Moncultures versus polycultures
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.
Other diversification strategies
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.
- David Tilman, Jason Hill, and Clarence Lehman. "Carbon-Negative Biofuels from Low-Input High-Diversity Grassland Biomass." Science Vol. 314. no. 5805, December 8, 2006, pp. 1598 - 1600.
- Michael P. Russelle et al, "Comment on 'Carbon-Negative Biofuels from Low-Input High-Diversity Grassland Biomass'." Science Vol. 316. no. 5831. June 2007, p. 1567.
- N. Jordan, G. Boody, W. Broussard, J. D. Glover, D. Keeney, B. H. McCown, G. McIsaac, M. Muller, H. Murray, J. Neal, C. Pansing, R. E. Turner, K. Warner, D. Wyse. "Sustainable Development of the Agricultural Bio-Economy." Science Vol. 316. no. 5831, June 2007, pp. 1570 - 1571.
- Joseph M. DiTomaso, Jacob N. Barney, Alison M. Fox. "Biofuel Feedstocks: The Risk of Future Invasions." Council for Agricultural Science and Technology (CAST), CAST CommentaryQTA 2007-1, November 2007.
- Mike Morris, National Sustainable Agriculture Information Service (ATTRA)
- Sharon Lezberg, Associate Scientist, Environmental Resources Center, University of Wisconsin-Madison