1Department of Animal Science, Faculty of Agricultural Sciences, University of Guilan, Rasht, Iran
2Graduated from Faculty of Agricultural Sciences, University of Guilan, Rasht, Iran
Calving records from April 1996 to December 2005 comprising 31863 calving events of 1173 dairy cows in a large commercial dairy herd were used to evaluate the effects of heat stress, using temperature-humidity index (THI) on milk yield and composition and reproductive performance of Holstein dairy cows under humid climatic conditions of Rasht in Iran. THI values were grouped into six groups: 30-40 (THI1), 41-50 (THI2), 51-60 (THI3), 61-70 (THI4), 71-80 (THI5) and 81-90 (THI6). Dairy cows in THI6 had lower milk and fat yields than other groups of THI, but cows in THI1 and THI2 had the greatest amounts of milk and fat yields, respectively (P<0.05). However, dairy cows in the second group of THI had lower percentages of fat than the sixth group of THI (P<0.05). Spring-calved cows had longer days open than cows calved in other seasons (P<0.05). Also, summer-calved cows had greater number of services per conception and lower conception rates than cows calved in other seasons (P<0.05). Dairy cows within the THI5 which calved in spring and/or calving years 2004-2005 had the longest days open (P<0.05). Cows in the second group of THI had the greatest and the lowest number of inseminations and conception rates compared with other groups of THI, respectively. The results of the present study indicated that summer heat stress negatively affected milk yield and composition and reproductive performance of dairy cows. Therefore, application of management interventions to ameliorate the effects of heat load on the performance of dairy cows could be needed in certain periods of the year under the climatic conditions.