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 determine the effects of porcine somatotropin (PST) and dietary P on growth performance and bone mechanical properties and mineralization during the finishing phase (58 to 105 kg) and a 35-d postfinishing phase. Gilts were injected daily with placebo (control) or 4 mg of PST and fed diets containing .4, .8, or 1.2% P in a 2 X 3 factorial arrangement. From 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 reached 105 kg, half the gilts were slaughtered and first rib, femur, and third and fourth metacarpals were collected for determination of mechanical 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 bending moment and modulus of elasticity increased as dietary P increased from .4 to 1.2% in pST-treated gilts. Administration of PST decreased (P < .05) stress of the rib, femur, and metacarpals compared with control gilts. Increasing dietary P resulted in a linear (P < .10) increase in bending moment, stress, and ash content for rib, femur, and metacarpal 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 observed (P < .03) for treatment combination received in the finishing phase on rib bending moment at the end of the postfinishing phase. Control gilts had similar rib bending moment values regardless of the dietary 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. Gilts that received higher P levels in the finishing phase also had increased (linear, P < .05) ash content at the end of the postfinishing phase. Gilts treated with pST and receiving the .4 or .8% P diet during the 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 postfinishing phase. We conclude that gilts administered pST during the finishing phase do not have greater daily requirements for Ca and P than control gilts to maximize growth performance but do have greater requirements than control gilts to attain comparable bone strength during the finishing phase.