Presented are results of a study on the influence of air temperature and humidity on milk production by 174 Holstein-Friesian cows kept in a free stall barn. Occurrence of heat stress in three technological groups of cows with help of temperature-humidity index (THI) was specified. The result of heat stress was a decrease in milk yield. There was a strong correlation found (P=0.05) between minimum THI, temperature and milk yield. Delay of decrease in milk production as related to maximum temperature (TMAX) in most cases was 2 days (P=0.05). Significant differences were shown between THI average hourly values (from 1 to 6 units) in particular areas of the barn when the average daily THI >68. Duration of total air temperature exceed 21°C in each technological group was investigated. It was concluded that the number of hours above the critical temperature for the cows in group 1 was 90 and 42-44 for the remaining groups, which resulted in the largest decrease in milk yield in group 1. The obtained results indicate the need to evaluate the microclimate parameters in several specific places related to the existence in the barn technological groups. Predicting the risk of heat stress should take into account the zoning in the barns related to variations of temperature and humidity conditions. This will enable the selection of appropriate functional, utility and technical solutions to maintain optimum welfare of cows in free stall barn.
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