Heat-stressed dry cows will produce less milk, but there are ways to help them. 
It’s estimated that heat-stressed dry cows account for $810 million in milk loss annually in the United States (J. Dairy Sci. 99:9931-9941). Conversely, taking steps to cool dry cows was shown to add to farm profits.

How does heat stress during the dry period affect milk yield? In part, it causes abnormal mammary development and lower pre-calving dry matter intake.

Immune response pre- and post-calving becomes compromised when dry cows are heat stressed (B.C. do Amaral et al., 2011), creating health problems after calving. Subclinical ketosis, for example, increases by 50%. Additionally, calves born from heat-stressed dry cows experience lower immunity and diminished milk and reproductive performance later in life.

Tips to help heat-stressed dry cows
Are your fans clean and well-maintained? Just one-eighth inch of dust can hamper performance by up to 30%. Stocking density matters as well. To read about nutritional strategies – and to see research on the many positive effects of B Vitamin supplementation – read our article Do Not Forget Your Dry Cows During Heat Stress.