Effects of crop rotation diversity
Crop rotation is a term used to describe the pattern of growing two or more crop species in a given field in some consecutive order. Soybean is commonly rotated with corn, wheat, rice, or grain sorghum in the U.S. The perception is that rotation of soybean with a grain crop provides positive agronomic, environmental, and economic benefits. This is based on long-term rotation research that has been conducted in the Midwest [Click here to access a White Paper on this website that summarizes this research].
Nationally, the majority of the corn and soybean acres [average of 81,890,200 and 83,556,800, respectively, during the 2018-2022 period--NASS] are rotated with each other. In the Midsouth, soybean is rotated with rice and cotton in addition to corn. There is evidence that a diverse rotation that involves more than corn and soybeans grown in rotation will improve both soil health and yield of rotated crops. This evidence is presented in results from research reported in the following articles.
More diverse crop rotations improve yield, yield stability, and soil health [Oct. 2021, Univ. of Nebraska, Lincoln] by Wagner, Jin, and Schmer. Results are reported from a long-term dryland no-till crop rotation and nitrogen [N] fertilizer systems study [started in 1972, converted to no-till in 2007-2013] that was conducted in Nebraska. Rotations consisted of continuous corn, continuous soybean, continuous grain sorghum, corn-soybean, grain sorghum-soybean, corn-soybean-grain sorghum-oat/clover, and corn-oat/clover-grain sorghum-soybean. Results showed that diverse crop rotations provided more agronomic and soil benefits than applying N fertilizer alone–i.e. fertilizer N was no substitute for crop rotation. Overall, rotating crops improved soil and crop yields with concurrent lower fertilizer-N costs.
Long-term research reveals advantages of diverse crop rotations from South Dakota Soil Health Coalition. Four-year crop rotations included combinations of corn, soybean, spring wheat, winter wheat, oats, field peas, and sunflower. Increasing rotation diversity resulted in more carbon [C] in the soil, more soil organic matter [SOM], and an overall improvement in soil health.
Increasing crop rotational diversity can enhance cereal yields by Smith et al. appears in Communications Earth & Environment [2023, 4:89, https://doi.org/10.1038/s43247-023-00746-0]. In this analysis, the authors use yield data of small grain cereals and corn from 32 long-term experiments across Europe and North America to show that crop rotational diversity [measured as crop species diversity and functional richness] enhanced grain yield, and the yield benefit increased with time. They showed that this enabled a lower dependence on N fertilizers, which in turn reduced greenhouse gas emissions and N pollution. Their results indicate that increasing crop functional richness rather than species diversity may be a strategy for supporting and stabilizing grain yields in multiple environments. The authors state that “individual farmers would need to assess this yield benefit against other aspects such as market value of the crops included in the more diverse rotation...”.
Responses of soil organic carbon, aggregate stability, carbon and nitrogen fractions to 15 and 24 years of no-till diversified crop rotations by Maiga et al. appears in Soil Research [Jan. 2019, https://doi.org/10.1071/SR18068]. Rotations consisted of corn-soybean [2-year rotation] and corn-soybean-winter wheat-oat [4-year rotation]. Results from the research that was conducted in South Dakota showed that use of diverse 4-year crop rotations for a long duration enhanced soil organic carbon [SOC], C and N fractions, and soil aggregation compared to those same variables under a 2-year rotation of corn and soybean.
Diversified no-till crop rotation reduces nitrous oxide emissions, increases soybean yields, and promotes soil carbon accrual by Lehman, Osborne, and Duke appears in the Soil Science Society of America Journal [Vol. 81, p. 76-83, Jan. 2017, https://doi.org/10.2136/sssaj2016.01.0021]. Rotations consisted of corn-soybean [2-year rotation] and corn-field peas-winter wheat-soybean [4-year rotation]. Results from the research conducted in South Dakota and presented in this article showed that diverse rotations covering 4 years can decrease nitrous oxide [N2O] emissions, increase or accelerate SOC gains, accrue soil C earlier and deeper in the soil profile, and increase soybean yields vs. those same variables in a 2-year rotation of corn and soybean.
Complex crop rotations improve organic nitrogen cycling by Breza et al. appears in Soil Biology and Biochemistry [177 (2023) 108911, https://doi.org/10.1016/j.soilbio.2022.108911]. Rotations consisted of corn-corn, corn-soybean, and corn-soybean-grain sorghum-oat/clover. Results from this research conducted in Nebraska showed that internal N cycling is stimulated by increased complexity of a cropping system. However, N fertilization suppresses some of the benefits of crop rotation diversity. The authors concluded that balancing reduced N fertilizer application with increased rotational cropping complexity has the potential to promote/increase internal N cycling while simultaneously reducing/minimizing environmental N losses.
Long-term rotation diversity and nitrogen effects on soil organic carbon and nitrogen stocks by Schmer et al. appears in Agrosystems, Geosciences and Environment [2020;3:e20055, https://doi.org/10.1002/agg2.20055]. An experiment with monocrops of continuous corn, soybean, and grain sorghum, plus 2-year rotations of corn-soybean and grain sorghum-soybean, and 4-year rotations of corn-soybean-grain sorghum-oat+clover and corn-oat+clover-grain sorghum-soybean was conducted in Nebraska. Fertilizer N effects on SOC and N soil stocks were primarily confined to the surface soil depth, while crop rotation complexity affected SOC and soil N stocks throughout the 0-60 in. soil profile. The positive effects of rotation on SOC and soil N stocks were only manifested after prolonged rotation complexity.
Long-term evidence shows that crop rotation diversification increases agricultural resilience to adverse growing conditions in North America by Bowles et al. [One Earth 2, 2020, https://doi.org/10.1016/j.oneear.2020.02.007] reports results from 11 experiments covering 347 site-years. More diverse rotations resulted in increased corn yields over time under both favorable and unfavorable growing conditions. The authors concluded that crop rotation diversification should be considered as a central component for risk reduction and crop yield resilience when growing commodity crops such as corn under changing climate conditions. They also concluded that a transition to crop rotation diversity is urgent and should be supported over the long term.
All of the results reported in the above-linked articles are from research conducted at non-midsouthern U.S. locations. While they do paint a positive picture of the environmental, yield, and soil benefits that will accrue from increasing crop rotation diversity, there are questions that must be addressed before increased rotation diversity can or will be adopted by Midsouthern U.S. producers.
• Is there now or will there soon be a market for the harvested products that will be forthcoming from the alternate crops [those other than soybeans and corn] that will be grown in more diverse rotations? In other words, the ability to market or use products from each crop in a diverse rotation for annual economic gain will affect a producer’s decision to increase rotation diversity.
• Will markets that support increased rotation diversity be available or can they be quickly developed for all soybean/corn growing regions of the U.S.?
• Will the income from the alternate crops grown to increase rotational diversity be sufficient to offset the income that may be lost from not growing only soybeans and corn in rotation? If the answer to this question is no, then producers may not have the time, inclination, and/or resources needed to transition to rotation diversification, or they may not be able to wait for development of suitable markets for products from alternate crops grown in more diversified rotations. After all, crop producers must have income every year that is sufficient to support their continued ability to produce marketable crops and pay the bills.
Midsouth soybean producers are encouraged to explore the use of rotational diversification in their operations. However, they must first determine if cash crops are available that can replace the current summer cash crops–e.g. corn and rice–now used in short-term rotations with soybeans. Research is needed to discern such alternate crops that can be grown and marketed profitably to increase rotational diversification in the region, and to determine and/or develop and/or enhance markets for such crops.
Composed by Larry G. Heatherly, June 2023, larryh91746@gmail.com