Persistence of heat stress eff ect in dairy cows

In order to determine the persistence of heat stress effect in dairy cows regarding the breed, parity and susceptibility to heat stress, over 1.9 million of test-day records from Holstein and Simmental cows reared in Croatia were used. The persistence of heat stress effect in the subsequent milk recordings was determined in cows that had significant decrease of daily milk yield at different THI threshold value (65, 70, 75, 80, and 85). The obtained results indicate that cows heat stressed at lower THI threshold value had higher proportional drop of daily milk yield in subsequent milk recordings. Also, primiparous and Simmentals experienced higher proportional drop when compared to multiparous and Holsteins. The negative effect of heat stress on daily milk contents that existed in all cows was more pronounced in the 2nd subsequent milk recording. The determined increase of somatic cell count in more heat stress sensitive Holsteins (THI in 65, 70, 75) indicates that Holstein cows, in terms of heat stress, also become more susceptible to mastitis. Finally, the lower was the THI threshold value, the stronger and more persistent was the heat stress effect. Considering the quite probable increase of the frequency of heat stress days, it is necessary to develop an adequate strategy for dairy farming. Aiming the accurate answer to the problem of heat stress in this region, it is necessary to conduct further research on genetic evaluation.