Estimated heat production, blood parameters and mitochondrial DNA copy number of Nellore bulls (Bos indicus) with high and low residual feed intake

Our hypothesis was that heat production, blood parameters, and mitochondrial DNA (mtDNA) copy number differed among Nellore bulls with high and low residual feed intake (RFI). Young Nellore bulls underwent a feed efficiency test after weaning to identify their RFI class by regression of dry matter intake (DMI) in relation to average daily gain (ADG) and mid-test metabolic body weight (BW0.75). Calculations of RFI were not corrected with any estimate of body composition. The 9 highest and the 9 lowest RFI bulls were classed as high and low-RFI groups and were individually fed in a feedlot for 98 days. The RFI values of Nellore bulls were then re-calculated at the end of the finishing period. Spearman correlations were then computed to compare ADG, DMI, BW0.75, and RFI across the two test periods (growing and finishing). During the finishing period, heart rate (HR) was monitored for 4 consecutive days and used to calculate the estimated heat production (EHP) by oxygen pulse methodology in 12 bulls (6 high versus 6 low RFI). To this end, electrodes were fitted to bulls with stretch belts and oxygen consumption was determined using a facemask connected to a gas analyzer and HR was simultaneously measured for 15 min. Daily EHP was calculated multiplying individual oxygen pulse (O2P) by the average HR of each animal, assuming 4.89 kcal/L of O-2. Blood samples were collected on the 45th and 90th days of the finishing period to measure parameters such as hematocrit, hemoglobin, and glucose. Immediately after slaughter, liver, muscle and adipose tissue (subcutaneous and visceral fat) samples were collected and, subsequently, the amount of mtDNA was determined by quantitative real-time PCR. The study showed that EHP (expressed in Mcal/day or in relation to BW0.75), O-2 consumption, and mtDNA copy number per cell were similar (P > 0.10) between RFI groups, while HR, hemoglobin, and hematocrit concentration were lower (P < 0.05) in the efficient phenotype (low RFI). Our results suggest that mitochondrial abundance is comparable between groups in liver, muscle, and adipose tissues, and the differences in feed efficiency observed in the animals (high versus low RFI) were not due to differences in daily EHP (regardless how it was expressed). Finally, hemoglobin, hematocrit cocentration, and HR may be useful traits to identify efficient animals for Nellore cattle breeding programs.