PORCINE SOMATOTROPIN AFFECTS THE DIETARY LYSINE REQUIREMENT AND NET LYSINE UTILIZATION FOR GROWING PIGS

This study was conducted to determine the effects of exogenous porcine somatotropin (pST) on the dietary lysine requirement and efficiency o f absorbed lysine utilization for pigs during the 20- to 60-kg phase o f growth. Seventy-two crossbred pigs (20 +/- 0.7 kg body wt) received daily intramuscular injections of either excipient (0 dose) or pST (15 0 mg/kg body wt) and were fed diets in which protein and lysine concen trations ranged from 6.4 to 23.5 g/100 g diet and from 0.40 to 1.48 g/ 100 g diet, respectively. Nutrient density was altered to compensate f or reduced feed intake with pST, but diets were approximately isocalor ic. Rate and efficiency of gain and whole-body protein accretion rate exhibited a dose-response improvement (P < 0.01) to increases in dieta ry protein for both excipient and pST-treated pigs. Pigs receiving pST grew more rapidly and more efficiently than control counterparts (P < 0.01). Treatment with pST improved the rate of protein accretion (P < 0.01) at all but the lowest level of dietary protein. The net lysine utilization for lysine accretion and maintenance was 0.46 for control pigs and 0.57 for pigs receiving pST, a 24% improvement in the efficie ncy. Thus, treatment with pST increased the maximum rate of protein ac cretion as well as the partial efficiency with which dietary lysine is used for protein accretion. Consequently, only a 9% increase in dieta ry lysine was required to maximize protein deposition in pST-treated p igs, because the metabolic efficiency of lysine utilization was improv ed.