Physiological stress response of heifers divergently ranked for residual feed intake following a bovine corticotrophin releasing hormone challenge

Abstract Text: The objective of this study was to determine whether beef heifers previously ranked on the basis of phenotypic RFI differed in their physiological stress response to an exogenous corticotrophin-releasing hormone (CRH) challenge. Yearling Limousin×Friesian heifers (n=86) were ranked on phenotypic RFI. The 15 highest [mean 0.47 kg/d; high RFI] and 15 lowest [mean -0.53 kg/d; Low RFI] ranking animals were used for this study. To facilitate intensive blood collection heifers were fitted aseptically with indwelling jugular catheters on day -1. To examine the response of the adrenal cortex, a standardised dose of bovine CRH (bCRH;0.3 μg/kg BW) was administered (day 0). Prior to heifers undergoing bCRH challenge dexamethasone (20 µg/kg BW) was administered intramuscularly on day -1. Baseline blood samples were collected into tubes containing lithium heparin as an anti-coagulant at -60 and 0 min prior to the administration of dexamethasone. On day 0, serial heparinised blood samples were collected at -40, -20, 0, 20, 40, 60, 80, 100, 120, 150, 180, 210, 240, 270, 330, and 390 min relative to the time of CRH administration (0 min) for plasma cortisol and DHEA concentrations. Data were analysed using a repeated measures mixed models ANOVA (PROC MIXED) in SAS incorporating terms for RFI group, sample time and their interaction, as appropriate. Residual feed intake ranged from -1.27 to 1.87 kg DM/d. (SD=0.93) representing a mean daily difference of 3.14 kg DM in feed consumed between the most and least efficient animals. Low RFI animals consumed 18% less feed than animals with high-RFI. Least square means for RFI and F:G were higher (P<0.05) for high RFI than for low RFI animals.  Neither a RFI × sampling time interaction nor a direct effect of RFI was detected for DHEA, cortisol or cortisol:DHEA concentrations in response to the exogenous bCRH challenge. No difference (P>0.10) in median plasma area under the curve (AUC) for cortisol, DHEA or cortisol:DHEA was observed between the two RFI groups. The maximum concentration and change in cortisol and DHEA concentrations owing to CRH were not different between the high and low RFI phenotypes. Furthermore, across animals, AUC, maximum concentration or change in cortisol or DHEA concentrations were not associated (P>0.10) with DMI, F:G or RFI co-efficients. These data suggest that the responsiveness of the hypothalamic-pituitary-adrenal axis is unlikely to contribute to appreciable variation in the efficiency feed utilisation of cattle.  

Keywords: Feed Efficiency, Stress, Cortisol