Influence of dietary protein and recombinant porcine somatotropin administration in young pigs: III. Muscle fiber morphology and shear force.

Sixty crossbred barrows (average 30 kg) were used in a 5 x 2 factorial treatment array to examine interactions between dietary protein concentration (11, 15, 19, 23, or 27% CP) and recombinant porcine somatotropin (rpST: 0, excipient buffer vs 100 micrograms.kg-1.d-1) for 42 d on muscle fiber morphology and meat tenderness. Diets were isocaloric (3.8 Mcal of DE/kg) and of equal lysine (4.9 g/Mcal of DE) achieved by diluting soybean meal with cornstarch and by addition of crystalline lysine. Dosage of rpST and feed intake (80% of predicted ad libitum) were adjusted weekly. Four muscles (longissimus = LM; semimembranosus = SM; semitendinosus = ST; triceps brachii = TB) were evaluated. Percentages of muscle fiber types (beta R, alpha R, alpha W) for the LM, SM, and TB were not influenced by rpST treatment. More alpha R and fewer alpha W fibers were found in the ST muscle of rpST-treated pigs. No interactions were observed between rpST treatment and dietary protein for muscle fiber type distribution. Dietary protein had no consistent influence on the distribution of muscle fiber types in all four muscles. Area of fibers generally increased in rpST-treated pigs compared with controls when diets contained 19% or more CP. The LM shear force was increased (13%) by rpST treatment for chops frozen after 5 d of storage in the cooler, but not in those chops frozen within 1.5 h postmortem. Dietary protein had a variable influence on tenderness. These data indicate that muscle fiber growth (hypertrophy) in pigs is positively influenced by rpST treatment. Marginal dietary protein intake reduces muscle fiber growth responses to rpST.