Components for a simulation model are described to predict feed intake, amino acid requirements and body composition changes over time on the basis of the inherent potential protein growth rate of pigs varying in genotype. The potential protein growth rate of the animal is predicted each day based on its genotype and state, from which the potential growth rate of the other chemical components can be predicted and hence the nutrient requirements can be calculated. By considering the potential growth rate, the nutrient requirements, the nutrient supply (the composition of the feed) and the environment simultaneously, the constrained food intake can be predicted. If this is less than the desired food intake, the actual growth rate will be less than the potential growth rate. In either case, the growth rate of each of the chemical components can be predicted for that day. This final state of the animal at the end of the day becomes the initial state on the next day, and the process is repeated.