Physiological responses to the endurance test of a 3-day-event during hot and cool weather.

This research investigated the physiological responses of horses during 3-day-event competitions under different weather conditions – hot and cool. It found significant differences in the heart rates and rectal temperatures of the horses under both conditions which affected their overall performance, with heat exposure resulting in higher exhaustion rates.

Study Background and Design

• The study was designed to collect physiological data during two different 3-day-event competitions – one labelled as H (hot condition), and the other as CL (cool condition).
• The ambient temperature and relative humidity during these events varied significantly: for H the temperature ranged between 2.5 to 35 degrees Celsius and humidity between 74-36%, while for CL, these were between 7.8 to 8.3 degrees Celsius and 46-41%, respectively.
• Measurements taken included rectal temperature, heart and respiratory rates at various points in the competition and recovery periods. The pauses during the Endurance Test were extended for horses competing under the hotter conditions (H), and these horses were also cooled aggressively using ice water during rests and recovery times.

Key Findings

• The heart rate was significantly higher at the end of Phase C for horses competing under hot conditions compared to their counterparts in cooler climates. Heart rates significantly decreased in both groups after 6 minutes into the rest-pause and 10 minutes after the completion of Phase D.
• Rectal temperatures were also significantly higher for horses competing at H than those at CL, at various points during the competition and recovery. Rectal temperatures decreased significantly for both groups during the rest-pause and after the finish of Phase D.
• A total of 69.6% of horses starting Phase A at H were able to complete Phase D, compared to a slightly higher completion rate in cooler climates – 82.1% of starters at CL.

Eliminations and Alterations Due to Heat Stress

• 10 horses were eliminated during the rest-pause at H due to reasons such as lameness, exertional rhabdomyolysis, and exhaustion. In comparison, two horses were eliminated at CL during the same period – one due to lameness and the other due to exertional rhabdomyolysis.
• There was significant individual variation observed in horses’ responses to heat at H. The effectiveness of the cooling measures was likely affected by the heat, causing thermal stress and contributing to higher exhaustion rates for horses unable to complete the Endurance Test.
• Aggressive cooling measures during rest-pause and recovery periods proved to be crucial for heat dissipation in horses competing under hot conditions. These measures were not necessary for horses competing in the cooler conditions at CL.