The conceptual bases of recent NRC publications of nutrient requirements to meet dietary energy needs of growing Holsteins, emphasizing heifers gaining 0.75 kg/d, are described and compared with several world systems and with body composition data. Requirements of Holsteins for net energy for maintenance are generally thought to be higher (from 7 to 20%) than those of British beef breeds. Total estimates of metabolizable energy requirements are quite similar for most energy systems for cattle at 200 to 400 kg of body weight. For heavier cattle, the energy density estimates based on beef breed equations project higher metabolizable energy for gain and total metabolizable energy requirements. Extrapolations from the chemical composition of growing Holstein heifers indicate that, at the slower growth that is typical of replacements, energy densities are lower than for more rapidly growing heifers and that net energy requirements are consistent with current US standards.