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Photo courtesy of Kentucky Equine Research
Photo courtesy of Kentucky Equine Research

By Kentucky Equine Research Staff

Many people are familiar with melatonin as a sleep aid, but horse owners also may be aware of its role in mare cyclicity and reproduction. Few of us, however, know that melatonin, a neurotransmitter, may affect a horse’s temperament.

In the past, melatonin has been related to aggression in some species. For example, studies show that Syrian hamsters with high melatonin levels demonstrated more aggressive behavior than those with lower melatonin levels. One human study also found that participants administered melatonin exhibited more aggression in specific situations than those receiving a placebo.

To evaluate the effect of melatonin on temperament in horses, Korean researchers recruited 32 horses, all residing in a single riding facility in Korea for a minimum of five years.* The horses were put through a series of tests by three different handlers to determine the following attributes and to assign a score from 0 to 5 with 0-1 being low, 2-3 medium, and 4-5 high:

  • Docility—the time it takes to catch the horse in an unfamiliar situation;
  • Affinity—the time it takes to become familiar with strangers;
  • Dominance—the act of gaining the upper hand with other horses over feed; and
  • Trainability—the time it takes to achieve a training goal.

The overall temperament was an average of the four scores, and those scores were compared to melatonin levels collected between 10:00 p.m. and 11:00 p.m.

No significant relationship between melatonin and any of the four temperament measures was identified, suggesting that melatonin cannot be used as a biomarker for horse temperament.

“However, melatonin levels were significantly lower in cold-blooded horses than in Thoroughbreds, which are considered hot-blooded. Those horses actively in a training program had higher melatonin levels than fully trained horses used for competition and riding lessons,” explained Catherine Whitehouse, M.S., a Kentucky Equine Research nutrition advisor.

Having a test available to potentially identify “safe” horses would be valuable to the equine industry, especially when purchasing horses for inexperienced riders or using horses as therapy partners. This could be used in combination with an evaluation from an experienced horse handler.

In addition to regular handling and training, diet and management strategies can help improve behavior-related issues in horses.

“Consider increasing turnout time with herdmates, maximizing forage availability, and supplying a concentrate feed that matches the horse’s energy needs. Some horses that tend to be hyperreactive may benefit from a feed higher in fiber and fat and lower in nonstructural carbohydrates when additional calories are needed beyond high-quality forage and a ration balancer,” advised Whitehouse.

Reprinted courtesy of Kentucky Equine Research. Kentucky Equine Research is an international equine nutrition, research, and consultation company serving horse owners and the feed industry. Our goals are to advance the industry's knowledge of equine nutrition and exercise physiology, apply that knowledge to produce healthier, more athletic horses, and support the nutritional care of all horses throughout their lives. Learn more at

There a more informative articles in our section on Health & Education.

*Song, Y., J. Kim, Y. Park, and M. Yoon. 2023. Association between the plasma concentration of melatonin and behavioral temperament in horses. Journal of Animal Science and Technology 65(5):1094-1104.

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