Temperature-humidity index values and their significance on the daily production of dairy cattle

The objectives of this study were to determine the microclimatic conditions in stables in three climactic regions (East, Mediterranean, and Central) of Croatia as well as to evaluate the effect of temperature-humidity index (THI) values on the daily production of dairy cattle. With that purpose, 1675686 test-day records collected from January 2005 until April 2010 were extracted from HPA (Croatian Agricultural Agency) database. For estimation of the effect of THI on daily production of dairy cows fixed-effect model that took into account the effects of lactation stage, breed, calving season, measuring season, and THI group (T(1) - THI <= 72; T(2) - THI>72) was used. Model was applied to each class of parity (P(1), P(2), P(3), P(4), and P(5)) and region. During the analysed period, the highest monthly averages of ambient temperature were determined in Mediterranean region, the highest monthly averages of relative humidity were observed in Central region, while the highest monthly mean values of temperature-humidity index (THI) were determined in Mediterranean region. Heat stress conditions indicated with mean daily values of THI>72 were determined during spring and summer season in all analysed regions. Absence of heat stress conditions during autumn and winter season also characterised all three regions. Highly significant (P<0.01) decrease of daily milk yield as well as of daily fat and protein content due to enhanced THI was observed in all cows regardless the parity class and in all three climatic regions. Furthermore, the most deteriorate effect of heat stress was observed in East region. During heat stress period, with the aim of minimization of the effects of heat stress, it is necessary to regulate management strategies in the dairy herd.