In a step toward understanding heat tolerance in horses, Brazilian researchers recently concluded that respiratory rate (RR), heart rate (HR), and rectal temperature (RT) are the most useful parameters for distinguishing equine adaptation to elevated temperatures. 

As part of an ongoing project on equine heat tolerance, the study authors investigated variations of numerous physiological traits in four breeds of horses: English Thoroughbreds, Brazilian Showjumpers, Breton draft horses, and Thoroughbred/Showjumper crosses.

Statistical analyses of these traits revealed that RR, HR, and RT were the most prominently differentiating characteristics under heat stress (other variables examined included the horse’s weight and various blood parameters such as packed cell volume and total plasma protein). Tree diagrams of the results indicated "clear distances" between groups, according to the report, which appeared last month in Tropical Animal Health and Production.

"It is both expensive and time consuming to keep measuring all characteristics every time a heat tolerance study is carried out, and it also stresses the animals," said head researcher Concepta McManus, DVM, PhD, professor at the Faculty of Agronomics and Veterinary Medicine at the University of Brasilia. "We ran this multivariate analysis to find out which traits were important and how useful they were. What we discovered was that traits such as blood parameters and sweating rates were not very revealing, whereas RR, HR, and RT are not only the most important characteristics but also the easiest to measure