A temperature-humidity index (THI) is a single value representing the combined effects of air temperature and humidity associated with the level of heat stress. The objective of this study was to evaluate the effects of THI on milk yield and milk composition of dairy cows in climatic conditions of 7 Isfahan dairy farms. The experiment carried out during in July 2012 to October 2013. In this study, eight THI with different weightings of dry bulb temperature and humidity were compared. Ambient temperature and relative humidity were measured every 15 min with data loggers located throughout the farm. According to THI values, from mid-October to early May the heat stress was lower than the threshold, with lowest values in January, then, THI was higher than threshold until mid-October, which was the highest level in July. Indices with higher weightings of humidity are more appropriate for humid climates, and indices with the most emphasis on ambient temperature are more suitable for semiarid climates. Since that Isfahan is located in the dry climate, THI 5 is the best indicator of its 8 temperature- humidity indicators. The thresholdforheat stress was 65 by THI 5. When THI increased one unit upper than treshold, average milk production decreased 0.5 Kg. Disintegration of estimated THI thresholds of heat stress into corresponding temperature and relative humidity revealed that heat stress occurred in Isfahan at temperatures ? 21°C and relative humidity of 25%. Results clearly showed a negative relationship between milk production and THI. Indeed, as THI increases from 49.1 in the winter season to 70.8 in the summer season, heat stress reduced total milk production by 38.5 to 36.1 kg, respectively. This decrease can be largely explained by the effect of summer heat stress, particularly in July, August and September when THI values are well above the critical threshold of 65. The reason for the drop in milk yield during the early fall could be explained by the carry over effect of the unfavorable conditions during the summer particularly in the absence of environmental control systems. Key words: temperature-humidity index, heat stress, milk loss
