Corn yields seem to rise every year, along with demand. But the question is whether, even with genetic tinkering, the yields will continue to increase. As farmers put the state's 2009 corn crop into the ground this month, they expect to grow more corn per acre than last year. And if history is any guide, they will. Farmers today harvest more corn than their parents did a generation ago from the same fields, a fact made evident in historical charts that show corn yield over the past several decades as a steadily rising line.

So reliable is corn's growth of about 2 extra bushels per acre per year that government analysts folded it into their forecasts for this year's 12.1 billion bushel crop. It's just expected.

And yet it's still not enough. The state's ubiquitous crop that's become a staple for feed, fuel, sugar and everything from drywall to shoe polish is in more demand than ever, with a third of the crop going to ethanol.

Some say farmers will rise to the challenge, that corn yields will grow even faster in years to come.

"We are projected to double corn yields in this nation in the next 20 years," said Jeff Broin, CEO of ethanol refiner Poet, a South Dakota-based company that produces 1 billion gallons of ethanol a year.

The reason for at least some of his optimism came last year when scientists cracked the corn genome, a string of 2.5 billion pairs of DNA bases, ushering in the promise of yet more genetic modification of a plant that since 1996 has become one of the world's largest transgenic crops. Seed companies are among those who, like Broin, predict a doubling of corn yields within a few decades.

But the history and hype surrounding corn yields has one staggering asterisk: The very best farms, blessed with the best weather and land, have posted the same yields for at least 20 years -- suggesting they have reached the limit of what the corn plant can produce.

"The odds, if we keep the status quo, of those yield trends slowing down is very high," said Roger Elmore, an Iowa State University professor of agronomy. (startribune.com)