Correlated responses in body composition and fat partitioning to divergentselection for yearling growth rate in Angus cattle

Body composition data were collected in a serial slaughter experiment on 91 Angus steers produced after 12 years of divergent selection for yearling g rowth rate. Steers born in 1986 and 1987 from lines selected for high (High Line) and low (Low Line) growth rate from birth to weaning, and from an un selected line (Control Line) were slaughtered at 0 (birth) and circa 7, 13, 27, 35 and 45 months of age. Weights were obtained for dissected carcass m uscle, bone, subcutaneous and intermuscular fat, non-carcass fat depots, vi sceral organs, hide and head, tail and distal legs. At the same stage of ma turity of empty body weight or total fat, body components and fat partition s were a similar proportion of their mean mature weights in all three selec tion lines. At the same mean empty body weight of 360 kg, High Line steers had smaller proportions of carcass fat and a higher proportion of bone than steers from the other two lines, and the Low Line steers had a smaller pro portion of muscle relative to the other lines. Relative to the High Line, C ontrol and Low Line steers required 85 and 175 additional days, respectivel y, to attain this carcass weight. Mean mature empty body weight (body weigh t minus urine and gut contents) was 666+/-17 (standard error, S.E.) kg, 588 +/-18 kg and 512+/-16 kg for High, Control and Low Line steers, respectivel y. There were no significant (P < 0.05) differences among the selection lin es for the pattern of growth of the different body components from birth to maturity. At maturity, no significant selection line differences were obta ined in body composition. Relative growth rates indicated that, with respec t to body weight, dissected fat was late developing and bone, muscle and vi scera early maturing, over the period from weaning to maturity. When compar ed at the same stage of maturity or level of fatness, steers from the line selected for fast growth rate would be heavier than unselected steers, but have the same body composition. (C) 2000 Elsevier Science B.V. All rights r eserved.