The cortisol concentration in horses follows a diurnal rhythm. The highest concentration can be found in the morning, declines over the day and has its minimum in the late afternoon/evening. Changes in management (for example start of pasture period), change of stabling or group mates, as well as equestrian exercise and veterinary procedures can also affect the cortisol concentration. In stallions the breeding season influences the cortisol concentration. After habituation or removal of the stressor, cortisol concentrations drop to their normal level. (Aurich et al., 2015)
All of this should be considered when interpreting cortisol concentrations after analysis.
Stressful stimuli can lead to the secretion of a releasing hormone from the hypothalamus. This hormone stimulates the anterior pituitary to release ACTH (adrenocorticotropic hormone). ACTH arrives in the blood stream and reaches the adrenal cortex that stimulates the endocrine cells to synthesize and secrete corticosteroids (mineralocorticoids and glucocorticoids). Glucocorticoids operate on skeletal muscle and lead to a breakdown of muscle proteins into amino acids. They are converted into glucose in the liver and kidney and released into blood which causes an increase in blood glucose. When glucocorticoids rise above the normal level, components of the immune system are suppressed. (Reece et al., 2011)
The most important glucocorticoid is cortisol. In general, the stress-induced cortisol secretion shows great variability because several factors (breed, age, gender, genetic components, individual influences) have an impact on endocrine changes. (May, 2007)
Aurich J., Wulf M., Ille N., Erber R., von Lewinski M., Palme R., Aurich C. (2015). Effects of season, age, sex and housing on salivary cortisol concentrations in horses. Domestic Animal Endocrinology, 52, 11-16.
May, A. (2007). Evaluierung von Stressparametern beim Pferd im Zusammenhang mit dem Klinikaufenthalt. Ludwig-Maximilians-Universität: München.
Reece, J.B., Urry, L.A., Cain, M.L., Wasserman, S.A., Minorsky, P.V., Jackson, R.B. (2011). Campbell Biology. San Francisco: Pearson.