The effect of level of protein supplementation on the performance and carcass composition of young bulls given grass silage ad libitum

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Abstract
Two experiments have been carried out to examine the effects of the level of protein supplementation given with grass silage-based diets on the performance and carcass composition of bulls, and to compare diets based on silage and dried forage. The five treatments used consisted of grass silage offered ad libitum and supplemented with 2·5 kg dry matter (DM) of barley-based concentrates containing (1) zero (2) 200 (3) 400 and (4) 600 g soya-bean meal per kg and (5) artificially dried grass and hay supplemented with 3·2 kg concentrate DM. The silages used in both experiments were well preserved, containing on average 200 g DM per kg; 140 g crude protein (CP) per kg DM; 63 g ammonia-nitrogen per kg total N and 731 g digestible organic matter per kg DM. The bulls were of late-maturing breed type and were initially 12 months old and 412 and 405 kg live weight in experiments 1 and 2 respectively. For treatments 1, 2, 4 and 5 in experiment 1 respectively (treatment 3 was not used) total DM intakes were 8·3, 8·3, 81 and 110 (s.e. 0·21) kg/day; CP intakes 1063, 1271, 1664 and 1539g/day; metabolizable energy intakes (MEI) 98, 99, 96 and 87 MJ/day; carcass weights 317, 316, 317 and 316 (s.e. 3·2) kg; carcass saleable meat concentrations 714, 712, 718 and 716 (s.e. 5·8) g/kg and carcass fat trims 73, 81, 73 and 68 (s.e. 3·9) g/kg. In experiment 2 for treatments 1 to 5 respectively total DM intakes were 8·3, 8·5, 8·3, 8·4 and 11·2 (s.e. 0·26) kg/day; CP intakes were 1090, 1329, 1504, 1720 and 1561 g/day; MEI 102, 106, 103, 103 and 94 MJ/day; carcass weights 318, 331, 330, 327 and 321 (s.e. 3·3) kg; carcass saleable meat concentrations 726, 721, 725, 721 and 732 (s.e. 60) g/kg and fat trims 71, 77, 78, 80 and 64 (s.e. 4·5) g/kg. It is concluded that protein supplementation of a silage-based diet did not affect performance or carcass fatness in experiment 1 or carcass fatness in experiment 2, but including 200 or 400 g soya-bean meal per kg concentrate increased performance in experiment 2. Animals given silage produced fatter carcasses than those given dried forage in experiment 2 but not in experiment 1.

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