Heat stress during the summer disrupts several reproductive processes, resulting in a pronounced depression of conception rate in dairy cows worldwide. The rise of internal body temperature during the summer is responsible for the impaired reproduction. A major cause for sustained hyperthermia during the summer is high milk production, which continues to rise. The processes of milk synthesis and secretion increase cows’ metabolic heat production. For instance, heat production of cows yielding 30 kg/day milk is twice as high as maintenance heat production of nonlactating cows, and that of high milk yielding cows giving 55 kg/day is about three times higher than maintenance heat production.