The growth rate, feed consumption and carcase composition of nine untreated wether lambs (controls) were compared with those of lambs in which a chronic pneumonia had been experimentally induced. Six pneumonic lambs (group 1) were killed with the controls on day 109 and eight (group 2) lambs were killed when they had a similar mean liveweight to the controls (42 kg) on day 172. The mean liveweight gain of infected animals to day 108 was 59 per cent, the mean feed intake 69 per cent and the mean carcase weight of group 1 was 73.5 per cent that of the controls. Group 2 lambs required 25 per cent more feed and nine weeks longer than the controls to reach similar live and carcase weights. This depression of appetite and growth rate was most marked in the first 35 days after inoculation, but growth rates of infected lambs continued to be lower than those of the controls throughout the experiment. At slaughter, all infected lambs had consolidated lesions of pneumonia and a significant correlation was noted between the extent of lung lesions and total liveweight gain in individual lambs. Carcases of group 1 lambs had a proportionately low fat, high lean meat and bone content, indicative of immature development and consistent with a lowered feed intake. Carcases of group 2 lambs showed a similar trend but differences from the controls were not significant.
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