Abstract: Temperature-humidity index (THI) was used to evaluate the relationship between THI and annual dynamic of performance. Respiration rate, rectal temperature and somatic cell count (SCC) kept stable when THI lower than 68, and increased sharply after THI exceed 68. Dry mater intake, milk yield, milk fat and milk protein kept stable when THI was lower than 70, and decreased sharply after THI exceed 70. The results indicated that the performance of dairy cow begins decreasing only when THI exceed 70. The average milk yield decreased at 0.4 kg per unit increase in THI when THI > 72. Milk fat and protein declined at a rate of 0.02% and 0.01%, respectively, while SCC increased 2.77×10⁴, for per unit increase in THI when THI increased from 68 to 79. All the parameters investigated above were cubically correlated to THI, with high fitness. THI was thus considered to be an efficient and convenient index for evaluating the effect of thermal environment on dairy cow performance. 

Key words: dairy cow; performance; temperature-humidity index