Equestrian competitions often take place in venues far from a horse’s home field, especially at elite levels. It is not uncommon for weather to pose unique challenges when it comes to conditioning horses in those environments. Olympic-level exercise, especially for eventing horses, in extreme heat and humidity has the potential to cause metabolic distress and heat exhaustion. The same challenges exist for all equestrian pursuits engaged in speed and/or distance. Exercise in ambient temperatures of 86-93 degrees Fahrenheit and relative humidity of 75-85% has significant effects on a horse’s ability to dissipate its thermal load, with serious impacts on performance. This not only affects horse health and welfare but, when horses are seen in distress due to competition, it also impacts public scrutiny of equestrians’ social license to operate. As such, it’s important that researchers continue studying ways to keep equine athletes safe from heat stress in hot climates.
Hot Weather Acclimatization for Horses
Acclimatization can take two to three weeks to elicit physiological adaptations that improve a horse’s thermoregulatory capacity and heat tolerance. In some cases, competitors opt to move their horses to the venue’s location for several weeks prior to competition to enable adaptations to high heat and humidity. This comes with extended stabling costs and training logistics. An alternative method is to simulate high heat and humidity conditions at home in the weeks prior to the event.
Researchers at Utrecht University’s Department of Veterinary Medicine evaluated the effects of acclimatization preparations by using a heated indoor arena to simulate high heat and humidity conditions expected at the 2021 Tokyo Olympic Games. This acclimatization period occurred three weeks before the Tokyo competition.
The study involved four Olympic-qualified warmbloods (three eventers and one paradressage) aged 11-15 years old that were trained for an hour a day in a heated indoor arena (~90 F, or 32 C, with 50-60% humidity) for 14 days. Other training efforts were conducted outdoors to condition at higher speeds or on hills. Following outdoor training, the horses were then exercised in the heated indoor arena. Standardized exercise tests compared each horse’s clinical variables before and after the training period. The researchers assessed heart rate, plasma lactate concentration, and deep rectal temperature. Attempts were made to measure sweat loss and composition and plasma volume but were not readily obtained in all cases.
Sixty minutes daily under the high heat and humidity conditions was sufficient to elicit beneficial physiological adaptations. The results indicate that after the acclimatization period, heart rate, rectal temperature, and lactic acid concentration all decreased. Lower levels of lactic acid might be associated with reduced workload on heart and muscles, thereby contributing to improved performance. In three of the four horses, sodium and chloride concentrations in sweat decreased. Sweat losses decreased by 42-45% in those three horses—from 3.8% preacclimatization to 2.1% following.
It also took longer for the acclimatized horses to reach a peak rectal temperature, indicating increased internal heat storage. While rectal temperature was lower during exercise, it was higher during recovery compared to readings prior to acclimatization. Two horses had a higher peak rectal temperature with no associated signs of fatigue, indicating they had developed improved heat tolerance or heat transfer. The authors pointed out that “greater heat storage after acclimatization may be evidence that a horse is functioning near its limits of adaptation.”
There were no changes in serum amyloid A (SAA) measurements during the acclimatization period, indicating this protocol did not elicit an inflammatory response.
These horses had various body dimensions, genetics, and fitness levels and worked under different loads. With this in mind, it is important to tailor acclimatization strategies to each individual, acknowledging that some horses need longer acclimatization periods than others.
All four horses in this study successfully completed their Olympic events with “exemplary performance.” The authors added that the riders were subjected to the same heat and humidity conditions during training and likely benefited with their own physiological adaptations.
Reference
Munsters C, Siegers E, Sloet van Oldruitenborgh-Oosterbaan M. Effect of a 14-Day Period of Heat Acclimation on Horses Using Heated Indoor Arenas in Preparation for Tokyo Olympic Games. Animals 2024, 14, 546. https://doi.org/10.3390/ani14040546
Cooling Strategies for Athletic Horses to Prevent Heat Stress
Athletic horses are subject to overheating during arduous exercise in hot and humid climates. Endurance horses compete for 12-24 continuous hours (with mandatory short rest breaks) depending on the ride length (50-100 miles). Applying cool or cold water helps mitigate heat stress or exhaustion by heat dissipation through evaporative cooling to conduct heat away from the body surface. Thai researchers examined whether adding salt to the water can improve cooling and heart rate recovery compared to using plain water.
The researchers used heart rate variability (HRV) to measure the efficiency of water cooling by quantifying skin surface cooling and heating and autonomic nervous system responses to cold-water immersion. They pointed out that HRV “is a measure of fluctuations in the interval between heartbeats, where the variation depends on the rhythmic oscillation of the autonomic nervous system acting on the sinoatrial node of the heart during the cardiac cycle.”
In the study, 12 healthy Arabian horses qualified for endurance competition were evaluated over a 27-mile novice endurance race. The ambient temperature on the study day was about 80 F with 63% humidity. The researchers used two types of water for cooling, prepared about 30 minutes before the anticipated finish time of the competition:
- Ice blocks in 200 liters of tap water to result in equivalent volumes of ice and water.
- 10 killigrams of table salt in 200 liters of water to create 5% weight by volume saline solution before adding ice. This produces a saline water concentration of 2.5-5%.
The horses were divided into two groups of six each—one group was assigned to receive nonsaline cooling and the other received saline cooling. After crossing the finish line, horses were repeatedly hosed with the prescribed water source, with the water scraped away as more was applied over nine minutes.
Adding salt to the water lowers its freezing point. Adding ice to salt water “causes it to melt continuously by heat absorption, leaving very cold salt water.” Water remained above 32 F to abide by FEI rules for water cooling. The cold-saline water temperature was lower than the cold-plain water: 11 C vs. 18 C ( 52 F vs 64 F).
In summary, the authors said cooling with cold water is effective at reducing heart rates and improving recovery. The saline water yields a faster decrease in heart rate and increase in parasympathetic nervous system responses by five minutes of cooling compared to plain cold water.
Reference
Wonghanchao T, Sanigavatee K, Poochipakorn C, et al. Impact of different cooling solutions on autonomic modulation in horses in a novice endurance ride. Animal 2024; DOI: 10.1016/j.animal.2024.101114
Related Reading
- Disease Du Jour: Heat Stress and Stroke in Horses
- Transport Effects on Endurance Horses
- Pre-Loading with Electrolytes
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