Do you group cows by production? If so, then you're a big step ahead in understanding why heat stress is worse than you may realize.
THI (temperature humidity index; the "feels like" combination of temperature and humidity) has been widely used in the dairy industry to measure when stress occurs and how severe it is. For years, mild heat stress was believed to begin at a THI of 72. But not today.
More research has found that negative effects actually begin at a THI of 68 and become progressively more severe. Dead cows can be expected at a THI of 99 or more.
But those numbers are just a guideline. Bob Collier, professor and former chairman of the department of animal sciences at the University of Arizona, said the reality is that high-producing cows experience heat stress at less than 68.
The reason is feed intake. Cows that make more milk eat more feed in order to do it; and as more feed is converted into milk, more internal body heat is produced. That means high-producing cows can start suffering from heat stress before you or your employees think about taking off your coats.
Collier says that means just-fresh and high-production groups need all the shades, fans, misters and drinking water they can get in order to minimize milk and reproduction losses due to heat stress.
