Determination of Climate Critical Periods in Management of Dairy Cows in Northeast of Iran Using Temperature Humidity Index (THI)

This study was carried out to investigate the possibility of prediction the occurrence of heat stress in dairy cows and determination of climate critical periods. The most common indexes for estimation the degree of thermal stress istemperature humidity index (THI). In this study Yousef’s THI index (1985)was used to analyze the yearly and monthly effects of temperature and humidity. Meteorological data were obtained from the Northeast Regional Climate Center of Iran.After theTHI was calculated, the variability of THI was analyzed using scatter charts and THI values were analyzed as two-factor factorial design. Statistically, the changes of THI had constant trend over the year. Effects of years were meaningful and THI monthly means had significant differences during the year. The highest values of THI were evaluated in July that THI values in many hours were higher than the threshold values of the occurrence of heat stress. Moreover, 186291 records of milk production from a large industrial dairy farm were analyzed. Data showed that milk production also had constant trend over the year but with opposite direction of THI. In other words, when the amount of THI increased and passed of the threshold values of the occurrence of heat stress, milk production reduced and reached to the lowest level in July.Generally THI index is effective for determination of the critical periods of climate and prediction the probability of heat stress in dairy cows, and it can be a perfect guidefordairy farmersto perform adequate preventive acts.