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The aim of this research was to analyze the effects of long-lasting heat stress on the milk traits of Simmental and Holstein cows in Central Croatia. For this purpose, 1.9 million of test-day records from 83,086 Simmental and 66,740 Holstein cows were available.

The studied traits were: daily milk yield, fat and protein content and somatic cell count in the milk. Data collection took place from January 2005 to December 2012. Based on the temperature-humidity index (THI) was determined whether a mild (THI value = 69-71), moderate (THI value = 71-79) or a strongly heat stress (THI value >= 80) existed for the animals. The evaluation (SAS 8.2) included stress-susceptible animals, which at mentioned THI values showed a significant reduction in milk yield. The effects of heat stress were analyzed in cows that had decrease of daily milk yield after heat stress reaction in the two subsequent milk recordings. The results showed extremely unfavorable climatic conditions in central Croatia, which had a negative effect on the investigated milk traits of both cows breeds. The daily milk yield of the cows decreased more at the low than at the higher THI thresholds. In the cows after the second lactation, the milk yield even increased, which may indicate an adaptability of the animals to a long-lasting heat period. The Simmental cows compared to Holstein proved to be more heat-tolerant animals in terms of increased somatic cells count in milk as a result of heat stress. The steady increase of hot days due to climate change requires a sustainable method for long-term heat-combating in cows. The analysis of genetic predisposition of the animals regarding the heat tolerance is necessary here.