Effects of heat environment on performance, behaviour, and physiological responses of dairy cows

In the trial with permanent exposure the cows were housed in climatic chamber so that the temperature was maintained at 34 degrees C for a three day period. The respiration rate was increased from 37 to 92. Rectal temperature remained unchanged during the first 6 h. It later began to increase gradually to 40.1 degrees C. Milk yield decreased in all cows as follows: by 5.2 % on the first day. 8.0 % on the second day, 16.5 % on the third one. Earing was significantly decreased on the third day. Rumination was decreased during the whole period of heal stress. Glucose level was increased under the influence of the thermic load. The content df nonesterified fatty acids increased markedly after 24 h of hyperthermia. Thyroxine and total proteinaemia levels were decreased during the hyperthermic load. In the second five-day trial where normal temperatures (23 degrees C), were alternated with high temperatures (34 degrees C). In the course of the experimental day the mean respiration rate increased maximally to 98 in comparison with 31.3 before treatment and rectal temperature to 39.4 degrees C in comparison with 38.6 degrees C. All values decreased to the pre-experimental level at night. The differences in milk yield and milk composition between pre-experimental and experimental period were statistically non-significant for all indices. Time eating was insignificantly shorter during the whole period and so was the time of rumination on the second to fourth days. The heat stress brought about significant increase in the content of nonesterified fatty acids as soon as twelve hours afterwords. The values of the observed parameters show that the alternating exposure to higher and lower temperatures does not cause a significant disturbance of homeosthasis contrary to the permanent hyperthermia.