The Virginia Beef Cattle Simulation Model: A bio-economic simulation program modeling the interactions among reproduction, forage availability, nutrition, growth, and marketing in beef cattle

The Virginia Beef Cattle Simulation Model (VBCSM) is a user-friendly, dynamic, stochastic computer program whose objective is to serve as a decision-aid for Virginia cattlemen dealing with complex management issues such as whether to retain weaned calves through the stocker growth stage. Its five source-code modules are reproduction, forage, nutrition, marketing, and a tool that randomly assigns values to variables from appropriate statistical distributions. The VBCSM contains production statistics for 12 breeds, 21 forage species, and three Virginia agro-ecological zones. It simulates at the animal level using information obtained from program dialog. Help can be activated on each dialog page. It is event-driven on a daily time increment. The reproduction module simulates puberty, conception, abortion, parturition, dystocia, lactation, pregnancy testing, culling, within-herd replacement female selection, open or pregnant replacement female purchases, cow and calf mortality, and weaning. The forage module simulates daily pasture growth dependent upon month, precipitation, erosion, pasture maintenance, grazing system, farm location, weed infestation, and slope. This module interacts with the nutrition module to calculate each animal's forage intake, supplemental feed requirements, and daily gain or loss using National Research Council equations. The marketing routine sells the weanling calves to the stocker herd and sells stocker calves, orphan calves, and cull cows through user-specified markets, including the Virginia Tel-O-Market auction. After simulating for eight years to achieve equilibrium conditions, the VBCSM provides an income statement for the cow-calf operation and a partial budget for net income or loss from the stocker herd for up to three years. VBCSM was rigorously tested using a mathematical model with two calving seasons, three lengths of breeding season, four culling policies, and a year effect. Descriptive statistics suggest that the program code works in a consistent manner. However, several potential programming inconsistencies were discovered. Simulation results indicate that fall calving may be more profitable for Virginia cattle producers than spring calving for weanling calf production, but a spring calved stocker program may be more profitable that a fall calved stocker program. Perhaps, VBCSM will help cattlemen to enhance their profits by more efficient market planning and utilization of production resources.