A total of 108 medium frame weaner steers, divided into 9 groups of 12 each with mean group masses of 200 kg, were used to determine the effect of three concentrate to roughage (C: R) ratios (80: 20, 55: 45, and 30: 70), fed at three feeding levels (ad libitum, 90% ad libitum and 80% ad libitum), on mass gain, dressing percentage and visual and physical carcass characteristics. Steers were slaughtered at a mean mass of 380 kg. Alimentary canal mass and contents, three subcutaneous fat thicknesses, carcass and buttock length, and cold carcass mass of each steer were measured. Carcasses were classified according to fatness and conformation scores into specific grades. Steers on the three feeding levels were respectively fed for 106, 114 and 174 days on the 80: 20 diet, 156, 161 and 191 days on the 55: 45 diet, and 197, 297 and 322 days on the 30: 70 diet. Contrary to expectation, OM intake decreased and then increased as the C: R ratio increased. Respective carcass gains were 792, 723 and 530; 579, 535 and 489; and 448, 329 and 285 g /d. A decrease in C: R ratio reduced both dressing percentage and carcass gain linearly, while a decrease in feeding level did not significantly influence dressing percentage, but it reduced carcass gain linearly. Subcutaneous fat thickness measurements at different sites were divergently influenced by main effects, but they tended to be reduced by a decrease in either C: R ratio or feeding level. Carcass fatness score decreased with a decrease in either C : R ratio (linear, non-linear) or feeding level (linear). Carcass fatness can be nutritionally manipulated at the assumed target mass within the constraints of the South African beef carcass classification and grading system.
Keywords: Beef steers, dietary energy concentration, feeding level, mass gain, visual and physical carcass measurements.