Livestock feeding systems calculate energy transactions as the sum of energy used for maintenance and growth/production, with associated efficiencies (km and kg ) for the conversion of feed energy toward these purposes (Freer et al 2007, NASEM 2007). These efficiencies reflect the amount of heat (HP) produced from using metabolizable energy from feed (MEI) for maintenance and gain. This approach attributes variation in HP to variation in MEI and dietary energy density (M/D). However, in growing animals, HP also arises from changes in body composition (fat and protein) and composition of gain. A simple way has been devised to combine these two approaches to calculate total HP in growing sheep.