A dynamic simulation model, Alberta Beef Production Simulation System (ABPSS), which includes herd inventory, nutrient requirements, forage production, and economic submodels, was used to compare bioeconomic efficiency in spring and fall calvings and different weaning ages (220, 200, 180, 160, and 140 d). Comparisons were made assuming a mature cow weight of 550 kg and a peak milk yield of 8.2 kg d−1. The first day of calving was assumed to be 28 March and 8 September for spring- and fall-calving cows, respectively. Bioeconomic efficiency was measured as the net return per cow (total return minus total cost). Fall calving in Alberta generally results in longer exposure of young calves to extreme cold weather after calving, and therefore total annual DMI and feed cost were higher in the fall-calving season group than in the spring-calving season group. Bioeconomic efficiency improved as weaning age increased from 140 to 220 d in both calving seasons. For weaning age of 200 d or less, spring calving was more efficient than fall calving. However, at a weaning age of 220 d, fall calving had higher bioeconomic efficiency than spring calving, primarily due to higher market prices for fall-born calves. This indicated that interactions of calving season by weaning age was an important factor affecting bioeconomic efficiency. It must be noted that the model was developed based on experimental results and data from the liteature, and due to the unavailability of suitable data the model could not be validated. We suggest that the ABPSS model has the potential for providing a useful decision-making tool for simultaneous consideration of many factors in an integrated system and for evaluating the effects of alternative management strategies on profitability of beef production systems. Key words: Beef cattle, simulation and modelling, production system, calving season, weaning age, bioeconomic efficiency
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