Over the long term, corn yields just keep edging up. It's mostly a steady, year-by-year ascent, occasionally interrupted by blips such as the drought that hammered parts of the Corn Belt this past year. But the relentless rise in yields promises to resume next summer.

The yielding ability of corn hybrids in central Iowa, for example, has risen steadily the past 70 years. So says Don Duvick, a retired corn research director for Pioneer, now a plant breeder at Iowa State University.

Duvick tracked the corn yield trend from 1934 through 2000 in a chapter of a soon-to-be-published book, Corn: Origin, History, Technology and Production (John Wiley & Sons, publisher), with chapter co-authors J.S.C. Smith and M. Cooper.

They report that yields have increased from about 70 bu/acre in the early 1930s to about 150 bu/acre in the late 1990s. That's about 1.1 bu/acre/year — a trend in much of the central Corn Belt.

“Fifty to 60% of U.S. corn yields have come from genetic improvements, with the remainder coming largely from agronomic practices,” Duvick points out. “We expect an increase in the proportion of yield gains that will come from genetics, while contributions from nutrients, herbicides and insecticides will likely plateau or decrease.”

Duvick's research shows that the maximum yield per plant has neither increased nor decreased since the early 1930s — when corn was grown stress-free at about 4,000 plants/acre. That means today's higher yields come from higher plant populations per acre, not from more production per plant.

What's behind these escalated populations and higher yields?

“It's greater stress tolerance,” says University of Illinois agronomist Emerson Nafziger. “Corn breeders have increased plant populations when screening best-yielding hybrids. That puts more stress on individual plants and increases stress tolerance without specifically creating stress conditions. This results in a better ability by plants to function (photosynthesize) even when weather is dry and temperatures may not be ideal.”

When testing at ultra-high populations, pressure is put on stalk strength, explains Joe Keaschall, a Pioneer corn research director. “We push as high as we can on population, and as we do so, stalks get somewhat smaller in diameter. We're looking for the practical optimum population.

“Our goal,” says Keaschall, “is to maintain stalk strength, ear size and disease-insect-drought tolerance while increasing plant population.

Corn breeders also are testing at more locations, says Duane Potrzeba of NC+ Hybrids. “The increase in test locations builds stability across environments and against the differing conditions that occur from year to year,” he notes.

What's the greatest risk to corn yields?

“Even though we have made great progress in drought tolerance, severe drought remains the biggest threat to yields,” says Potrzeba.

Another potential hazard: high nighttime temperatures that reduce yields by limiting available sugars during grain fill.

Where will genetic gains come from over the next five-10 years?

From traditional programs that screen and select for plant populations, disease resistance, drought tolerance and other stresses. From transgenic traits, such as those for rootworm resistance and Roundup resistance. And from the use of genetic markers that allow native diversity in corn populations, says Keaschall.

Duvick says exotic germplasm also has a place in future genetic progress. But in the end, he adds, the key to higher yields will be new hybrids with new levels of stress tolerance.