PSVIII-23 Devising a plant-animal interface model for simulating the daily gain of grazing stockers in the southeastern United States.

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Abstract Beef cattle production is a major economic activity in the southeastern US. Managing a grazing system efficiently requires understanding the complex system and its interconnected components such as plant and animal communities and the interface between them. This necessitates the development and use of mathematical models. Weather conditions and plant maturity vary continuously, and so do forage nutritive values and animal performance. To account for these phenomena, the grazing system models that can simulate animal performance on a daily basis are essential. Our work developed a system for simulating the daily gain of stockers grazing bermudagrass in the southeastern US by coupling a popular crop growth model, called Decision Support System for Agrotechnology Transfer (DSSAT), to a beef cattle growth model, called Stocker Daily Gain Model (SDGM), through the incorporation of Plant-Animal Interface Model (PAIM). The PAIM and SDGM are essentially two different subroutines in the DSSAT system. The plant attributes associated with the grazing of the forage simulated by DSSAT are passed by PAIM to SDGM, which, in turn, computes DMI and stocker gain and sends them back to PAIM. The PAIM then prints daily gain and the updated BW values, while sending back the updated daily values of plant attributes to the DSSAT forage model. The PAIM contains data on stocking rate, stocker’s initial BW, starting and termination dates of grazing, and the daily minimum forage mass to be retained on the pasture after grazing. In PAIM, grazing occurs on a day only when the above-ground forage mass on that day is greater than the minimum amount specified. The DSSAT forage model starts simulation on a specified day. Once simulation is started, the forage model computes values for several plant attributes that are associated with grazing with a daily time-step and terminates on the specified grazing termination day. Meanwhile, when simulation reaches the specified grazing start day, PAIM is activated. The PAIM then receives relevant data from the DSSAT forage model for that day and continues receiving them on a daily basis until the specified grazing termination day is reached. Once PAIM obtains the relevant data from the DSSAT forage model, it sends them to SDGM on a daily basis. The SDGM, in turn, computes values for several animal attributes, including weight gain, and returns them back to PAIM. On a day of no grazing, SDGM returns weight loss. The grazing simulation system containing the DSSAT forage model, PAIM, and SDGM was evaluated using data for bermudagrass DM, stocker BW, and stocker ADG for 24 seasons during 1987-2020 obtained at Texas A&M AgriLife Research at Overton. As the Willmott Index values showed, the performance of the system was satisfactory (ADG: 0.84, DM: 0.77, BW: 0.85).

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