Acute Physiological Stress Response of Horses to Different Potential Short-Term Stressors

In order to avoid stress in horses, it has to be known to what extent the animals perceive a challenge as stressful. In this study, salivary cortisol, heart rate, and heart rate variability parameters SDRR (standard deviation of the beat-to-beat interval) and RMSSD (root mean square of successive beat-to-beat differences) were determined in Shetland ponies (six stallions, five mares) in response to a flashlight, exercise without a rider, road transport, and a nontreatment control. Saliva was collected from 1 hour before to 24 hours after the tests, and cardiac activity was recorded from 1 hour before to 2 hours after tests. Salivary cortisol concentration increased in response to transport (P < .001) and remained unchanged in response to exercise, flashlight, and no treatment (P < .001 among tests). Heart rate increased during exercise (150 ± 7 beats/min), followed by transport (99 ± 12 beats/min) and remained unchanged in response to flashlight exposure and no treatment (over time P < .001, among tests P < .001). The SDRR decreased during exercise (P < .01 over time) but not flashlight and control treatment (P < .001 among tests). Changes in RMSSD were similar (P < .001) except for a lack of changes in response to the flashlight. The SDRR differed between mares and stallions (P < .01). In conclusion, horses were not stressed by exposure to the flashlight and exercise without a rider, whereas road transport was perceived as stressful. The response did not differ markedly between stallions and mares.