Adding alfalfa or clover to a traditional corn-soybean rotation dramatically reduces energy use without affecting profit, say Iowa State University researchers.

In a six-year study in Boone County, IA, they compared energy use and production efficiencies of a traditional corn-soybean rotation with a three-year rotation (corn-soybeans-small grain+red clover) and a four-year rotation (corn-soybeans-small grain+alfalfa-alfalfa).

“The three- and four-year rotations are representative of low-external-input cropping systems in the region that are integrated with cattle production through the feeding of crops to livestock and the application of manure to crop fields,” the scientists point out.

They found that extended rotations required significantly less energy investment than the two-year rotation. Adding a legume disrupted the life cycles of weeds, so less herbicide was needed, and nitrogen fixed by the legume and provided by manure greatly reduced the amount of nitrogen fertilizer needed by the corn crop.

Compared with the two-year system, nitrogen fertilizer inputs were reduced by 66% and 78%, respectively, in the three- and four-year rotations. Herbicide use was reduced by 80% in the three-year rotation and 85% in the four-year system. Despite those input reductions, corn and soybean yields in the extended rotations equaled or exceeded those harvested in the conventional two-year system.

The extended rotations did require about twice as much labor, primarily for cutting, raking and baling hay.

If fossil energy prices rise significantly in coming years without comparable increases in grain prices, more diversified crop rotations like these may be used more widely, the scientists conclude.