December 2, 2011 | Andrew Burger

Results of a long-term study conducted by the Leopold Center for Sustainable Agriculture at Iowa State University to compare the agronomic, economic and ecological benefits of conventional versus low external-input (LEI) cropping systems showed that low-input, diversified crop rotation systems can produce high yields of corn and soybean, improve soil quality, suppress weeds effectively, reduce the need for fossil fuel inputs as well as requirement for synthetic N fertilizer.

Synthetic fertilizers represent a major expenses for farmers in Iowa and throughout the US Corn Belt, and they’re often linked to environmental damage and land degradation. Such fertalizers are also tied to the use of fossil fuels, the reduction of which is seen as critical to improving agricultural sustainability.

“Today’s high prices for corn and soybeans and minimal incentives and regulations to promote environmental protection discourage widespread adoption of diversified LEI (Low External Input) systems in Iowa and much of the central Corn Belt,” the report authors wrote. “However, if fossil energy costs increase substantially relative to crop prices, the diversified LEI systems tested in this project could serve as models for the types of cropping systems that will have to be considered if Iowa agriculture is to remain productive and profitable.”

Looking to discover ways to reduce farmers’ reliance on these high cost and often volatile external inputs, the Leopold Center research team compared the agronomic, ecological and economic performance of three different cropping systems over a period of four years (2007-2010): a conventionally managed, two-year corn-soybean rotation, and two diverse low external input (LEI) cropping systems comprised of a three-year corn-soybean-oat/red clover rotation, and a four-year corn-soybean-oat/alfalfa-alfalfa rotation.

The field experiment was conducted on a 9-hectare (22 acre) field in Iowa’s Boone County. Use of synthetic nitrogen fertilizer was 89% and 93% lower in the three-year and four-year LEI systems, respectively, than in the two-year conventional cropping system. Herbicide use was reduced by 96% and 97% in the three-year and four-year old LEI systems.

Researchers measured crop yields, weed dry matter production, weed seed densities in soil, economic costs and returns, fossil energy use and soil organic matter concentrations. Corn and soybean yields in the two lower input, LEI rotation systems were higher than in the conventional rotation system, while weed biomass in corn and soybean was low in all of the systems.

Concentrations of soil particulate organic matter (POM-C) were found to be significantly greater in both the three-year and four-year LEI rotation systems. That’s indicative of healthier soil, according to the researchers, who noted that the results suggest that, “soil organic carbon is increasing in the more diverse LEI (Low External Input) systems.”

Levels of potentially mineralizable nitrogen (PMN) – another indicator of soil health – also were higher in the three-year and four-year systems. According to the study’s authors, the higher PMN levels indicate that the more diverse, alternative rotations had greater capacity to supply crops with nitrogen.

Economic Impact

With respect to the comparative economics of the three crop rotation systems, the researchers found that at $966 per hectare per year, net returns to land and management was highest for the three-year LEI rotation system, and lowest for the four-year LEI system at $884 per hectare per year. The conventional, two-year rotation system resulted in a return of $911 per hectare per year.

Labor costs were higher for the alternative, lower external input systems due to their longer rotation periods, but they “were only a small fraction of total production costs,” according to the report authors.

Not surprisingly, the conventional two-year system used the largest amount of fossil fuel energy, the four-year LEI system used the least, and the three-year LEI system’s fossil fuel energy use fell somewhere between the two extremes.

For more details on the results of this study click on the following link: http://www.leopold.iastate.edu/grants/e2007-09