Effects of exogenous porcine somatotropin administration between 30 and 60 kilograms on longissimus muscle fiber morphology and meat tenderness of pigs grown to 90 kilograms.

Twenty-four barrows were used to investigate the effects of exogenous porcine somatotropin (pST) administration (0 and 100 micrograms.kg-1.d-1) between 30 and 60 kg on longissimus muscle morphology and meat tenderness of pigs grown to 90 kg. Administration of pST was by daily i.m. injection. Pigs were fed a fortified diet in restricted amounts between 30 and 60 kg and had ad libitum access to this diet from 60 to 90 kg. Excipient pigs had fewer alpha R fibers and more alpha W fibers than pST-treated pigs (P less than .05). Administration of pST increased (P less than .05) muscle fiber area for all three fiber types at both the 60 kg (34%) and 90 kg (29%) slaughter weights. Furthermore, pST administration increased shear-force (30% at 60 kg; 19% at 90 kg) of the longissimus muscle, indicating that pST administration reduced meat tenderness. All the pST-treated pigs exhibited the "giant fiber syndrome" and a high proportion (62%) of pST-treated pigs exhibited pale, soft, exudative muscle. Whether the giant fiber anomaly resulted from increased muscle protein accretion rate or fiber degeneration is unknown. Results indicated that the stimulatory effects of pST on muscle growth are sustained following cessation of hormone treatment. However, the incidence of pale, soft, exudative muscle in 62% of the pST-treated pigs indicates that pST potentially has undesirable side effects.