Development of adjustment factors for backfat and loin muscle area from serial real-time ultrasonic measurements on purebred lines of swine.

We recorded serial real-time ultrasonic measurements of backfat and loin muscle area to assess the rate of change per unit of live weight and growth rate measured as the change in weight per day of age during the finishing stage of production. Barrows (648) and gilts (459) representing eight major U.S. pure breeds of swine were evaluated in the study. Real-time ultrasonic measurements of backfat and loin muscle area were collected at average live weights (LW) of 67.4, 80.3, 93.4, and 104.9 kg. Backfat was measured at the midline over the last rib (LR) and last lumbar vertebra (LL) and off the midline at the 3/4 point (BF10) over the 10th rib. Loin muscle area (LMA) images were collected from the right-hand side of each pig at the 10th rib. Deposition rates were calculated on an intrapig basis for the dependent scan variables BF10, LMA, LR, and LL in models that included the independent variables LW and LW2. For growth rate (WDA), LW was regressed on age and age. Intrapig linear regression coefficients and y-intercepts were further analyzed as dependent variables in a mixed model that included the fixed effects test group, sex, and breed and random effects of sire and dam nested within breed. Significant breed effects were present for deposition rates of BF10, LMA, LR, LL, and WDA. Across the eight breeds, BF10 was deposited at a mean of .271 +/- .008 mm/kg, a rate faster than the average for LR (.201 mm/kg) and LL (.206 mm/kg). The LMA deposition rate across breeds averaged .304 cm2/kg. Average WDA across the breeds was .774 kg. Barrows deposited fat faster at all locations, deposited muscle slower, and grew faster than gilts in this study (P < .001). The results of this study indicate a need for breed- and sex-specific adjustment factors for rates of backfat and loin muscle deposition and for weight per day of age in swine.