Holstein steers were given milk from birth until they reached a weight of 100 kg. Beginning at 60 kg, they received gradually increasing amounts of one of three different diets and these became their sole diet after milk was withdrawn. The three diets were: (a) 85% grass hay and 15% concentrate (group 1); (b) the same diet with enough rumen undegradable protein (formaldehyde-treated canola meal) added to raise the crude protein content by 10 g kg−1 (group 2); and (c) 85% concentrate and 15% grass hay (group 3). The amounts of the three diets offered were designed to provide 0.95 MJ of digestible energy per day per unit of liveweight0.75. Steers were slaughtered at about 500 kg liveweight and the right sides of their carcasses were dissected into separable muscle, adipose tissue, and bone. This information was used to derive estimates of the daily gains of protein, fat, and energy in the empty bodies and the carcasses of the steers. Empty liveweight gain of group 3 steers exceeded that of group 1 steers by 42% and they required only 58% as much extra-maintenance energy per unit of gain. Daily gains of carcass muscle, adipose tissue, bone, protein, fat, and energy, and of empty body protein, fat, and energy, were greater in steers in group 3 than in those in group 1. The extra gain of group 3 steers, compared with group 1 steers, was due largely to gain of extra body fat. The overall efficiency of energy gain was 50% greater in steers in group 3 than in those in group 1. Supplementing the forage diet (group 1) with rumen undegradable protein (group 2) increased daily empty liveweight gains by 13% and reduced the extra-maintenance energy requirement per unit weight gain by 12%. Daily gains of carcass muscle, bone, and protein, and of empty body protein, were greater in steers in group 2 than in those in group 1. The extra gain of group 2 steers, compared with group 1 steers, was due largely to gain of extra body protein and its associated water. There were no differences between steers in these two groups in efficiency of energy gain (energy gain per unit extra-maintenance energy intake). It was concluded that the higher energetic efficiency of empty body weight gain of steers in group 2, as opposed to those in group 1, was due to a higher rate of net protein synthesis as a consequence of the increased amounts of protein absorbed from the intestine, and that the higher energetic efficiency of weight gain of steers given the concentrate diet (group 3), as opposed to those given the forage diet (group 1), was due to a higher efficiency of conversion of the energy-yielding products of digestion into fat. Key words: Energetic efficiency, steers, protein, undegradable, hay, concentrate