Persistency of heat stress effect in dairy Simmentals in Eastern Croatia

In order to determine the persistency of heat stress effect in dairy Simmentals in Eastern Croatia in accordance to the parity and susceptibility to heat stress, over 0.5 million of test-day records were analysed. Only cows with detected statistically significant decrease in daily milk yield were included in the further analyses. The persistency of heat stress effect in the subsequent milk recordings (1st and 2nd) was determined in cows that had significant decrease of daily milk yield at different temperature-humidity index (THI) threshold value (68 and 72). The obtained results indicate that cows heat stressed at lower THI threshold value had higher drop of daily milk yield in subsequent milk recordings. Also, primiparous experience higher drop than multiparous cows. The negative effect of heat stress on daily milk contents that existed in all cows was more pronounced in the 2nd subsequent milk recording. Finally, the lower THI threshold value, the stronger and more persistent was heat stress effect. Having in mind the quite probable increase of the frequency of heat stress days, it is necessary to develop an adequate strategy for dairy farming. Further research on genetic evaluation on heat stress should provide necessary informations.