Adrenal cortex reactivity in dairy cows differs between lactational stages and between different feeding levels

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The ACTH stimulation test was established more than 60 years ago in human medicine to diagnose adrenocortical insufficiency and hypothalamo–pituitary–adrenal (HPA) axis function (Jenkins, Forsham, Laidlaw, Reddy, & Thorn, 1955; Thorn, Goetz, Streeten, Dingman, & Arons, 1953). ACTH challenge tests are experimentally used in farm animals to assess stress response (Otovic & Hutchinson, 2015; Trevisi, Bertoni, Lombardelli, & Minuti, 2013; Verkerk, Macmillan, & McLeay, 1994; Weiss, Helmreich, Möstl, Dzidic, & Bruckmaier, 2004). Munksgaard, Herskin, Løvendahl, and Andersen (2006) summarized findings in non‐ruminant species with evidence that diet composition and energy supply impact HPA‐axis regulation and stress response. Whereas HPA‐axis reactivity changed in growing heifers and steers, adrenal reactivity of lactating cows seemed not to be affected by energy intake (Munksgaard et al., 2006).
However, the importance of investigating changes in ACTH challenge test characteristics (i.e., total capacity of cortisol release, peak height, time of peak, etc.) in terms of repeatability in dairy cows changing their endocrine and metabolic status was not considered so far. Instead, ACTH tests are mostly performed only once and animals grouped retrospectively as high and low responders (Van Reenen et al., 2005; Weiss et al., 2004). Fisher, Verkerk, Morrow, and Matthews (2002) emphasized the role of cortisol for the short‐term stimulation to mobilize body reserves. Hence, the adrenal cortex sensitivity could be indicative for the flexibility in metabolic adaptations to nutrient and energy deficiencies. Furthermore, the explanatory power of ACTH test outcomes at different lactational stages needs more analysis to determine appropriate dates for conducting those.
The objective of this study was to investigate relationships between ACTH test response characteristics and metabolic parameters at different lactational stages in dairy cows. Based on these experimental approaches, we tested the hypothesis that responses to ACTH challenges in dairy cows are repeatable among various lactational stages. We further hypothesized that individual performance and enhanced metabolic load affect adrenal cortex reactivity.
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