The effects of body composition on fasting heat production in pigs.

Ten sets of three litter mate barrows from each of three genetic stocks, Beltsville Highfat (HF) and Lowfat (LF) Duroc-Yorkshire composites and a Hampshire X Large White cross (CX), were used to investigate effects of body composition on fasting heat production (FHP). Pigs from each set were fasted for 24 h at 10, 17 or 24 wk of age before measuring respiratory exchange for 24 h in an open-circuit calorimeter. Average heat production during night hours (8 h) in the calorimeter was used to estimate FHP. Each pig was then slaughtered and dissected into three fractions: carcass, head and feet, viscera and blood. Each fraction was frozen, ground, sampled and analyzed for protein, fat, water and ash. Correlations within ages of FHP with calorimeter live weight (CLWT) were lower than with total weight of lean, protein or water, but near zero with fat at all ages. Coefficients of determination (R2) for linear and nonlinear predictors of FHP from lean were: .58, .89 and .91 at 10, 17 and 24 wk, respectively, but from CLWT were: .52, .74 and .82, respectively. Negative partial regressions of FHP on fat improved R2 slightly at 10 and 17 wk but not at 24 wk, suggesting that fatter pigs were better insulated. Viscera and blood influenced FHP more than carcass or head and feet. Estimated exponents relating lean mass to FHP ranged from .79 to .86 and FHP (kcal/d) = (86 +/- 20) X lean kg X 84 (+/- X 06) at 24 wk. There was some evidence of stock differences in FHP independent of body composition.