Response to selection for lean tissue growth in sheep as assessed by X-raycomputer tomography

A population of Dorset Down sheep were selected for nine years to increase lean tissue growth using a three trait index combining information on livew eight, muscle depth and fat depth. Response to selection was assessed by X- ray CT for weight and composition of the carcass by comparison of animals s ampled from a selected line (n=12) and a control line (n=10). Carcass tissu e weights were assessed at 266 days of age from 16-19 slices evenly spaced along the long axis of the body of each animal. Selection led to increases in liveweight (+11%, P<0.05) and carcass weight (+7%, P<0.098). At the same liveweight, selection line animals had a lower carcass fat proportion (-15% P<0.01) and a higher carcass muscle proportion (+4%, P<0.05). It is concluded that selection has acted to increase mature size rather than appetite.