A Wheat Grazing Model for Simulating Grain and Beef Production: Part I- Model Development

It is a common practice to grow winter wheat (Triticum aestivum L.) as a dual-purpose crop in the U.S. Southern Great Plains to decrease production risk and to increase profit margin through cattle (Bos taurus) production. Crop management of the dual-purpose wheat is complex because of the tradeoffs between beef production and wheat grain yield. A wheat grazing model helps in making optimal decision. The objective of this study was to develop and incorporate a grazing and metabolizable energy-based cattle growth module into the Decision Support Systems for Agrotechnology Transfer (DSSAT) to simulate beef and wheat grain production. The wheat grazing model was comprised of wheat growth, wheat-cattle interaction, and cattle growth components. Wheat growth was simulated by the cropping system model (CSM) of DSSAT. For the wheat-cattle interface, removals of canopy biomass and leaf area by grazing were estimated daily. Predicted grain yield was also reduced by 50 kg ha per day for each day of grazing past the first hollow stem stage. Cattle growth rate was based on a metabolizable energy intake. Maximum voluntary daily intake was estimated based on stocker body weight and forage quality, and is further adjusted for actual forage availability, temperature, and adaptation status during the first two week of grazing to estimate the actual daily intake. Changes in wheat growth processes brought about by grazing, including a grazing effect on the delay of plant phenological development, are not simulated in the model. Field experiments to characterize any such effects are needed to help fine-tune the model. 1 X-C Zhang, W.A. Phillips, J.D. Garbrecht, and J.L. Steiner, USDA-ARS Grazinglands Research Laboratory, 7207 W. Cheyenne St., El Reno, OK 73036; L.A. Hunt, Dep. of Plant Agric., Univ. of Guelph, Guelph, Ontario, Canada N1G 2W1 (retired). * Corresponding author ([email protected]).