Twenty-eight inbred (I, inbreeding coefficient 0-375 and 0-5) and 28 matching outbred (O) sheep, aged 26 weeks, were housed and allocated in equal number to two planes of nutrition, a high plane (H), involving ad libitum feeding on a pelleted diet, the other a low plane (L), restricted to an amount of the same diet which would maintain live weight. Food consumption, live weight, linear body dimensions and wool growth were recorded for 19 weeks, and for 4 weeks later in the experiment when L as well H animals received ad libitum feeding. Observations were also made for a final 9 weeks when the sheep were at grass.
The I sheep were substantially smaller in weight and body dimensions than the O sheep throughout the experiment. On ad libitum feeding, the I and O lambs grew in parallel and by the same amount per unit of food consumed. There was also no difference between I and O sheep in the amount of food per kg metabolic live weight (M0·73) required to maintain the live weight of the sheep on the restricted intake. The two types of sheep were therefore similar in efficiency.
During the 4-week period when L sheep were fed ad libitum they showed some compensatory growth relative to the H group, but I and O groups behaved similarly. At grass, a tendency for slightly faster growth continued for the O group formerly on a low plane, but there were no significant interactions between plane of nutrition (as originally allocated) and inbreeding.
Wool growth mirrored the results on body growth, with substantial effects of plane of nutrition, but wool growth per unit area of skin did not differ between I and O groups.