Body composition and fasting heat production from birth to 14 months of age for three biological types of beef heifers.

: Thirty heifers were randomly sampled at five ages from each of the Hereford, Charolais, and Simmental herds at the U.S. Meat Animal Research Center to estimate association of fasting heat production (FHP) with body composition. Replicated measures of respiratory exchange were obtained for six heifers per breed at ages 2 d, 3 mo, 7 mo, 10 mo, and 14 mo using open-circuit calorimetry. Regression adjustment of FHP/live weight.75 to zero activity (AFHP) reduced the mean by 12% and variance among periods for the same animal by 42%. Daily AFHP (kcal/kg.75) was highest at 2 d (122), lowest at 10 mo (92) (P less than .01), and intermediate (103 to 106) at other ages and averaged 109, 106, and 102 for Charolais, Simmental and Hereford over all ages (P less than .05). Pooled within-age correlations of AFHP were .77 with weight of carcass (CAR) nonfat or water and .75 with live and empty body weight (EBW) but were only .13 with fat weight. Prediction of AFHP within age groups was most accurate from multiple regression on the nonfat weight in visceral organs and blood (VOB), gastrointestinal tract (GIF), head, hide, and shanks (HHS), and CAR fractions (R2 = 61%, error SD = 21.5), from regression on nonfat in CAR alone (60%, 21.6), or from regression on chemical components in each of the four fractions (59%, 22.0), relative to EBW (55%, 22.9) or its four chemical components (58%, 22.3). Partial regressions were largest for water or nonfat (P less than .01) and were negligible for fat. Importance in predicting AFHP was two to eight times greater for nonfat in CAR than in other fractions because CAR was 60 to 65% of EBW. Lean mass is clearly a major predictor of nutrient requirement that is useful to evaluate effects of body composition on the efficiency of beef production.