Investigating the impacts of heat stress on production of Holstein cows in Gavdasht research station

The aim of this study was to assess the impact of heat stress on production of dairy cows of National Research and Development Station for Dual-Purpose Cattle (Gavdasht, located in Babol city). In this order from 3876 test-day records which have been collected from 159 heads of Holstein cattle during the years 2015 to 2018 were used. Using meteorological data of the nearest synoptic station, two climate index with different weightings of dry bulb temperature and humidity were calculated and compared. Because longer periods of heat stress might have a more severe effect than shorter periods, 1-, 2-, and 3-d periods were also considered, by averaging the weather data measurements. Generally, regression analysis with a generalized linear mixed model was used to study the change in test day milk yield in response to climate conditions. Goodness of fit of models were determined using coefficient of determination (R2) and mean square error (MSE). Based on these results, the index with the temperature and relative humidity (THI) was determined as the best indicator for studying the climatic conditions of this study area. It was also observed that the average weather data 2 d before the test date (TD) explained more of the variability of milk yield than weather data 1 or 3 d before TD or on the TD itself. The starting point of heat stress for the Holstein cattle of Gavdasht Research Station was around 74 and their yield decreased by -0.12 kg per unit increase of THI ≥ 74.