Summer Heat Stress and Reduced Fertility in Holstein-Friesian Cows in Arizona

SUMMARY Significant reduction in fertility occurred during the summer in heat-stressed Holstein-Friesian cows in Arizona. The fertility rate decreased to less than 20% during the very hot months of July, August, and September, as determined by the number of live calves born per number of breedings per month. The period between calving and subsequent pregnancy was greatly extended in cows exposed to hot weather. Estrous cycle length, ovulation time, and corpus luteal development were not affected by hot weather in cows which were bred but which did not become pregnant; however, estrus was significantly shortened. Dexamethasone given to cows early in the cycle did not increase fertility. Frequencies of dystocia, retained placenta, parturient paresis, and postparturient uterine infection did not increase with hot weather; however, severity of the disease processes was usually increased.