The Thai Journal of Veterinary Medicine


The objectives of this experiment were to study the effect of heat stress on daily rhythmicity of body temperature in dairy cows under hot and humid climatic conditions and to evaluate the efficacy of improved cooling system by monitoring body temperature. Sequential measurements of body temperature, air temperature, relative humidity and THI were monitored every 5 min. Four cross-bred Holstein-Frisian (93.75% HF) cows were assigned randomly to be used in two trial periods to determine the effects of heat stress and improved cooling system on body temperature. The experiment was conducted in the barn for a total of 6 days, 3 days without a supplemental cooling system (control) and 3 days with a supplemental cooling system (treatment). All cows were housed in the same barn throughout the experiment, but the treatment animals were exposed to the improved cooling system with sprinklers and fans in the holding pen 7 times a day. In this study, air temperature was negatively correlated (r = -0.983, p < 0.0001) with relative humidity, but positively correlated with THI (r = 0.996, p < 0.001). In addition, THI and air temperature were positively correlated (r = 0.709, p < 0.0001 and r = 0.714, p < 0.0001, respectively) with body temperature of the control animals. The mean level of body temperature of the control animals was higher (p < 0.05) than the treatment animals. Body temperature of the control animals consistently rose during the day, reaching a peak in the afternoon (1400 h), after which it remained relatively stable until midnight and then fell throughout early morning. The daily rhythmicity of body temperature had a mean level of 39.3±0.30C with a maximum-minimum range of 0.90C in the control animals. In contrast, body temperatures of the treatment animals fell by 0.3-1.00C after the improved cooling occurred at 0500, 0900, 1100, 1400, 1600, 1900 and 2200 h. Thus, their body temperatures were maintained in the range of 37.9-39.40C. The mean level of body temperature of these animals was 38.5±0.30C. In conclusion, climatic conditions have a significant influence on body temperature. Body temperature of dairy cows under tropical conditions has a daily rhythmicity, with a period of rising body temperature and increasing heat load during the day (hyperthermia), followed by a period of heat dissipation and falling body temperature during the night. Improved intensive cooling with sprinklers and fans has the potential to decrease the severity of heat stress and maintain a normal body temperature of dairy cows under hot and humid climatic conditions.



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