Farmers have utilized rotations of multiple crops over a several year period for hundreds or perhaps even thousands of years. Archeological evidence suggests that farmers in the Fertile Crescent region (northern regions of the modern countries of Syria and Iraq) used such practices as far back as 6,000 B.C. Medieval farmers in Northern Europe grew multiple crops in sequence, such as wheat, followed by barley or oats, then legumes for livestock fodder. A year of fallow (leaving the field bare during the growing season to allow the soil to recover) was often included in the rotation to help the soil recover.
Native farmers in North America used complex cropping systems, often raising corn, pole beans, and squash (or pumpkins) on fields at the same time. Archeological evidence indicates that this practice emerged in North America about 1070 A.D, although it had been used by Indigenous peoples in Central and South America far earlier. Some East Coast tribes such as the Iroquois and Cherokee referred to this combination of crops as the Three Sisters, planting the corn on small hills throughout the field, surrounded by beans, with squash plants interspersed throughout the field. The climbing beans, typically tepara or common bean varieties, naturally absorb nitrogen from the air and convert it to nitrates, fertilizing the soil for the corn and squash. In return, they are supported by winding around the corn stalks as the corn grows. The squash leaves provide ground cover between the corn and beans, preventing weeds from taking over the field and help with water retention in the topsoil. These three plants yield better together than when they are planted on a mono-crop basis.
When eaten together, the Three Sisters are a complete and balanced meal, rich in carbohydrates, protein, vitamins, and minerals. Another benefit is that all three crops can be stored for long periods of time.
The Puritan (or Pilgrim) immigrants who founded the Plymouth Colony in what is now the Commonwealth of Massachusetts in 1620, those settlers adopted the Three Sisters practice, and elsewhere in the original colonies farmers utilized rotational cropping practices that often included a year of fallow, although their primary crops were the small grains (wheat, barley, rye etc.) imported from Europe.
In the early 1900’s, George Washington Carver’s work at Tuskegee University explored a number of agronomic areas which revolutionized agriculture, especially in the Southern United States. Among his more important contributions was to promote crop rotations that included legumes such as peanuts and cowpeas, which helped to replenish the nitrogen content of soils in fields that had been exhausted of that component due to continuous cotton production.
Soybeans were first grown as a forage crop as part of American farmer’s’ rotations in the mid-19th century, after having been brought from China as a novelty crop to the colony of Georgia in 1765. It was viewed primarily as a food ingredient for several decades. However, this crop was not planted widely in the United States until early in the 20th century when the technology was developed to use chemical solvents to separate meal and oil from the whole bean, making the meal component more digestible for livestock than the whole bean was. USDA first reported soybean acres grown for processing into meal and oil in the 1939 Census of Agriculture, with about four million acres harvested. Today, a two-year rotation of corn and soybeans is the dominant cropping system in the United States, especially in the Midwest. An analysis of recent precision agriculture data shows that around 75 percent of U.S. farmers plant different crops on the same field in consecutive crop years, though most are likely using a corn-soybean rotation.
A 2019 study found that while corn-soybean rotations do generate certain benefits for farmers, such as allowing them to reduce overall nitrogen fertilizer use, saving costs and reducing fertilizer runoff. However, the research also indicates that use of this rotation leads to lower soil carbon content compared to continuous corn production or using more complex crop rotations. This work and similar research strongly suggest that it would be beneficial for the environment for farmers to consider adding a third crop to their rotation. Other research finds that more complex rotations improve resilience to extreme weather events and also help with weed control.
There are several candidates for what that third crop could be. A 2020 study published in the Agronomy journal found that adding winter wheat to a corn-soybean rotation increased net returns by 10 percent over the four-year rotation. Other small grains such as barley or oats would generate similar benefits. Adding a third crop to the rotation, especially one that is planted in the winter rather than the spring can spread out fieldwork and harvest time, both of which can help reduce risk.
With the legalization of growing and marketing industrial hemp accomplished in the 2018 farm bill, that decision added another candidate to the list of potential third crops. Hemp has several benefits that make it worth considering–it can enhance soil health by enriching biodiversity, can reduce the need for chemical inputs such as pesticide and herbicide, and is quite water-efficient. However, the robust market for industrial hemp to produce CBD products that was anticipated by farmers a few years ago has not emerged due to lack of regulatory approval, and many farmers are waiting to see if the upcoming farm bill can address some of the regulatory uncertainties that cloud the market prospects for this crop.
One of the barriers to farmers who might consider this modification to their cropping system is the lack of markets for that third crop. In recent years, Practical Farmers of Iowa has held several workshops and conferences for farmers, sharing information about where they might find buyers for their small grain crops. They also offer a program for farmers in five Midwest states who grow a small grain followed by a cover crop, to provide them either with cost share assistance to make the change or a connection to a small grains market program. Enrollment in this program is limited, but it might provide a roadmap for other groups to follow a similar approach.
