The vast majority of farm-based bioenergy production currently relies on monocultures: pure stands of a single plant species such as corn or soybeans. Yet researchers have long known that plant mixtures or polycultures offer numerous advantages, including resistance to plant diseases, insect pests, weeds and other invasive species.
In 2006, a study led by David Tilman at the University of Minnesota found that degraded agricultural land planted with highly diverse mixtures of prairie grasses and other flowering plants produced 238 percent more bioenergy, on average, than the same land planted with monocultures of switchgrass or other prairie plant species.
The study found that mixed-grass plots restored biodiversity, were highly resistant to plant diseases and invasive species, required far less inputs of fertilizer and pesticides than corn or soybeans and stored more carbon in the soil. Moreover, these plots were superior to corn ethanol and soybean biodiesel in their energy yield per acre and in their impact on reducing greenhouse gases. Because these prairie grasses grow well on degraded land, they may also avoid competing with food crops for the best agricultural land.
Other researchers have also been looking closely at the advantages of polycultures for bioenergy production. For example, long-term studies at the Great Lakes Bioenergy Research Center and W.K. Kellogg Biological Station in Michigan are comparing biomass production in a wide variety of cropping systems and prairie ecosystems.
In addition to diversifying plant mixtures, crop rotation over time and the use of cover crops are also important strategies for reducing disease and pest problems and making farming systems more sustainable.