Purebred lambs from three breeds (East Friesland, Oxford and Texel) were fed to appetite and treated with recombinant DNA-derived bovine somatotropin (BST) at 0·05, 0·10 or 0·20 mg/kg per day or a placebo between 19 and 27 weeks of age.
BST maintained growth over the later stages of treatment but did not affect average daily live-weight gain over the whole treatment period. Food intake was decreased and food conversion efficiency was improved by BST treatment. However, carcass weight and killing-out proportion were decreased by BST treatment in all breeds. Consistent breed differences were observed. Oxford lambs had higher live weights and carcass weights than the other breeds but no breed × treatment interactions were apparent. Ultrasonic backfat depth (mean of measurements at 13th rib and 3rd lumbar vertebra) was reduced by BST treatment in the Oxford (the fattest) and Texel lambs but not in the East Frieslands (the leanest). Ultrasonic muscle depth was not affected by BST; East Friesland lambs had considerably smaller muscle depth than the other breeds.
In the loin joint, BST decreased fat cover and subcutaneous fat proportion and increased longissimus dorsi proportion. There were also marked breed differences with Oxfords fatter than the other breeds. BST treatment did not affect any of the meat quality indicators but some differences between breeds were apparent in ultimate pH and colour of the muscle. Fatty acid composition of the subcutaneous fat was affected by breed but not by BST. East Friesland lambs had higher concentrations of unsaturated and branched chain fatty acids than the other breeds.
A mammogenic response (increased mammary development and some secretory activity) to BST was observed in some female East Friesland and Texel lambs but not in Oxford lambs.