Studies show that milk production in dairy cows begins to be negatively affected at a THI of 68. It has recently been suggested that reproduction can start to be affected at a THI of 65.
There are many different tools to measure heat stress such as THI, Apps, data loggers that can be placed in areas of investigation, and breaths per minute.
THI is the Temperature Humidity Index, which measures the air temperature and relative humidity. Both of these are important to measure heat stress so you can use the appropriate tools to keep them cool.
Some indicators of heat stress are panting, increased respiration, sweating, standing, and changes in manure consistency.