A one phase diet during gestation will not be able to provide the correct energy and nutrients to sows. During the beginning of gestation sows are still repairing their own tissue from the previous weaning, and during late gestation energy and amino acid requirements are higher as both are allotted to fetal and mammary gland growth. As well, gilts will have higher requirements than older sows because they are still growing. The NRC nutritional model uses the conceptus (and associated structures) and maternal protein requirement pools. The conceptus pool has stable requirements until day 70, and then rapidly increases. Maternal protein deposition depends on energy intake, age of the sow, and other time-dependent factors. The GfE evaluates nutritional requirements in a similar way to the NRC, but absolute values for amino acids may differ due to using different patterns. As well, the energy requirement is different because it uses a factorial approach. Nutritional experiments have found increased requirement for lysine, threonine, isoleucine, and trytophan between early and late gestation. Determined and modeled requirements generally agree, although there can be some difference in the final values possibly due to the time-dependent factors. Amino acid requirements increase more than energy requirements in late gestation, so a single phase diet is unlikely to be able to provide sufficient amino acids. A solution to this is to have a high and low amino acid diet, and to mix at a ratio based on the stage of gestation and parity. Phase feeding allows more accurate levels of nutrients and energy to be provided, and can save costs by reducing overfeeding.