Effect of Heat Stress on Reproductive Performance and Milk Yield of Holstein Dairy Cows in Egypt

Heat stress ( HS) adversely affects reproduction . This study aimed to investigate the effects of HS on the reproductive and productive indices during summer and winter seasons in Holstein cows. Metrological data as well as reproductive and some productive parameters are collected during the period from 2021 to 2024 in summer and winter seasons. The temperature-humidity index (THI) was calculated and used as an indicator for HS. The statistical significance of differences as well as a correlation between various parameters such as meteorological data, some reproductive and productive indices in summer and winter were investigated. THI was remarkably higher during summer than in winter. Higher THI in summer significantly increased the services per conception and prolonged the period of a calving interval, days open, and gestation period than that in winter. The heat detection, conception and pregnancy rates significantly dropped during the summer season compared to that in winter. Meanwhile, the abortion rate significantly increased compared to that in winter. Daily milk yield significantly decreased during summer than in winter, although 305-day milk yield didn’t differ significantly between the two seasons. A positive correlation was noticed between THI and services per conception, calving interval, and days open. Conversely, a negative correlation was recorded between THI and gestation period, heat detection rate, conception pregnancy rates and abortion rate as well as daily milk yield. In conclusion, increased THI over 75 deteriorated the reproductive indices and daily milk yield of Holstein cows during summer in Egypt.