Influence of bovine somatotropin administration to Holstein steers on growth, lipid metabolism and carcass characteristics

Two experiments were conducted to study the effectiveness of bovine somatotropin (bST) to alter the growth, carcass characteristics and lipid metabolism of Holstein steers. The first evaluated the effectiveness of long-term (354 d) bST administration to growing-finishing Holstein steers at the beginning, end or during the entire feeding period on lean, skeletal, and carcass measurements. One hundred and sixty-eight Holstein steer calves (185 kg) were blocked into four body weight (BW) groups and randomly allocated to initial harvest (8 steers) or four treatments (10 steers/treatment) within a block. Treatments were control, no bST; bST d 0 to 182; bST d 183 to harvest; and bST d 0 to harvest. Doses were 320 mg bST/14-d injection from d 0 to 112 and 640 mg bST 1/14-d injection from d 113 to harvest. Two steers from each block were harvested on d 199 with the remaining steers harvested when block BW averaged 615 kg. The last treatment was administered 31 d before harvest. Administering bST to young, light-weight Holstein steers increased skeletal growth and reduced carcass fat content producing a leaner product. Somatotropin increased noncarcass components without increasing carcass weight. Steers receiving bST during the entire study showed the greatest effects with the majority of lipid reduction occurring during the latter part of the study. Somatotropin increased protein accretion and therefore red meat yield.; The second study evaluated the effect of bST administration to Holstein steers on measures of lipogenesis and lipolysis during the latter half of the feeding period when fat deposition was the greatest. Twenty-eight Holstein steers (460 kg) were blocked by weight and randomly assigned to an initial harvest group (8 steers), or to 116 daily treatments of control (no bST, 10 steers) or bST (100 mug bST/kg BW, 10 steers). Three perirectal adipose tissue (AT) biopsies were collected from each steer (5 to 16 d before treatment and 40 to 49, and 103 to 112 d after treatment). Lipogenesis was measured in triplicate in vitro as tritium incorporation into fatty acids, fatty acid synthase activity, and NADP-isocitrate dehydrogenase activity. Lipolysis was measured in triplicate in vitro by glycerol release from AT and in vivo by a seven-dose epinephrine challenge administered intravenously beginning on d --24, 32 and 95. It was clear that somatotropin improved the efficiency of carcass protein accretion and content, while drastically reducing carcass lipid accretion and content. The decrease in lipid accretion was a result of depressed lipogenesis through inhibiting lipogenic enzymes and enhancing lipolysis in the adipocyte. Bovine somatotropin was effective in reducing carcass fat and increasing edible lean in both experiments. Administering bST to young, light-weight steers increased skeletal growth and noncarcass weight, but did not increase the total carcass weight and reduced the quality grade of the carcass. Somatotropin decreased lipogenesis and increased lipolysis in AT of finishing-Holstein steers in positive energy balance.