The effects of plane of nutrition and slaughter weight on the performance and carcass composition of continental beef bulls given high forage diets

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Abstract
Bulls (½ Blonde d'Aquitaine ⅜ Charolais) were used in a 2 × 3 factorial design experiment with two planes of nutrition in the finishing period and three slaughter weights. High and low planes of nutrition were based on diets with similar forage to concentrate ratio (0·40 of dry matter (DM)) offered ad libitum or at 0·78 of ad libitum DM intake at equal live weight respectively. The target slaughter live weights were 550, 625 and 700 kg. Twelve bulls were taken to each slaughter point and an additional five animals were killed as a pre-experimental slaughter group. Mean initial live weight was 412 (s.e. 5·3) kg at a mean age of 342 (s.e. 2·5) days. No significant interactions were found between 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 nutrition had no effect on live-weight or carcass gain per unit of energy intake. The low plane of nutrition produced significant decreases in body cavity fat depots, subcutaneous fat in the sample joint and increased 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 conformation 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 decreased. 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 grass silage, with high rates of growth and acceptable carcass leanness.

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