Economic modelling to optimize dairy heifer management decisions

The objective of heifer rearing within a dairy farm is to produce high quality dairy replacements at low costs. Heifer management decisions interact with the biological aspects of growth, thereby influencing the future profitability of the heifer. To maximize profitability farmers need to be aware of the potential impacts of these management decisions. However, this is a field of research that received only little attention. The objective of the research described in this thesis was to obtain insight into the technical and economic consequences of various dairy heifer rearing strategies by means of a stochastic dynamic optimization model.The research was started with a literature review to identify the key issues in heifer rearing and a large field survey to evaluate the present heifer rearing performance of dairy farmers in the Netherlands. Based on the insights obtained, a stochastic dynamic optimization model was developed, using the hierarchic Markov process technique. Within the model heifers were described in terms of age, season, body weight, reproductive state and prepubertal growth rate. Management decisions with respect to growth rate, time of insemination and replacement were optimized under maximization of discounted net returns per heifer place per year. Model behaviour was studied by the evaluation of a basic Dutch production situation. Within the default situation, the optimal rearing policy resulted in an average calving age of 22.6 months and an average calving body weight of 564 kg. Discounted net returns coincided with Dfl 337 per heifer per year. Extensive sensitivity analyses were carried out to evaluate the consequences of variation in price and production variables on the optimization outcomes.Additionally, the basic model was extended to determine the extent to which heifer rearing activities influence the environmental problems on a Dutch dairy farm and to study the sensitivity of the optimal rearing policy for environmental measures as the Dutch mineral accounting system (MINAS) or the use of least mineral rations. In the final phase, application of the model under other production circumstances was studied by an evaluation of a rearing situation typical for a Pennsylvanian (US) dairy herd.