Heat production of growing heifers that differ in composition of Bos indicus and Bos taurus

Previous studies using indirect-calorimetry have reported that heat production scaled to body weight of Bos indices cross cattle is lower than that of Bos taurus cattle; however, in a comparative slaughter study, estimated fasting heat production of Bos indices x Bos taurus steers was not lower than that of Bos taurus steers. We hypothesize that fasting heat production decreases at different rates as the proportions of Bos indices and Bos taurus parentage changes. Calves were produced by artificially inseminating F 1 (Brahman-MARC III; n = 55) and MARC III (n = 60) cows to either Brahman (n = 9) or MARC III (n = 9) sires. Thirty-one heifers were produced 7 - 0% Brahman:100% MARC III (0B:100M), 13 - 25% Brahman:75% MARC III (25B:75M), 5 - 50% Brahman:50% MARC III (50B:50M), and 6 - 75% Brahman:25% MARC III (7513:25M). The indirect-calorimetry measurements were made when heifers were 30.2 +/- 0.3 week of age and then every six weeks for a total of six measurements. Heat production (kcal/kg) decreased linearly over the 30 weeks of the development phase and heifers with differing ratio of Brahman and MARC III differed in the rate of decrease (P < 0.001). The heat production (kcal/kg) as a function of age (x, weeks) was f(x) = -0.268 (0.034)x + 38.7 (1.6) for OB:100M, f(x) = -0.211 (0.023)x + 35.9 (1.1) for 25B:75M, f(x) = -0.164 (0.033)x + 33.6 (1.51) for 50B:50M, and f(x) = -0.068 (0.047)x + 29.4 (2.1) for 75B:25M heifers. The current data set supports our hypothesis that fasting heat production decreases at different rates as the proportion of Bos indices and Bos taurus parentage changes and supports that apparent discrepancies between indirect-calorimetry studies and comparative slaughter studies is due to the age at which heat production is measured and the inference space over which it is applied.