Influence of Ambient Temperature and Relative Humidity on Young Performance Horses

Heat stress is a concern in performance horses due to the daily exercise they receive. To determine the influence of ambient temperature and relative humidity on young performance horses, twelve Quarter Horses (2-3 yr, 444 kg) were utilized in a randomized complete block design for a 5-wk study. Horses were stratified by age, sex, and weight between exercise treatments. Exercise treatments consisted of a morning (AM; n = 6) and an afternoon (PM; n = 6) exercise bout to evaluate the effects of differing temperatures and humidity on physiological characteristics of horses. The mean ambient temperature for the AM exercise bout was 16˚C with a mean relative humidity of 81%, while the mean ambient temperature for the PM exercise bout was 29˚C with a mean relative humidity of 38%. Whole blood lactate (LAC), heart rate (HR), respiration rate (RR), rectal temperature (RT), and circumferences and temperatures of the carpal and metacarpal joints, were measured immediately prior and immediately following the standardized exercise bout on d 14, 21, 28, and 35. HR, RR, and RT were measured 30 min into the recovery period and LAC was measured 2 and 24 h into the recovery period. Circumferences and temperatures of the joints were also measured 24 h into the recovery period. Differences in parameters measured were determined using the GLM procedure of SAS and the 0 min data was used as a covariate to account for differences among horses that existed prior to exercise. RR, RT, and HR were greater (P < 0.01) in the PM group after exercise and in the recovery period compared to the AM group. Two hours into the recovery period, the PM group had a greater LAC (P < 0.05). This indicates that the horses may have had an impaired ability to dissipate heat during the recovery period due to the higher ambient temperatures. Understanding the physiological responses of horses during recovery at different ambient temperatures, may enable industry professionals to modify daily exercise regimens to allow the equine athlete to perform at their full potential and prevent injury or harm to the animal.