The Effects of Bovine Growth Hormone and Thyroxine on Growth Rate and Carcass Measurements in Lambs1

The effects of bovine growth hormone (GH) and thyroxine (T4) on growth and carcass characteristics were assessed in Dorset ram lambs. Lambs in four groups (n = 10/group) were treated for 30 d as follows: controls, 3.33 mg (6 IU) GH/d (s.c.); 5-mg T4implant (s.c.) on d 1 and a 10-mg T4implant 21 d later; GH + T4. Blood samples were collected at 3-d intervals for analysis of GH, T4, triiodothyronine, somatomedin-C and testosterone concentrations. Six lambs/group were slaughtered for carcass measurements and composition. Daily GH injections increased (P< .005) baseline plasma GH levels 10-fold, whereas plasma T4concentrations were increased 10% (P< .10) by the implants. Somatomedin-C increased with time in all groups, but the increments from d 0 to d 30 were higher (P< .05) with GH treatment. Average daily gain (mean = 352 g/d), feed consumption and feed to gain ratio were not affected (P> .1) by GH or T4treatment in ram lambs. Hot carcass weight and dressing percentage were increased (P< .05) by T4. Growth hormone increased carcass protein content (P< .005) and muscle weights while reducing carcass fat (P< .05). Carcass composition was not altered by T4alone, and the T4× GH interaction was not significant; however, the combination of T4and GH resulted in greater muscle and protein weight than did either hormone alone or no hormone administration. There were no differences in bone length or in the metacarpal growth plate width among groups. The beneficial effects of GH on carcass composition were not further enhanced by administration of thyroxine.