Summer pulses as sources of green manure and soil cover in the U.S. Southern Great Plains

Authors: BAATH, GURJINDER - Texas Agrilife; SARKER, SAYANTON - Texas A&M University; Northup, Brian; SAPKOTA, BALA - Texas A&M University; Gowda, Prasanna; Flynn, Kyle

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/4/2023
Publication Date: 4/8/2023
Citation: Baath, G., Sarker, S., Northup, B.K., Sapkota, B., Gowda, P.H., Flynn, K.C. 2023. Summer pulses as sources of green manure and soil cover in the U.S. Southern Great Plains. Agronomy Journal. 2(2):66-74. https://doi.org/10.1016/j.crope.2023.04.001.
DOI: https://doi.org/10.1016/j.crope.2023.04.001

Interpretive Summary: Winter wheat has been a primary crop for producers in the U.S. Southern Plains for many decades. In 2020 to 2022, over 16 million acres per year were planted and grown across Kansas, Oklahoma, and Texas to produce grain, hay, straw, or grazeable pasture making it a flexible crop for growers. Most winter wheat in the Southern Plains is grown in continuous rotations each year, separated by periods of summer fallow. These rotations have issues that affect long-term sustainability, including low efficiencies in use of precipitation and increased soil erosion. Replacing summer fallow with a legume grown as a green manure could help reduce soil erosion, improve efficiency in use of soil water in growing crops, and add nitrogen (N) credits for wheat. We conducted a two-year field study to test how three different summer-grown grain legumes (pulses) functioned in providing green N and soil cover. We tested soybean as a control, mothbean (a vining pulse from India), and tepary bean (a vining pulse from the U.S. Desert Southwest). Mothbean and tepary bean are both capable of producing grain and forage crops within 75 days and less than 6 inches of rainfall. We examined their performance for two row spacings (15 and 30 inches) and two moisture regimes (rainfed conditions and supplementally irrigated). The 15-inch row spacing allowed all three pulses to provide greater amounts of canopy cover, biomass, and N accumulation than the 30-inch row spacing. Tepary bean demonstrated a capacity to provide greater, and more consistent, amounts of canopy cover during the early growing season, and provided more soil protection. Mothbean provided the second-most amounts of canopy cover, and effective by mid-growing seasons. Soybean produced the most biomass (4750 to 7905 lb./acre) which allowed more N to be accumulated (105 to 240 lb./acre) among the three pulses. Tests showed that using canopy height and cover we were able to explain 62% and 55% of the variation in biomass production by the three pulses. One issue we found in growing these pulses for green N and cover was the challenge generated by uncertainty in the amount and timing of rainfall. Future studies should combine crop models with long-term weather scenarios and simulate how these pulses grow and perform in different areas of the Southern Plains.

Technical Abstract: Winter wheat (Triticum aestivum L.) – summer fallow rotations in the U.S. Southern Great Plains (SGP) have numerous sustainability issues, including low precipitation use efficiencies and increased soil erosion. Replacing summer fallow with a legume grown as a green manure would help reduce soil erosion, improve precipitation use efficiency, and add nitrogen (N) credits for subsequent wheat crops. A two-year field experiment evaluated the capabilities of three summer pulses (grain legumes) as green manures and soil cover: soybean [Glycine max (L.) Merr.], mothbean [Vigna aconitifolia (Jacq.) Marechal], and tepary bean (Phaseolus acutifolius A. Gray). We examined their performance across two row spacings (38- and 76-cm) and two moisture regimes (rainfed and supplemental irrigated). Across all pulses, the narrow row spacing (38-cm) provided greater canopy cover, aboveground biomass, and N accumulation during the early growing season than the broad row spacing. Among species, tepary bean demonstrated consistent and higher canopy cover earlier in the growing season. Soybean produced the most aboveground biomass (5327-8855 kg/ha) and N accumulation (115-269 kg/ha) among the three pulses. Multilinear regression (MLR) models suggested both canopy height and canopy cover could estimate aboveground biomass (R^2 = 0.62) and N accumulation (R^2 = 0.55) for the tested pulses. Uncertainty in amount and timing of rainfall posed a serious challenge in defining the function of these species. Modelling studies that simulate growth of these crops under long-term weather scenarios are encouraged, to identify the most reliable cover crop for different areas of the SGP.