The effect of diets with three divergent concentrate to roughage (C : R) ratios (80: 20, 55: 45 and 30: 70), fed at
three feed intake levels (ad libitum, 90% ad libitum and 80% ad libitum), on carcass composition and tissue gain of implanted medium frame weaner steers was studied. Steers (initially 200 kg) were slaughtered at the same mean group live target mass of 380 kg. Each treatment consisted of 12 group-fed steers. Percentage carcass fat, based on the prime rib cut, for the three intake levels adjusted to the same carcass mass were respectively 21.7, 17.6 and 17.8% for the 80:20 diet, 20.1, 19.5 and 17.7% for the 55:45 diet and 14.5,16.5 and 13.3% for the 30:70 diet. Carcass fat was reduced (P ≤ 0.01) by both a decrease in the C: R ratio and feed intake level. Corresponding carcass muscle percentages were 61.4, 64.9 and 63.6; 62.8, 62.9 and 63.5; 66.2, 65.1 and 66.3%. Carcass muscle was increased (P ≤ 0.05) by both a decrease in C: R ratio and feed intake level. Muscle gain (g / d) increased, but at a declining rate when carcass gain increased, while fat gain (g / d) increased at an accelerating rate. Fat gain of steers on the 80: 20 diet when fed at 90% ad libitum was lower (P ≤ 0.05) than when fed ad libitum, while protein gain was significantly higher (P ≤0.05) and muscle gain (P = 0.07) tended to be higher. Maximum lean deposition, therefore, did not occur at maximum carcass gain and maximum energy intake. Efficiency with which metabolizable energy was utilized for carcass gain (MJ ME/ g) improved with an increase in the C: R ratio. Steers fed sub-ad libitum feeding levels on the 80: 20 and 55: 45 diets tended not to differ from ad libitum-fed steers, while those fed the 30: 70 diet was less efficient than ad libitum-fed steers. Diet and feed restriction influenced carcass composition primarily through the influence energy intake has on the rate of fat deposition.