding rates, saving a few dollars per acre.

Optimum corn plant population doesn't change with row width or hybrid maturity, either. In five years of southern Minnesota trials, “yield response to plant population was similar for both 20- and 30-in. rows,” Coulter says, showing that “optimum plant population is not affected by row spacing.” Final stands of 32,000-34,000 plants/acre will earn the highest return — whether you plant in conventional or narrow rows, he says.

However, narrow rows did boost corn yields about 4% in 2009 in the Red River Valley of Minnesota and North Dakota, where growers who have sugar beets in their rotation often plant corn on 22-in. rows. “We have a lot of narrow-row corn,” says Phil Glogoza, a U of M Extension crops specialist in the Red River Valley. “Up here, narrow rows appear to be a good choice.” But farther south, there was no advantage to narrow rows, Coulter says.

Likewise, Coulter found no evidence that the optimum plant population changes for different hybrid maturities. He tested 95-, 101- and 105-day RM hybrids in southern Minnesota and 80-, 89- and 92-day RM hybrids in northern Minnesota.

Of course, planting date is still a key factor in corn yield potential. Many studies from around the Midwest show that there is a greater risk of yield loss from planting late than from planting too early.

University of Illinois research from 2005 to 2007, for example, showed that when planting is more than 20 days past the optimum date, yield losses approach 1 bu./acre/day. Coulter's research in 2008 and 2009 showed that yields in southern Minnesota dropped by just 3% when corn was planted in mid-May, but yields fell 17% when planting was delayed until late May.

IN HINDSIGHT…

Andy Beyer raises corn, soybeans, spring wheat and sugar beets on 2,400 acres near Breckenridge, MN, in the southern Red River Valley. Last spring, planting was delayed by heavy rains and flooding, which compounded problems from the late harvest in 2008. Growers ran out of time to do fall tillage, and Beyer and many others had to resort to burning corn residue before planting in 2009.

Beyer wasn't able to start seeding corn until the second week of May. He finished at the end of the month, three to four weeks late.

Planting in 22-in. rows, he sent 34,000 seeds/acre down the seed tubes, aiming for a final stand of 30,000-31,000 plants/acre. He didn't back off on seeding rates for his late-planted corn because “planting conditions weren't that good. It was real wet,” and he had compaction problems on some fields, too.

Beyer usually plants 87-92-day RM hybrids. In mid-May, he dropped back to 82-day corn. But after harvesting the 2009 crop at 30% moisture or more, he'd do things differently next time, he says. “I'd stop planting spring wheat on May 1, and I'd stop planting corn mid-May, and only plant beans after that.”