Energy Costs of Protein and Fat Deposition in Pigs Fed Ad Libitum

Ten sets of three littermate barrows from each of three genetic stocks (Beltsville Highfat and Lowfat Duroc-Yorkshire composites plus a Hampshire × Large White cross) were used to estimate the energy costs of protein (bp) and fat (bF) deposition in the pig. At 10, 17 and 24 wk of age, one pig from each set was slaughtered and analyzed for fat, protein, water and ash. Pigs retained from 10 to 17 or 24 wk were fed a 16% crude protein diet ad libitum. Protein (ΔP) and fat (ΔF) deposited were calculated from final and initial weight and composition, using initial composition of sibs. Metabolizable energy (ME) content of the diet was determined from a N-energy balance experiment conducted with five additional barrows from each line at 12 and 19 wk of age. The regression of ME intake (MEI) on the sum of daily lean mass (Σleani·85, kg), ΔP and ΔF was calculated for periods 10 to 17 wk + 17 to 24 wk to minimize dependence between Σleani and ΔP in deriving predicted MEI (Mcal) = .116 (±.017) Σleani·85 + 11.7 (±2.3) ΔP + 15.3 (±.8) ΔF, with R2 = .94. Assuming maintenance requirements (MEM) to be .116 leani·85/d, the regression of (MEI-MEM) on ΔP and ΔF for all the data yielded the following prediction: MEI = MEM + 11.0 (±1.6) ΔP + 16.2 (±.6) ΔF, with R2 = .98. Lean mass, water and protein were of equal value in predicting MEM· Alternative regression methods showed bp to be very sensitive to prior estimates and(or) assumptions relative to MEM· Estimates of bF may be biased downward when MEM is predicted from live weight·75, because MEM of fatter pigs is overestimated. There was some evidence of genetic variation in ME utilization independent of differences in growth rate and (or) body composition.