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Over two-thirds of a horse’s body is comprised of water. Endurance horse owners are concerned about dehydration and electrolyte imbalances during competition and fight the long-held belief that you can’t make a horse drink. In contrast, Standardbred and Thoroughbred racehorses frequently have water withheld prior to competition. But how do we know if any of these horses are actually dehydrated?

Dehydration is a welfare issue for all horses competing in athletic events. Typically, a clinical examination of the horse’s gums, skin-tent response, sweat production, and some blood cell count values help veterinarians determine if a horse is dehydrated,” explained Kathleen Crandell, Ph.D., a Kentucky Equine Research (KER) nutritionist.

In a research study*, Sibthorpe and colleagues withheld water from a group of Standardbred racehorses at 5 a.m. and exercised the horses for 30 minutes at 4 p.m. Packed cell volume (PCV) and total protein of blood were measured. PCV is a measure of the percentage of red blood cells in a volume of blood. A high PCV indicates dehydration because the amount of water in blood is low. Total protein involves a similar concept. Additionally, a multi-frequency bioelectrical impedance analysis (MFBIA) was used to estimate total water in the horses’ bodies.

The researchers found that there was no change in PCV or total protein between horses that had water withheld and those who didn’t. This finding suggested that the horses were not dehydrated. Further, MFBIA did not detect a difference in total body water between the two groups of horses.

“Ensuring the safety and welfare of racehorses is a priority in the equine industry. Although this particular study did not detect dehydration after exercise when water was withheld, the researchers suggested that additional studies are needed,” noted Crandell.

Further, the researchers proposed that MFBIA could prove to be a valuable tool for assessing hydration in addition to the classic methods.

According to Crandell, the results of this study should not serve as evidence that horses do not require free-choice access to water. Instead, it is a message that horse owners don’t need to panic if there is a brief lapse in supply.

“It is also important to understand that in this study the horses were maintained in a thermoneutral environment, and they were not heat-stressed or sweating excessively. Further, they were not treated with a dehydrating medication like furosemide that could have an effect on dehydration. For recovery from dehydration due to furosemide administration or more strenuous workouts, particularly in high temperatures, supplementation with Race Recovery or Restore Paste can help achieve a normal electrolyte balance following exercise,” recommended Crandell.

*Sibthorpe, P., M.P. Coyle, L. Bayliss, et al. 2016. The physiological effects of withholding water prior to exercise in horses. Proceedings of the Australasian Equine Science Symposium. Volume 6.

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