Milk composition responses amplify economic damages from heat stress

Studies analyzing the impact of climate change on agriculture mostly emphasize effects on production quantity, overlooking potential impacts on quality and nutritional content. We explore this question in the context of U.S. milk production, which represents about 20% of national animal-products. We link individual lactation records for over 6.5 million cows over 2007–2016 with high-resolution weather data to estimate the nonlinear effects of heat on both milk yield and composition (fat and protein content). While milk yield declines sharply only when the temperature-humidity index (THI) exceeds 70 (primarily in the summer), milk composition deteriorates steadily with rising THI (throughout the year). Combining these findings indicates that estimates of heat damages that ignore composition effects common in the literature underestimate total economic losses by at least a factor of two. We also find that the sensitivity of both milk yield and composition to heat varies little with cow age, farm size, region of the country, or period in the sample, indicating no detectable change in the aggregate heat sensitivity of U.S. dairy systems over the study period.