Stress and immunological response of heifers divergently ranked for residual feed intake following an adrenocorticotropic hormone challenge

Abstract

Background: When an animal is exposed to a stressor, metabolic rate, energy consumption and utilisation increase primarily through activation of the hypothalamic-pituitary-adrenal (HPA) axis. Changes to partitioning of energy by an animal are likely to influence the efficiency with which it is utilised. Therefore, this study aimed to determine the physiological stress response to an exogenous adrenocorticotropic hormone (ACTH) challenge in beef heifers divergently ranked on phenotypic residual feed intake (RFI). Results: Data were collected on 34 Simmental weaning beef heifers the progeny of a well characterized and divergently bred RFI suckler beef herd. Residual feed intake was determined on each animal during the post-weaning stage over a 91-day feed intake measurement period during which they were individually offered adlibitum grass silage and 2kg of concentrate per head once daily. The 12 highest [0.34kg DM/d] and 12 lowest [-0.48kg DM/d] ranking animals on RFI were selected for use in this study. For the physiological stress challenge heifers (mean age 605±13 d; mean BW 518±31.4kg) were fitted aseptically with indwelling jugular catheters to facilitate intensive blood collection. The response of the adrenal cortex to a standardised dose of ACTH (1.98IU/kg metabolic BW0.75) was examined. Serial blood samples were analysed for plasma cortisol, ACTH and haematology variables. Heifers differing in RFI did not differ (P=0.59) in ACTH concentrations. Concentration of ACTH peaked (P<0.001) in both RFI groups at 20min post-ACTH administration, following which concentration declined to baseline levels by 150min. Similarly, cortisol systemic profile peaked at 60min and concentrations remained continuously elevated for 150min. A RFI×time interaction was detected for cortisol concentrations (P=0.06) with high RFI heifers had a greater cortisol response than Low RFI from 40min to 150min relative to ACTH administration. Cortisol response was positively associated with RFI status (r=0.32; P<0.01). No effect of RFI was evident for neutrophil, lymphocytes, monocyte, eosinophils and basophil count. Plasma red blood cell number (6.07 vs. 6.23; P=0.02) and hematocrit percentage (23.2 vs. 24.5; P=0.02) were greater for low than high RFI animals. Conclusions: Evidence is provided that feed efficiency is associated with HPA axis function and susceptibility to stress, and responsiveness of the HPA axis is likely to contribute to appreciable variation in the efficiency feed utilisation of cattle.