THE EFFECT OF RACTOPAMINE ON MYOFIBER DISTRIBUTION AND MORPHOLOGY AND THEIR RELATION TO MEAT QUALITY IN SWINE

One hundred and twenty-eight purebred Lacombe pigs (equal number of barrows and gilts) were assigned to one of four treatments at 64 kg and fed a commercial grower-finisher diet ad libitum to which ractopamine (RAC) was incorporated (0, 10, 15 or 20 mg kg-1). Pigs were slaughtered at 100 kg. Histological samples were collected from the semimembranosus (SM) and the psoas major (PM) muscles from the 0 and 20 mg kg-1 RAC groups. Meat quality and muscle composition data were collected for all pigs. A significant 3-way interaction for muscle weight involving treatment, muscle and gender (P = 0.01) indicated that for the SM, gilts appeared to respond to RAC at a lower level of incorporation (10 mg kg-1) than barrows. Despite these differences in response at the gross muscle level, responses were similar at the cellular level for both muscles. The proportion of red fibres did not change (P = 0.21), the proportion of intermediate fibres decreased (P = 0.01) and the proportion of white muscle fibres increased (P = 0.02) in the 20 mg kg-1 RAC group compared to the controls. In addition, when compared to controls, average fibre diameters in the 20 mg kg-1 RAC group remained constant in the red fibres, and increased significantly in both the intermediate and white fibres (P = 0.01 and 0.02, respectively). Over both muscles Kramer Press values were significantly higher (P = 0.03) in the RAC-fed pigs than in the controls. Incorporation of RAC into the diet did not result in any changes to crude protein or total and soluble hydroxyproline levels. Crude fat tended to increase in the 15 mg kg-1 RAC treatment in the SM and decreased in the 20 mg kg-1 RAC treatment in the PM. The shift in fibre type, and the associated changes in fibre size, may contribute to the increased shear values found in RAC-fed pigs.