EFFECTS OF RACTOPAMINE, GENOTYPE, AND GROWTH-PHASE ON FINISHING PERFORMANCE AND CARCASS VALUE IN SWINE .1. GROWTH-PERFORMANCE AND CARCASS MERIT

A replicated factorial experiment using 183 individually fed crossbred barrows was conducted. Pigs were fed an 18.5% CP (.95% lysine) diet with 3,594 kcal of ME/kg. The effects of five genotypes (GT): 1) Hampshire (H) x (H x Duroc [D]), 2) synthetic terminal sire line, 3) (H x D) x (Landrace [L] x [Yorkshire (Y) x D]), 4) L x (Y x D), and 5) Y x L; two levels of ractopamine (RAC) treatment: 0 and 20 ppm; and three treatment weight periods (WT): 1) 59 to 100, 2) 73 to 114, and, 3) 86 to 127 kg live weight on growth performance and carcass traits were evaluated. Ractopamine increased (P < .0001) amount (FSL) and percentage (PFSL) of carcass lean standardized to 10% fat content, loin eye area (LEA), and dressing percentage (PDRES) and reduced (P < .01) amount (DF) and percentage (PDF) of dissected fat. Magnitudes of RAC effects were smaller than those reported by other researchers. Effects of GT and WT on all growth and carcass traits were highly significant (P < .001) except for those of WT on ADG (P < .05) and GT on average feed intake (AFI) and backfat thickness at the last rib (BFLR; P < .05). Genotype 3 performed better for most economically important traits than did GT 1 and 4, suggesting that heterosis existed in GT 3, which essentially was obtained by crossing GT 1 and 4. Among the three treatment periods, WT 2 had the highest ADG. As BW increased from WT 1 to 3, AFI and AFI/ADG ratio (FCR) increased and lean percentage decreased. No significant interactions among RAC, GT, and WT were observed for ADG, FCR, and muscle quantity traits, indicating that the effects of RAC, GT, and WT were additive for these traits.