Effects of ractopamine, genotype, and growth phase on finishing performance and carcass value in swine: II. Estimation of lean growth rate and lean feed efficiency

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Abstract
A replicated factorial experiment using 183 individually fed crossbred barrows was conducted. The 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) × (H × Duroc [D]), 2) synthetic terminal sire line, 3) (H × D) × (Landrace [L] × [Yorkshire (Y) × D]), 4) L × (Y × D), and 5) Y × 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 ADG of dissected lean (ADLG) and fat standardized lean adjusted to 10% fat content (ADSLG) and feed efficiency of ADLG (LFE) and ADSLG (SLFE) were evaluated. Initial carcass lean quantity of each individual animal was determined by a regression equation (R2 = .95) generated from 30 additional barrows (six per GT) slaughtered at 59 kg and 30 (six per GT) untreated pigs slaughtered at 100 kg average live weight. Logarithmic and reciprocal transformations of dependent variables were used to stabilize heterogeneous variances and to improve normality of the residuals. Ractopamine increased (P < .0001) ADLG, ADSLG, LFE, and SLFE, respectively, by 19.5, 25.0, 19.6, and 25.5%. Differences (P < .001) were observed among genotypes for all traits, showing that considerable variation existed in the data and indicating that genetic improvement can be realized through the identification and selection of superior genotypes. For ADLG and ADSLG, a RAC × GT interaction (P < .05) was observed, demonstrating greater responses to RAC in leaner genotypes. Pigs achieved greatest lean growth rate in the 73 to 114-kg period among the three growth phases and likely reached a plateau in lean growth rate at 70 to 90 kg. Lean feed efficiency declined as BW increased. These results clearly indicate that greater benefits can be expected if pigs from leaner genotypes are treated with ractopamine and slaughtered at or before 114 kg live weight.

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