Warm weather in the US varies in length and intensity depending on the state and can lead to heat stress in dairy cows. It has been estimated that dairy production alone accounts for 37.5% (ca. $900 million) of the annual livestock production losses associated with heat stress. These losses are multi-factorial and mostly attributed to milk production, changes in milk composition, less than optimal reproductive performance, and culling rates.
There’s plenty of research on practices to cool-off cows by evaporative heat losses such as shade, fans, sprinklers, and tunnel cooling. Cows cope with excessive environmental heat conditions by several physiologic strategies such as evaporative heat loss (perspiration), increasing respiration rate (panting), reducing heat of fermentation (caloric increment) by cutting-off intake, and feed sorting to choose more easily digestible feed particles. There has been consensus in the past however, that a decrease in intake to reduce the caloric increment is the primary driver leading to decreased milk yield in cows subjected to heat stress.