EFFECTS OF PORCINE SOMATOTROPIN AND DIETARY PHOSPHORUS ON GROWTH-PERFORMANCE AND BONE PROPERTIES OF GILTS

One hundred eight gilts (initial weight = 58.5 kg) were used to determ ine the effects of porcine somatotropin (pST) and dietary P on growth performance and bone mechanical properties and mineralization during t he finishing phase (58 to 105 kg) and a 35-d postfinishing phase. Gilt s were injected daily with placebo (control) or 4 mg of pST and fed di ets containing .4, .8, or 1.2% P in a 2 x 3 factorial arrangement. Fro m 58 to 105 kg, administration of pST increased (P < .01) ADG and G/F and decreased (P < .01) ADFI. When mean weight of the gilts in a pen r eached 105 kg, half the gilts were slaughtered and first rib, femur, a nd third and fourth metacarpals were collected for determination of me chanical properties and bone ash. A pST x P interaction was observed ( P < .05) for rib bending moment and modulus of elasticity; maximum rib bending moment was attained by control gilts at .8% P and rib modulus of elasticity values remained constant across P levels, whereas rib b ending moment and modulus of elasticity increased as dietary P increas ed from .4 to 1.2% in pST-treated gilts. Administration of pST decreas ed (P < .05) stress of the rib, femur, and metacarpals compared with c ontrol gilts. Increasing dietary P resulted in a linear (P < .10) incr ease in bending moment, stress, and ash content for rib, femur, and me tacarpal bones. The remaining 54 gilts were individually fed 1.8 kg/d of a common diet for 35 d postfinishing. A pST x P interaction was obs erved (P < .03) for treatment combination received in the finishing ph ase on rib bending moment at the end of the postfinishing phase. Contr ol gilts had similar rib bending moment values regardless of the dieta ry P level fed during the finishing phase, and pST-treated gilts that received the higher dietary P in the finishing phase still had greater rib bending moment values at the end of the postfinishing phase. Gilt s that received higher P levels in the finishing phase also had increa sed (linear, P < .05) ash content at the end of the postfinishing phas e. Gilts treated with pST and receiving the .4 or .8% P diet during th e finishing phase showed a compensatory increase in bone strength to a value equal to or exceeding that of control gilts by the end of the p ostfinishing phase. We conclude that gilts administered pST during the finishing phase do not have greater daily requirements for Ca and P t han control gilts to maximize growth performance but do have greater r equirements than control gilts to attain comparable bone strength duri ng the finishing phase.