Abstract
This study was conducted to investigate the effect 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. During the experiment, 44 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. These results confirmed that RT is an indicator of the response in dairy cows to hot environmental temperatures. However, they also showed signs of stress, which were reflected in higher levels of cortisol and in certain physiological responses.
Highlights
Heat stress is defined as an event that affects an animal’s homeostasis and health owing to a physiologically harmful heat load (Gaughan et al, 2012)
According to Pinto et al (2020), when temperature-humidity index (THI) exceeds a threshold value of 72, heat stress begins for dairy cattle
Thermal stress may manifest differently for physiological measures, with respiration rate (RR) being affected at a lower THI and rectal temperature (RT) increasing at a higher THI (Pinto et al, 2020)
Summary
Heat stress is defined as an event that affects an animal’s homeostasis and health owing to a physiologically harmful heat load (Gaughan et al, 2012). 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 (Rosenkrans et al, 2010). Heat stress influences a cow’s health negatively by altering the normal physiological functions of the cow, which results in a higher incidence of udder health problems during summertime (Turk et al, 2015). Animal discomfort because of heat stress has been found to be a primary cause of production losses in the global dairy industry, especially for high-producing cows (Thatcher et al, 2010; Dunshea et al, 2019). The usual on-farm mitigation technique to combat heat stress for dairy cows is to control the animal’s thermal environment (Mader et al, 2007)
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