Gross energy is the energy released following combustion of a feed sample. It provides little useful information about the value of this energy because it fails to account for that portion of the energy unavailable to the pig. Looking at digestible energy (DE) or metabolizable energy (ME) can be more useful in defining energy as they account for the loss of faeces and urine respectively. More useful is the use of the net energy (NE) system for feed formulation. Net energy, the efficiency of use of ME, is therefore defined as energy retained in the body or as the ME minus the energy lost as heat. The NE is usually divided into energy for maintenance (NEm) and retained energy (RE) or the energy used for growth (Neg). Several factors influence net energy; these are divided into three categories. Animal factors such as genotype can influence NE. An animal with a lean high growth rate potential responds to higher energy intakes by increasing a