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How much fuel went into producing the food on your plate? Chances are, it was a lot more energy than you will ever get out of eating that meal.

By some estimates, it takes about 10 calories of fossil fuels to get each calorie of food from farm to fork in the American food system. But it doesn’t have to be that way, according to a study published Monday in the May/June edition of Agronomy Journal.

Farmers can slash their fossil fuel use, while still growing bumper crops and turning a profit—all with the help of a little more crop rotation, concluded the team of researchers from Iowa State University after a six-year study.

In tests on a research farm in Iowa, the team mixed oats, alfalfa, and other crops into the rotation along with corn and soybeans, the two mainstays of the U.S. Corn Belt.

Fertilizing naturally

With a more diverse set of crops, the farms needed only a fraction of the normal amounts of synthetic fertilizers and herbicides, both of which are typically manufactured from natural gas.

One key was using alfalfa, which captures nitrogen from the air and stores it in the soil.

Thanks to this natural fertilization, the fields planted with alfalfa needed only about one-quarter the usual amount of nitrogen from chemical fertilizers.

The approach would be a major departure for most U.S. grain farms, which are accustomed to a two-year rotation—planting corn one year and soy the next, then repeating the cycle.

Agronomy professor Matt Liebman and a team of researchers at Iowa State University found that a farm could cut its fossil fuel use in half by shifting to a four-year cycle—adding a year of another grain, such as oats, and a year of alfalfa, a legume, to the typical corn-soy rotation.

These low-energy fields produced as many calories worth of crops, and generated about the same amount of money. "Our interest was not just how much corn we might be able to produce, but how much income might be generated on the farm," Liebman said.

The alternative approach required planning, labor, and one other important element—livestock. The researchers fed cows on corn, oats, and the hay from alfalfa, and then spread the cows' manure back on the fields. This contributed to fossil fuel savings by further reducing the need for synthetic fertilizers to provide received key nutrients like potassium and phosphorous.

"Manure is an excellent way to recycle these back to the fields," Liebman said.

However, the alternative methods did take twice as much labor, the study found,. ”We're not talking about huge amounts of back-breaking toil," Liebman said. "It's driving a tractor, and cutting hay, and then raking and baling it—and that's all done with machinery."

Incentive to Change

These complex rotations of crops have many benefits besides saving fossil fuels, said crop scientist Bill Deen of the University of Guelph in Ontario, Canada. The approach increases soil carbon levels and reduces erosion, he said.

There is a downside, however: "While complex rotations are desirable, there is limited market incentive to use them," Deen said, because there's a small market for crops such as alfalfa.

Deen said that another approach to saving fossil fuels is to grow alfalfa and other plants as "cover crops," which are left on the field rather than being harvested. "I think cover crops make more sense than trying to introduce crops with limited

market opportunity," Deen said.

Liebman’s team notes that Iowa had a long history of farms that mixed crop and livestock production. But a combination of low energy prices and high labor costs contributed to the adoption of the conventional two-year corn and soybean rotation the researchers conclude.

But grain farmers have found their reliance on fossil fuels can turn costly when oil prices spike. Also, farmers have been watching closely the possibility of federal action on global warming, which could cause fossil fuel prices to rise even more.

The Iowa State researchers argue that if fossil energy prices rise, but crop values don't keep up, farmers might have the incentive they need to shift to a more diversified cropping system.

"As fossil energy becomes more expensive,” Liebman said, “the conventional systems will be at a disadvantage."