THE EFFECTS OF PLANE OF NUTRITION AND SLAUGHTER WEIGHT ON THE PERFORMANCE AND CARCASS COMPOSITION OF CONTINENTAL BEEF BULLS GIVEN HIGH FORAGE DIETS

Bulls (1/2 Blonde d'Aquitaine 3/8 Charolais) were used in a 2 X 3 fact orial design experiment with two planes of nutrition in the finishing period and three slaughter weights. High and low planes of nutrition w ere based on diets with similar forage to concentrate ratio (0.40 of d ry matter (DM)) offered ad libitum or at 0-78 of ad libitum DM intake at equal live weight respectively. The target slaughter live weights w ere 550, 625 and 700 kg. Twelve bulls were taken to each slaughter poi nt and an additional five animals were killed as a pre-experimental sl aughter group. Mean initial live weight was 412 (s.e. 5.3) kg at a mea n age of 342 (s.e. 2.5) days. No significant interactions were found b etween the main factors. For the high and low planes, live-weight and estimated carcass gains were 1251 and 989 (s.e. 47.7), and 816 and 668 (s.e. 35.3) g/day respectively, the reduction in gains being similar to the proportional degree of nutritional restriction. Plane of nutrit ion had no effect on live-weight or carcass gain per unit of energy in take. The low plane of nutrition produced significant decreases in bod y cavity fat depots, subcutaneous fat in the sample joint and increase d the proportions of both saleable beef and high-priced joints in the carcass. With increase in slaughter weight, energy intake per unit of live weight0.75 and rates of both live-weight and carcass gain tended to decline. The dressing proportions were 583, 579 and 609 (s.e. 9.1) g/kg for the slaughter live weights of 550, 625 and 700 kg. Carcass co nformation improved while fat depots in the body cavity and estimated concentration of separable fat in the carcass increased with increase in slaughter weight, and both estimated lean and bone concentrations d ecreased. Forequarter as a proportion of total side tended to increase with increase in slaughter weight. Slaughter weight had no effect on concentration of saleable meat or ultimate pH of carcass muscle. It is concluded that bulls of this genotype can be taken to high slaughter weights on diets having a relatively high proportion of forage as gras s silage, with high rates of growth and acceptable carcass leanness.