Endocrine and hematological responses of beef heifers divergently ranked for residual feed intake following a bovine corticotropin-releasing hormone challenge

Abstract

The objective of this study was to determine if beef heifers divergently ranked on phenotypic residual feed intake (RFI) differed in their physiological stress response to an exogenous bovine corticotropin-releasing hormone (bCRH) challenge. Yearling Limousin × Friesian heifers (n = 86) were ranked by RFI. The 15 highest (mean 0.66 kg DM/d; high RFI) and 15 lowest (mean −0.72 kg DM/d; low RFI) ranking animals were used for this study. During the study period, heifers (mean age 485 ± 13 d; mean BW 408 ± 31.4 kg) were housed in a slatted-floor facility. To facilitate intensive blood collection, heifers were fitted aseptically with indwelling jugular catheters. All heifers received dexamethasone (DEX; 20 µg/kg BW i.m.) 12 h before the bCRH challenge (d 0). Heparinized blood samples were collected at −60 and 0 min before administration of DEX, and 12 h after DEX administration. Following DEX administration, cortisol and dehydroepiandrosterone (DHEA) concentrations similarly decreased (P ≥ 0.22) between high and low RFI groups. The response of the HPA axis to a standardized dose of bCRH (0.3 μg/kg BW) was examined. On d 0, serial blood samples were collected at −20, 0, 20, 40, 60, 80, 100, 120, 150, 180, 210, 240, 270, 330, and 390 min relative to the time of bCRH administration (0 min) and were analyzed for plasma cortisol and DHEA concentrations. Blood hematology variables were also determined at −20, 0, 20, 80, 150, 270, 330, and 390 min relative to bCRH administration. Neither an RFI × sampling time interaction nor a direct effect of RFI were detected (P ≥ 0.36) for plasma cortisol, DHEA concentrations, or cortisol:DHEA ratio. An effect of sample time was observed for cortisol (P < 0.001), DHEA (P = 0.04), and cortisol:DHEA (P = 0.02), with cortisol concentration peaking at 60 min post-CRH administration. The maximum concentration and rate of change in cortisol and DHEA concentrations following bCRH administration were not different (P ≥ 0.20) between the high and low RFI phenotypes. Similarly, an effect of RFI was not evident (P ≥ 0.16) for any of the hematology variables examined including neutrophil, lymphocyte, and leukocytes counts; white and red blood cell numbers; and hemoglobin concentrations. Our results suggest that responsiveness of the HPA to stress, at least at the level of the anterior pituitary gland, is not a significant contributor to variation in RFI among growing beef heifers.