Abstract
The paper presents the influence of temperature, relative air humidity, air movement velocity and solar radiation on forecasting the risk of heat stress in dairy cows in a free-stall barn during hot weather seasons. THI and adjusted THI indexes were calculated for the periods in which the air temperature exceeded 28°C in the barn, and when it reached 25°C. Based on the THI values obtained during the heat, which ranged from 63 to 83, the occurrence of heat stress in cattle was stated. Heat stress also caused a decrease in milk yield, from about 22 kg to about 18 kg in the technological group. The calculated THIadj values (83 to 90), taking into account the solar radiation and air movement velocity, indicated also the occurrence of health-threatening conditions for cattle. On the fourth day of heat the maximum daily value of THI in the first measurement point was 83 with THIadj of 90, while in the second point THI was 82 and THIadj was 87. This indicated the occurrence of significant differences in microclimatic conditions in the occupied zone of the same technological group. The influence of investigated microclimate factors on the formation of THIadj in the researched zone of the barn was stated. No proper method of its determination was also stated. The analysis of the impact of air movement velocity on THIadj value (P<0.05) and the solar radiation (P<0.05) demonstrated the importance of these parameters in forecasting heat stress in cows in the barn.