Rotational complexity increases cropping system yields under poorer growing conditions

Authors: Bybee-Finley, Kristine; Muller, Katherine; White, Kathryn; BOWELS, TIM - University Of California Berkeley; Cavigelli, Michel; Han, Eunjin; Schomberg, Harry; SNAPP, SIEG - International Maize & Wheat Improvement Center (CIMMYT); VIENS, FREDERI - Rice University; DEISS, LEONARDO - The Ohio State University

Submitted to: One Earth
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/2024
Publication Date: 9/20/2024
Citation: Bybee-Finley, K.A., Muller, K.E., White, K.E., Bowels, T., Cavigelli, M.A., Han, E., Schomberg, H.H., Snapp, S., Viens, F., Deiss, L. 2024. Rotational complexity increases cropping system yields under poorer growing conditions. One Earth. 7(9):1638-1654. https://doi.org/10.1016/j.oneear.2024.07.008.
DOI: https://doi.org/10.1016/j.oneear.2024.07.008

Interpretive Summary: The benefits and trade-offs of complex crop rotations, those with greater numbers of crops and/or different types of crops,have not been well-quantified. ARS and university scientists compiled data from 20 long-term cropping systems studies from across North America that grew every crop each year to create a database with 460 site-years of crop yields. We converted yield to gross returns in inflation-adjusted dollars for cash crops in the rotations (corn, soybean, small grains, and perennial forages). Whole rotation-level returns were determined as the average return for all crops in a rotation per year. The complexity of the rotation was defined with a continuous variable that accounted for the number of species and the length of the rotation. The influence of rotational complexity on returns was evaluated across growing conditions for each individual cash crop and entire rotations. Individual crop returns increased or stayed the same with more complex rotations. The effect of rotational complexity on rotation-level returns differed across growing conditions. Under poor growing conditions, rotational complexity was more beneficial. Future research can study how farmers consider opportunity costs with more complex management. These results will be of interest to crop scientists, producer groups, and policy makers.

Technical Abstract: Crop rotations have simplified as technology has changed, diets have shifted, and agriculture has become more specialized. While simplified crop rotations often reduce yields of individual crops, especially during stressful growing conditions, assessing productivity of the whole rotation is critical to understand trade-offs associated with increased rotational complexity. Further, how management factors and growing conditions moderate differences in whole-rotation productivity remains largely unknown. Using multi-level Bayesian analysis of data from 20 long-term cropping systems experiments in North America, we quantified the impacts of rotational complexity on individual maize (Zea mays L.), soybean [Glycine max (L.) Merr], spring small grains [barley (Hordeum vulgare L.), oats (Avena sativa L.), spring wheat (Triticum aestivum L.), and triticale (x Triticosecale Wittmack)], and winter wheat crop yields, as well as whole rotation yields, normalized to gross crop returns, a metric representing yield and market valuation. Rotational complexity often improved crop returns compared with continuous monocultures. For every unit of a rotational complexity index (RCI, an index consisting of the number of species and length of rotation), median returns changed by +$101 (maize), +$74 (soybean), +$40 (spring small grains), +$6 (winter wheat), and -$35 (whole rotations) ha-1. When growing conditions, quantified by an environmental index (EI, average yield across treatments within a site-year) were considered, rotational complexity improved rotation-level returns under poor conditions by $94 ha-1 RCI-1 for each unit decrease in EI ($1000 ha-1). Rotational complexity improved maize and soybean returns under good conditions by $30 and $19 ha-1 RCI-1 for each unit increase in EI, respectively. Rotational complexity increased returns across crops and whole rotations in low nitrogen fertilizer conditions, but reduced tillage had no impact. Based on a metric that favors the current industrial system, rotation-level analysis highlighted the opportunity costs of increasing rotational complexity with small grains or perennial forages, particularly if the perennial forage stand was in place for less than two years. However, as the frequency and severity of stressful weather increases with climate change, greater returns in more complex rotations during poor growing conditions show the value of diversifying cropping systems to reduce risk.