Effect of heat stress on vital signs in high-yield dairy cows

Abstract

The objective of this work was to examine the influence of outer temperature on values of the vital signs (temperature, pulse, respiration) in high-yield dairy cows in early stages of lactation, as well as to establish, on the grounds of the degree of correlation between the values for the vital signs and the temperature humidity index (THI), possibilities for using the examined physiological parameters of the organism as an indicator of heat stress. The experiment covered 10 high-yield dairy cows in the first phase of lactation. The investigations were carried out in the course of July and the first half of August. During the course of the experiment, the average THI was determined daily. An average daily THI higher than 70 indicated that the animal had been exposed to heat stress on that day. Vital signs were measured on twelve occasions during the period of investigation (June 30, July 4, July 7, July 10, July 14, July 17, July 21, July 24, July 29, August 5, August 11, and August 14). It was established on the grounds of the THI values that the examined animals were exposed to heat stress on June 30, July 4, July 7, July 14, July 17, July 21, July 29, August 5, and August 14, while there was no heat stress on July 10, July 24, and August 11. The average body temperature during all the periods of examination, with the exception of July 24, was above the upper physiological limit. A high degree of correlation was established between body temperature and the heat index (r = +0. 509; p = 0.05). The number of respiratory movements per minute was above the physiological values during the entire period of investigation. There was a high degree of correlation between the number of respiratory movements and THI (r = + 0.625; p<0.05). The average pulse values and number of contractions of the rumen did not vary significantly during the period of investigation. No significant correlation was established between the pulse and THI values, or between motoric activities of the rumen and THI. The results obtained in this work demonstrate that, in high-yield dairy cows exposed to moderate heat stress, the body temperature and the number of respiratory movements are above physiologically permitted values. On these grounds, it can be concluded that these two parameters of the vital signs can be used as physiological indicators of heat stress. .