Abstract
Feedlot performance, carcass composition and efficiency of muscle gain were compared in 33 young beef bulls and 33 beef steers of different mature body size (35 small or mainly British breed crosses; 31 large or Continental crosses). All cattle were fed a high-energy diet based on corn silage and high-moisture corn from weaning to slaughter. Slaughter was carried out once 6 mm of fat was attained over the 11/12th ribs determined ultrasonically. Large crossbred animals had higher average daily gains, greater slaughter weights and spent longer time on feed than small crossbred animals and bulls. Similar results were found for all bulls compared to all steers. Feed conversion rate expressed on a live weight basis was superior for small crossbreds compared to large crossbreds, but there were no corresponding differences between bulls and steers. At the same proportion of dissected carcass subcutaneous fat, carcasses from large crossbreds and all bulls had greater proportions of muscle, less bone and similar proportions of total fat than small crossbreds and all steers. There were generally no differences for gender and size in the distribution of muscle within each of the wholesale cuts except in the case of the chuck, where bull carcasses had a greater proportion of muscle compared to those from steers. Bone distribution, however, did vary significantly across several wholesale cuts with carcasses from large animals and bulls having lower proportions of bone than those from small animals and steers. Feed conversion rate (g muscle/kg DM) was superior for bulls compared to steers (63 and 56 g muscle/kg DM, respectively), but similar for large and small animals (60 and 59 g muscle/kg DM, respectively). The present data suggest that the evaluation of beef cattle for feed efficiency on a liveweight and muscle basis may yield different conclusions. There were no significant size × castration interactions for any of the growth, efficiency and carcass variables examined. Key words: Beef, mature size, castration, carcass composition, efficiency
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