Physiological responses in nonheat acclimated horses performing treadmill exercise in cool (20 degrees C/40% RH), hot dry (30 degrees C/40% RH) and hot humid (30 degrees C/80% RH) conditions.

This research study explores how different environmental temperatures and humidity levels impact the physiological responses of non-heat acclimated horses when they engage in an exercise routine that simulates competition conditions. The findings suggested that the horses managed relatively well in both cool and hot dry conditions but exhibited significant distress, including in some cases an inability to complete the test, in hot, humid conditions.

Study Design

• The researchers carried out an exercise routine on four winter-acclimatised, non-heat acclimated horses in three different environmental conditions: cool (20 degrees C, 40% relative humidity), hot dry (30 degrees C, 40% relative humidity), and hot humid (30 degrees C, 80% relative humidity).
• The exercise test was designed to mirror a One Star Speed and Endurance test, also referred to as a competition exercise test (CET). It was divided into different phases, and performance and physiological responses were noted for each.

Findings of the Study

• All four horses successfully completed the CET in both cool and hot dry environments. Only one horse managed to complete the same test in the hot, humid conditions. The other three were unable to do so; two due to visible fatigue and one because of exceedingly high right atrial temperature (43 degrees C).
• Oxygen consumption was consistent across all phases in the cool and hot-dry settings, but was notably higher in the hot-humid environment during Phases B, C, and D.
• Peak temperatures for both the right atrium and the rectum were highest in the hot, humid condition, followed by hot-dry, and then cool. These temperature increases did not correlate significantly with the time it took to reach these peak temperatures across the different conditions.
• Heat dissipation, the process by which the body cools itself down, was most efficient in the cool condition, less so in the hot-dry condition, and least in the hot-humid setup.
• A visible correlation was observed between significant weight loss in the horses and the surface area of their bodies and body weight across all conditions.
• The overall exercise intensity, in terms of oxygen consumption (VO2), was significantly higher in both hot conditions compared to the cool setup.
• To combat exercise-induced hyperthermia and a decreased capacity for heat dissipation, the horses exhibited a boost in their minute ventilation, especially during Phase C where horses were trotting. The manner of this increase differed; frequency-centric in the hot-dry condition and volume-centric in the hot-humid one.

Major Conclusions

• The horses showed a high tolerance to heat, indicating a robust thermoregulatory capacity. Despite this, their performance did suffer, especially in hot, humid conditions.
• This suggests that while non-heat acclimated horses can withstand and adapt to varying environmental conditions, extremes (especially severe heat and humidity) may limit their performance capabilities. This is an important consideration for the health and performance of horses in competitive or high-intensity scenarios in various climates.