Productivity and water use in intensified forage soybean-wheat cropping systems of the US Southern Great Plains

Authors: BAATH, GURJINDER - Oklahoma State University; Northup, Brian; RAO, SRINIVAS - Retired ARS Employee; KAKANI, VIJAYA - Oklahoma State University

Submitted to: Field Crops Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/28/2021
Publication Date: 3/2/2021
Citation: Baath, G.S., Northup, B.K., Rao, S.C., Kakani, V.G. 2021. Productivity and water use in intensified forage soybean-wheat cropping systems of the US Southern Great Plains. Field Crops Research. 265:108086. https://doi.org/10.1016/j.fcr.2021.108086.
DOI: https://doi.org/10.1016/j.fcr.2021.108086

Interpretive Summary: Winter wheat is a key part of agriculture in the U.S. southern Great Plains (SGP). It is grown for grain, hay, grazing for weight gain by cattle, or combinations of these products. Winter wheat in the SGP is largely produced in continuous rotations, with wheat crops separated by periods of summer fallow to conserve moisture for the next wheat crop. However, the wheat-summer fallow system is not efficient in using soil moisture, and their limited diversity in cash crops can affect the income of farmers. There is a thought that replacing summer fallow with growing crops can improve water use efficiencies in crop production, so combining soybean and winter wheat in a cropping system could be a useful approach to both improve use of soil water, and in diversifying crop production. We undertook a study to test the yield and water use of three cultivars of forage soybean that belonged to three maturity groups (MG V, MG VI, and MG VII) at different dates during the summer, their influence on the performance of following wheat crops, and the overall function of soybean-wheat systems compared to the fallow-wheat system. We used crop yield and soil moisture data collected during 2002-2006 from experiments near El Reno, Oklahoma, to develop computer models for soybean and wheat production within the Decision Support System for Agrotechnology Transfer-Cropping System Model (DSSAT-CSM), and validate their performance. Long-term simulations were then conducted with weather data from 1994-2019. Model results suggested the MG V soybean could produce greater forage yields and water use efficiencies than later-maturing soybeans, when soybean forage was harvested 90-120 days after planting. Double-cropping forage soybean with winter wheat also reduced wheat grain and forage yields by 19 to 29 bushels/ac and 2.1 to 2.5 tons/acre, respectively, and resulted in more water removed from the soil (3.0-5.2 inches) than the summer fallow-winter wheat system. However, improved yields in trade-offs between amounts of forage soybean and winter wheat produced were possible when summer rainfall exceeded 7.0 inches during the first 60 days of the growing season of soybean. Despite yield losses in winter wheat, double-cropped forage soybean-wheat systems could be a viable option for the SGP, depending on the economic values of soybean hay, wheat hay and grain, and other ecological benefits.

Technical Abstract: Intensifying continuous systems of dryland winter wheat (Triticum aestivum L.) by growing forage soybean [Glycine max (L.) Merr] during summers could provide nutritious forage for livestock, reduce requirements for inorganic N fertilizer for winter wheat, and ameliorate sustainability issues associated with summer fallow, such as reducing soil erosion and improving precipitation use efficiency (PUE). This modeling study utilized the Decision Support System for Agrotechnology Transfer-Cropping System Model (DSSAT-CSM) to assess the yield and water use of cultivars of forage soybean from three maturity groups (MG), harvested at three different dates [60, 90, and 120 days after planting (DAP)]; their influence on performance and water productivity of subsequent winter wheat, and overall function of double-crop systems in comparison to the fallow-wheat system. Crop yield and evapotranspiration (ET) data, collected from continuous field experiments at El Reno, Oklahoma (35.57N, 98.03W) during 2002-2006, were used for calibration and validation of soybean and wheat models. The long-term simulations were conducted using historical weather data (1994-2019). Results suggested mid-MG soybean (MG V) could result in greater forage yields and higher water use efficiency (WUEB) than later-maturing soybeans (MG VI & MG VII) when harvested at 90-120 days after planting (DAP). The double crop of forage soybean and winter wheat caused reductions of 1.4-2.1 Mg/ha and 4.7-5.6 Mg/ha in wheat grain and biomass yields, respectively, and higher seasonal ET losses (77-132 mm) than the summer fallow-winter wheat system. However, improved yields in trade-offs between production of forage soybean and winter wheat could be expected when summer precipitation exceeds 180 mm during the first 60 d of the growing season of soybean. Despite yield losses in winter wheat, double-cropped forage soybean-wheat systems could be a viable option, depending on the economic values of the different commodities produced by soybean and winter wheat, and other ecological benefits.