Significant increases in milk-per-cow over the past several decades may force us to modify the temperature-humidity index.
The index was developed using a retrospective analysis of studies from the 1950s and early 1960s. But today's cows produce a lot more milk than cows from the 1950s and 1960s did. Today's cows presumably have a lower temperature threshold for when milk losses begin to occur, since metabolic heat output is increased as production levels rise. To study this further, researchers at the University of Arizona looked at eight different modern-day experiments involving heat stress and dairy cows. From those studies, they concluded that the temperature-humidity index is currently underestimated for today's high-producing dairy cows. More specifically, they say a THI of 68 is where heat stress begins to affect cows adversely rather than the standard THI threshold of 72.
"Therefore, cooling methods on commercial dairy farms should be implemented earlier to prevent these effects," the researchers told those attending the recent Southwest Nutrition & Management Conference.