But this year is different. Now it is not just the demand for food that is driving the decision, it is also the demand for ethanol, the fuel that is made from corn.
Some states are requiring that ethanol be blended in small amounts with gasoline to comply with anti-pollution laws. High oil prices are dragging corn prices up with them, as the value of ethanol is pushed up by the value of the fuel it replaces.
"We're leaning more toward corn," said Garold Den Herder, a farmer who cultivates 2,400 acres in a combination of corn and soybeans and is on the board of directors of the Siouxland Energy and Livestock Cooperative, which opened an ethanol plant here in late 2001. Last year a bushel was selling for about $2 here, but near the plant it was about 10 cents higher.
Farmers expect it to go higher soon if oil prices stay high. Ethanol was up to $1.75 a gallon, last year, from just over $1 the year before.
The rising corn prices may be good news for farmers, but they are worrying some food planners.
"We're putting the supermarket in competition with the corner filling station for the output of the farm," said Lester R. Brown, an agriculture expert in Washington, D.C., and president of the Earth Policy Institute. Farms cannot feed all the world's people and its motor vehicles as well, Mr. Brown said, and the result is that more people will go hungry.
Others say that the price of goods that have corn as an ingredient, including foods like potato chips or Danish pastries, will rise.
But Robert C. Brown, a professor of mechanical engineering at Iowa State University and a specialist in agricultural engineering, said the use of corn for nonfood purposes sounded harsher than it was. "The impression is that we're taking food out of the mouths of babes," Professor Brown said. In fact, corn grown in Iowa is used mostly to feed farm animals or make corn syrup for processed foods.
And Bernie Punt, the general manager of the Siouxland plant, said, "It's not as big a loss as what it seems like," pointing out that the corn remnants that come out of the other end of the plant were used for animal feed.
A global shift to farm-based fuel could reduce the need for oil and slow climate change. But Lester Brown is not alone in worrying about the effect on world hunger. For 20 years, the International Food Policy Research Institute, a nonprofit group in Washington, has maintained a computer model to predict food supplies, based on population changes, farm policies and other factors.
Until now, the institute's analysis had included the price of oil and natural gas only as a factor in production costs, including the price of making fertilizer, running a tractor or hauling food to markets. But last year, after Joachim von Braun, the director of the institute, went to Brazil and India, both of which make vehicle fuel from plants, he told his economists to change the model, taking into account the demand for energy from farm products.
Even a small shift could have big effects, Mr. von Braun said, because "the mouth of your car is a monster compared to your family's stomach needs."
"I do not just expect somewhat higher food prices, but new instability as well," he said in an interview. "In the future, instability of energy prices will be translated into instability in food prices."
Gustavo Best, the energy coordinator at the United Nations Food and Agriculture Organization, said growing crops for energy could provide new opportunities for small farmers around the world and finance the development of roads and other valuable infrastructure in poor rural areas.
But, Mr. Best added, "definitely there is a danger that the competition can hit food security and food availability."
Some experts scoff at the idea of corn shortages, but others say it is possible. Wendy K. Wintersteen, the dean of the College of Agriculture at Iowa State University, said that possibly as early as this summer, "we will have areas of the state we would call corn deficient," because there will not be enough for livestock feed - the biggest use of corn here - and ethanol plants.
"It's a hard thing to imagine in Iowa," Ms. Wintersteen said. Eventually, experts say, American corn exports could fall.
Nationwide, the use of corn for energy could result in farmers' planting more of it and less wheat and cotton, said Keith J. Collins, chief economist of the Department of Agriculture. But the United States is paying farmers not to grow crops on 35 million acres, to prop up the value of corn, he said, and much of that land could come back into production.
A change is under way that experts say will tightly tie the price of crops to the price of oil: ethanol plants are multiplying.
Iowa has 19 ethanol plants now and will have 27 by the end of the year, said Mr. Punt, a former president of the Iowa Renewable Fuels Association. The Siouxland Energy and Livestock Cooperative showed a $6 million profit for 2005, Mr. Den Herder said, driven in part by the price of ethanol.
Many farmers here in the corn belt say they have the ability to grow the material for vast amounts of fuel. Another biofuel is a diesel substitute made from soybeans, which still leaves about 80 percent of the bean for cattle feed, advocates say.
Joe Jobe, executive director of the National Biodiesel Board, a trade group, predicted that more demand for soy oil as a diesel substitute would force production of meal, pushing down its price and thus making cattle feed cheaper.
"I think there's a historical shift under way, not to grow more crops for energy and less for food, but to grow more for both," Mr. Jobe said.
Nick Young, the president of an agriculture consulting firm, Promar, in Alexandria, Va., pointed out that corn products have been used for nonfood purposes for years, including to make fluids used to help drill oil wells. Mr. Young said it was an exaggeration to say that nonfood use of crops will make the world's poor go hungry, but he added that the use of vegetable oil as a substitute for diesel fuel had already driven up the price of canola oil.
"These markets are linked," Mr. Young said. "Inevitably, there's going to be some interaction on food prices."