Continuous production of the same crop often leads to a decline in yield for a variety of reasons, even with sufficient nutrient inputs. For centuries, farmers relied on crop rotation to maintain or enhance crop yield. Using forage grasses and legumes in rotation with summer annual or winter annual row crops can supply nutrients to subsequent crops that can decrease the need for purchased inputs.
Perennial forage crops protect the soil from wind and water erosion and use nutrients more efficiently than row crops that are usually only growing during a fraction of the growing season. Using perennials to establish permanent grasslands on highly erodible soil can eliminate almost all soil erosion.
Crop rotations including perennial forages usually have soils with higher organic matter because continuous root formation, growth, and death contribute carbon to the soil. Furthermore, land in perennial forages is not tilled, which lowers oxidation losses of soil organic matter. Organic matter inputs help increase the soil water holding capacity, which can help maintain crop growth during periods with below-average rainfall.
Despite these benefits, production of perennial forages dropped in the United States during the 20th century. Reasons for this decline include the development of pesticides, the expansion of fertilizer manufacturing, and changing rations for ruminants—animals, such as cattle and sheep, with a four-chambered stomach digestive system—the primary consumers of forages.
External inputs for crop production substitute for the ecological role crop rotation provides by breaking pest cycles and using forages to supply nitrogen to subsequent crops. The environmental effect of this paradigm shift has resulted in the creation of federal agencies and policy to regulate, research, and promote environmental stewardship. It remains unclear how emerging and future markets for agricultural products will affect the environment. Potentially, landscapes covered with perennial grasses and legumes can play a dominant role in stabilizing soil and water resources, provide feed for ruminants and herbivores, and contribute biomass as a source of biorenewable energy.
Feature adapted from Chapter 7, “Grass-based Farming Systems: Soil Conservation and Environmental Quality” by Jeremy W. Singer, Alan J. Franzluebbers, and Douglas L. Karlen in the book: Grassland Quietness and Strength for a New American Agriculture, Walter F. Wedin and Steven L. Fales, editors.