A teleonomic model describing performance (body, milk and intake) during growth and over repeated reproductive cycles throughout the lifespan of dairy cattle. 2. Voluntary intake and energy partitioning

This is the second of two papers describing a teleonomic model of individual performance during growth and over repeated reproductive cycles throughout the lifespan of dairy cattle. The model described in the first paper is based on the coupling of a regulating sub-model of the dynamic partitioning of a female mammal’s priority over a lifetime with an operating sub-model of whole-animal performance. The model provides a reference pattern of performance under normal husbandry and feed regimen, which is expressed in this paper in a reference dynamic pattern of energy partitioning adapted to changes in nutrient supply. This paper deals with the representation of deviations from the reference pattern of performance. First, a model of intake regulation, accounting for feed allowance, physical limitation of the digestive tract and energy demand, is used to determine the actual intake, which may generate a deviation from the energy input under the reference pattern of partitioning. Second, a theoretical model is proposed to apportion the energy deviation between flows involved in performance and thus simulate lifetime performance when actual intake is above or below requirements. The model explicitly involves a homeorhetic drive by way of the tendency to home on to the teleonomic trajectory and a homeostatic control by way of the tendency to maintain an energy equilibrium in response to nutritional constraints. The model was evaluated through simulations reproducing typical feeding trials in dairy cows. Model simulations shown in graphs concern the effect of dietary energy content on intake, body weight and condition score, and milk yield. Results highlight the ability of the model to simulate the combination of physical and energetic regulation of intake, the accelerated, retarded and compensatory patterns of growth and the short- and long-term residual effects of pre-partum feeding on lactation.