Administration of porcine somatotropin by daily injection: growth and endocrine responses in genetically lean and obese barrows and gilts.

Influences of genotype and sex on responses to porcine somatotropin (pST) administered by daily injection were examined in genetically lean and obese gilts and barrows. Pigs (59 +/- 1.4 kg BW, eight per pST dose x line x sex) were injected daily with pST at doses of 0, 2, and 4 mg/d for 6 wk. Administration of pST induced dose-dependent decreases in feed intake and rate of backfat deposition, whereas rate and efficiency of gain were increased in a quadratic manner indicating that 4 mg of pST/d approached the optimal dose. These live measures of growth were also influenced (P < .05) by sex and line. Line, sex, and pST dose effects (P < .05) were noted for most offal components and carcass components. Quantity of lean and bone in the ham was increased (P < .01) and that of fat decreased (P < .01) with dose of pST. Throughout the trial pST induced increases (P < .05) in serum concentrations of IGF-I, IGF-II, insulin, and glucose and decreases in urea nitrogen. Concentrations of pST 24 h after injection were lower (P < .05) in pST-injected pigs than in buffer-injected pigs, indicating negative feedback on endogenous pST secretion. The pST-induced increases (P < .05) in insulin, IGF-I, and IGF-II and the decrease in urea nitrogen were evident at 24 h after the initial injection. Although pST administration improved the growth performance and composition of lean and obese barrows and gilts, the influences of line and sex were not expunged by administration of pST.