Discipline: stress; Keywords: dairy cows, heat stress, physiology, rectal temperature, temperature-humidity index.
Heat stress is defined as an event that affects an animal’s homeostasis and health owing to a physiologically harmful heat load. Heat stress may prompt physiological dysfunction, which affects an animal’s production and reproduction capacity negatively, and causes economic losses that are estimated to be billions of dollars worldwide. Heat stress also influences a cow’s immune system which may manifest in a higher incidence of udder health problems during summertime. It is therefore essential to understand the mechanisms through which heat stress affects dairy cows adversely to develop abatement strategies. Consequently, the objectives of a study by the authors cited below were to evaluate the effects of heat stress on the physiology of dairy cows and to detect the relationship between rectal temperature (RT) and respiration rate (RR), heart rate (HR), and plasma concentrations of cortisol, thyroxine, and prolactin.
Forty-four Holstein cows were allocated to two groups for each season. The average temperature-humidity index (THI) values were 55 ± 2.31 in winter and 78 ± 1.9 in summer. As the THI values increased from 55 to 78, RR rose by 35 inspirations per minute, HR by 3 beats per minute, and RT by 1.2 °C. In addition, the average concentration of cortisol increased from 19.30 to 21.04 nmol/L, and prolactin from 58.52 to 129.79 ngm/L, whereas free thyroxine decreased from 15.43 to 14.01 pmol/L. Plasma sodium and potassium concentrations were similar in the two seasons.
Conclusions: It was shown that heat stress influenced the physiological status and productive functions of the study cattle. Rectal temperature was an important indicator of homeostatic regulation of body temperature and the relationship between cortisol levels and RT provided an indication of the relationship between heat stress and metabolic function. Therefore, the concentrations of serum cortisol, thyroxine, and prolactin could be used to quantify the level of heat stress that is being experienced by Holstein cattle.
H. Djelailia, N. M’Hamdi, R. Bouraoui & T. Najar, 2021. Effects of thermal stress on physiological state and hormone concentrations in Holstein cows under arid climatic conditions. S. Afr. J Anim. Sci. 51, 452- 459.