We quantify the impact of heat stress on the dairy industry throughout the Midwestern United States in the years 2012-2016 using animal-level production data. When temperature and humidity increase above critical levels, dairy cows become heat stressed and eat less which causes a drop in milk production. Crucially, certain types of dairy cattle are more susceptible than others: dairy cows that have given birth multiple times and are early in their production cycle are the most productive but also the most vulnerable to heat stress. We estimate that these cattle lose about between 2-5% of their milk production in a heat wave as opposed to at most 1% for other cattle. One low-cost form of adaption dairy farmers can use to mitigate these losses is changing the time of year that cattle can give birth. Using a back of the envelope calculation, we estimate that $950 million in lost profits could have been avoided in this period if all cows gave birth in the Fall instead of the Spring.