Modern sows may have different nutritional needs due to their increased size, lean mass, and litter size. The lactation model is a mathematical model for the nutritional requirements based on the litter size and piglet growth. Whole animal models originally assumed identical milk production in all sows in a group, but improvements allow sow variation to be taken into account and the protein requirement distribution curve created. Other models exist, but they do not yet have the option to include inter-sow variation. Mathematical models are a valuable tool for predicting nutrient requirements of sows, and some are able to consider inter-sow variation.