Since 2003, Iowa State University agronomist Matt Liebman and other researchers have been comparing two-year corn-soybean rotations with two longer-term rotations on plots west of Ames, Iowa. Economic analysis shows one of those rotations – three years of corn, soybeans, and oats/red clover – had the profit edge in 2008 through 2010. The other long-term rotation in the study was corn, soybeans, oats/alfalfa, and a fourth year of alfalfa hay.
In those more recent years of 2008-2010, the study included both genetically modified (GMO) corn and soybeans and non-GMO corn and beans in the rotations. The top net return to land and management was a three-year average of $379 an acre for the corn, soybean, and oats/clover rotation with non-GMO corn and beans. The same rotation with GMO crops netted nearly as much – $376 an acre. The next two highest returns (also averaged from 2008-2010) were $343 per acre for the four-year alfalfa rotation with non-GMO crops and $336 per acre for the two-year GMO corn-bean rotation.
This fall, ISU Extension economist Craig Chase and program specialist Ann Johanns are releasing averages of a six-year summary of the same trials, from 2006 through 2011. It, too, shows the three-year rotation on top. Looking at just return to management, it netted $194 an acre. Two-year rotations netted $187 an acre, and the longer alfalfa rotation was last with a return of $171 per acre. They simplified the comparison, looking only at GMO crops in the two-year rotations and non-GMO crops in the longer rotations.
In some years, the results were striking. During the drought of 2012, the alfalfa rotation looked attractive. “We already harvested, in three cuttings, $840 worth of alfalfa,” Liebman said in midsummer. “There's some advantage in diversity, even in a drought year.”
Also in 2012, the oat cover crop (planted with first-year alfalfa and with clover) averaged 118 bushels an acre. It was worth $3.90 a bushel.
In wet years, differences were dramatic, too. In 2010, fewer than 9% of the beans grown in three- and four-year rotations showed sudden death syndrome (SDS). GMO soybeans in the two-year rotations had 27% incidence of SDS. Nearly all (96%) of non-GMO beans in a two-year rotation were affected by SDS.
Why rotations pay off
At first glance, it might have seemed more profitable to raise corn and soybeans. Even a good oats yield was only about half that of a normal corn crop, and the prices were much lower. The red clover was plowed down as green manure for nitrogen and organic matter, not harvested. That last year in the three-year rotation certainly had lower revenue.
In fact, gross returns for GMO corn and soybeans were higher in all three years of the most recently published study of the ISU work. The two-year GMO crop rotation's revenue averaged $673 an acre. The three-year rotation with non-GMO corn and beans and oats/clover had the lowest average revenue of $576 an acre. Higher costs for the corn-soybean rotation made it slightly less competitive with the three-year rotation. Costs for corn and beans averaged $316 an acre, while the three-year rotation cost an average of $207 an acre. (These results were published this year in the journal Renewable Agriculture and Food Systems.)