Physiologic responses of acclimatized or non-acclimatized mature reining horses to heat stress: I. Heart rate, respiration rate, lactate, rectal temperature, cortisol and packed cell volume

An experiment was conducted utilizing twenty mature Quarter Horses to establish physiologic responses to reining training under conditions conducive to heat stress. Ten of the horses were acclimatized to ambient conditions [30°C, 80% relative humidity (RH)] for 28 days while the other ten were acclimatized simultaneously to 20°C and 50% RH in an air-conditioned facility. On day 28 standard exercise testing (SET) was conducted in ambient conditions (30°C, 80% RH) for both groups of horses and was repeated on day 30 and day 32 of the protocol. Heart rate and plasma lactate concentration revealed that galloping circles, spinning and stopping were more taxing maneuvers for the unacclimatized horses on day 28. However, these differences were less significant on day 30 and were not observed on day 32 indicating that it took the horses approximately five days to become acclimatized to ambient conditions. Respiration rate and rectal temperature were higher in the cool-treated horses during rest and the recovery period on day 28. These differences were only seen in the early stages of recovery on day 30 and totally disappeared on day 32. Packed cell volume was lowest in the cool-treated horses on day 28 during the SET and most of the recovery period, which is likely reflective of the absence of a substantial amount of sweating on the first day of acclimatization. This difference was still present, however, less apparent on day 30 and completely absent on day 32. Plasma cortisol concentrations were significantly higher during recovery in the cool-treated horses on day 28 and day 30, but they were not different on day 32.