Kelly A K, Lawrence P, Earley B, Kenny D A, McGee M
School of Agriculture and Food Science, College of Health and Agricultural Sciences, University College Dublin, Belfield, Dublin 4, Ireland.
Livestock Systems Research Department Teagasc, Animal & Grassland Research and Innovation Centre, Grange, Dunsany, Co. Meath, Ireland.
J Anim Sci Biotechnol. 2017 Aug 8;8:65. doi: 10.1186/s40104-017-0197-x. eCollection 2017.
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).
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 2 kg of concentrate per head once daily. The 12 highest [0.34 kg DM/d] and 12 lowest [-0.48 kg 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 ± 31.4 kg) were fitted aseptically with indwelling jugular catheters to facilitate intensive blood collection. The response of the adrenal cortex to a standardised dose of ACTH (1.98 IU/kg metabolic BW) was examined. Serial blood samples were analysed for plasma cortisol, ACTH and haematology variables. Heifers differing in RFI did not differ ( = 0.59) in ACTH concentrations. Concentration of ACTH peaked ( < 0.001) in both RFI groups at 20 min post-ACTH administration, following which concentration declined to baseline levels by 150 min. Similarly, cortisol systemic profile peaked at 60 min and concentrations remained continuously elevated for 150 min. A RFI × time interaction was detected for cortisol concentrations ( = 0.06) with high RFI heifers had a greater cortisol response than Low RFI from 40 min to 150 min relative to ACTH administration. Cortisol response was positively associated with RFI status ( = 0.32; < 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; = 0.02) and hematocrit percentage (23.2 vs. 24.5; = 0.02) were greater for low than high RFI animals.
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.
当动物暴露于应激源时，代谢率、能量消耗和利用主要通过下丘脑 - 垂体 - 肾上腺（HPA）轴的激活而增加。动物能量分配的变化可能会影响其利用效率。因此，本研究旨在确定表型剩余采食量（RFI）差异较大的肉用小母牛对外源性促肾上腺皮质激素（ACTH）刺激的生理应激反应。
收集了34头西门塔尔断奶肉用小母牛的数据，这些小母牛来自一个特征明确且RFI差异较大的肉用哺乳牛群。在断奶后阶段的91天采食量测量期内，测定每头动物的剩余采食量，在此期间，每天单独给它们提供随意采食的青贮草和每头2千克精饲料。选择RFI排名最高的12头[0.34千克干物质/天]和最低的12头[-0.48千克干物质/天]的动物用于本研究。对于生理应激挑战，小母牛（平均年龄605±13天；平均体重518±31.4千克）无菌植入颈静脉导管，以便密集采血。检查肾上腺皮质对标准化剂量ACTH（1.98国际单位/千克代谢体重）的反应。对系列血样进行血浆皮质醇、ACTH和血液学变量分析。不同RFI的小母牛ACTH浓度无差异（P = 0.59）。ACTH给药后20分钟，两个RFI组的ACTH浓度均达到峰值（P < 0.001），之后浓度在150分钟时降至基线水平。同样地，皮质醇全身水平在60分钟时达到峰值，并在150分钟内持续升高。在皮质醇浓度方面检测到RFI×时间交互作用（P = 0.06），相对于ACTH给药，高RFI小母牛在40分钟至150分钟的皮质醇反应比低RFI小母牛更大。皮质醇反应与RFI状态呈正相关（P = 0.32；P < 0.01）。RFI对中性粒细胞、淋巴细胞、单核细胞、嗜酸性粒细胞和嗜碱性粒细胞计数无明显影响。低RFI动物的血浆红细胞数量（6.07对6.23；P = 0.02）和血细胞比容百分比（23.2对24.5；P = 0.02）高于高RFI动物。
有证据表明饲料效率与HPA轴功能及应激易感性相关，HPA轴的反应性可能导致牛饲料利用效率的显著差异。
