In order to determine the effect of breeding region on differences in persistency of heat stress effect in first parity Simmentals test-day records provided by the Croatian Agricultural Agency were analysed. Only cows with detected statistically significant decrease in daily milk yield at set temperature-humidity index (THI) threshold value (65, 70 and 75) were included in the further analyses. The persistency of heat stress effect regarding the daily milk traits was determined as a drop in the subsequent milk recordings (1st and 2nd). The research results indicate significant difference in animals’ response to heat stress effect due to breeding region and individual’s susceptibility to heat stress. The most pronounced and persistent negative effect of heat stress was determined in cows reared in Eastern region. Also, the negative effect of heat stress was more pronounced and more persistent in Simmentals that were heat stressed at the lower THI threshold values (cows that are more susceptible to heat stress).
