Method: Modelling resource acquisition and allocation – extension and calibration of a cow model to a sheep

Simulation models are suitable to investigate how complex systems respond to changes. This is of particular interest regarding animal feed efficiency as this trait must be evaluated throughout the entire lifetime and thus is affected by trade-offs between physiological functions. The aim was to extend and calibrate the dynamic, mechanistic simulation model “Acquisition and Allocation” ( AQAL ) from dairy cows to reproductive ewes. This model was originally developed for investigating the effects of resource acquisition and allocation potentials on feed efficiency but also allows investigation of trade-offs between life functions. The model represents an individual female from birth to death or herd exit and uses four input parameters to describe the resource acquisition ability and allocation potential. The obtained energy is split between life functions such as maintenance, growth, reproduction and lactation. By including reproductive management rules, it allows for shifts between physiological stages, which then feedback and affect the current acquisition ability and resource allocation. To adapt the model to a reproductive ewe, we have included a litter size effect, an acquisition capacity linked to gestation, and a seasonal conception probability. The litter size is influenced by the proportion of fat in empty body weight at conception, and it affects the acquisition linked to gestation, the allocation to gestation and the allocation to lactation. We also incorporated the energetic costs of the gravid uterus depending on litter size. We use three different acquisition-allocation profiles to test the consistency of the litter size effect. We show that the model simulates consistent lifetime trajectories of reproductive ewes and that the effect of litter size adequately reflects the demands of increased litter size within the different acquisition/allocation profiles.