Persistency of the Effect of Heat Stress in Simmental Cows in Eastern Croatia

Aiming to determine the persistency of effect of heat stress in Simmental cows reared in Eastern Croatia regarding the parity and susceptibility to heat stress, 539,993 test-day records from 59,821 cows collected in the period from January 2005 to December 2012 were analysed. The cows with determined statistically significant decrease in daily milk production at set temperature-humidity index (THI) threshold value (68 and 72) were included in the further analyses. The persistency of heat stress effect was defined as the drop in daily milk traits in the subsequent milk recordings (1st and 2nd) after the heat stress. The obtained results indicate that cows heat stressed at lower THI threshold value had higher drop in daily milk yield in subsequent milk recordings. Furthermore, cows in first parity experience higher drop comparing to older cows. The negative effect of heat stress on daily milk contents that existed in all cows was more pronounced in the 2nd milk recording. Finally, the lower was THI threshold value (cows more susceptible to heat stress), the stronger and more persistent was heat stress effect. Given the very likely increase in the frequency of days with heat stress in the future, it is necessary to develop a proper mitigation strategy for dairy cattle production.