Effect of temperature-humidity index on daily milk yield of dairy cows
The objective of this study was to evaluate the effects of temperature-humidity index on daily milk yield of dairy cows under climate conditions in Croatia. In this study 103.569 individual test-day milk yield records of first lactation cows from Central data base of Croatian Livestock Centre were analysed. Data were collected in regular milk recording by alternative milk recording scheme from January 2005 to December 2005. According to lactation stage, cows were divided into five groups, L1 (180 days). Average temperature-humidity index, during spring, autumn and winter period was under chritical THI, so, lack of heat stress conditions characterized these periods. During the summer period, average ambient temperature and relative humidity were 22.71±4.76 °C and 70.30±9.78%, while the average THI was 70.30±7.12. Only, during July, chritical THI was exceeded. In the first 120 days of lactation, daily milk yield was higher in spring period than in the other periods, while if lactation was longer than 120 days, the highest daily milk yield production was in summer period. The highest daily fat and protein content was obtained in winter period, for all lactation stages. The linear regression of daily milk yield, fat and protein content to temperature-humidity index indicates that daily milk yield, fat and protein content slightly decreases as THI increases. Further investigations, in which effect of feeding could be eliminated, is needed.