Effect of thermal environment on dairy cows

Due to global warming, heat-induced stress is becoming an increasingly important problem for dairy cows in Lithuania. A temperature-humidity index (THI) is a single value representing the combined effects of air temperature and humidity associated with the level of heat stress. The aim of this study was to evaluate the effects of THI on the level of stress hormone cortisol in cows and productivity of dairy cows in climatic conditions of two dairy farms in Lithuania. The study was conducted in 2019–2020 in two selected dairy farms in southern Lithuania. Necessary data on environmental conditions and productivity of cows on farms were collected, averages of temperaturehumidity index were calculated. 20 urine samples were tested to determine the level of stress hormone in cows. Statistics were processed using statistical data analysis. The largest differences between the average air temperatures (⁰C), relative humidity (%), air velocity (m/s) and temperature-humidity index values in the farm regions were observed in the winter season. A statistically significant dependence of cortisol concentration in the body on THI was also found (r = 0.902, P < 0.05). In both farms, the highest temperatures-humidity index averages were recorded in 2019 June (74.2 and 73.9). The statistical analysis demonstrated that the temperature-humidity index had a negative effect on the milk yield, fat content and protein content of cows (P < 0.05).