Horses engage in a variety of recreational and competitive pursuits, many of which require them to be quiet and nonreactive to diverse stimuliâ€”from shooting guns and carnival rides to exotic animals and unfamiliar surroundings. In an effort to quell anxiety and to induce tractability, horse owners often use nutraceutical products, even though evidence of effectiveness is in short supply.
â€œHorse owners are quite keen on behavior-modifying supplements; in fact, the question comes up a great deal when I speak to owners one-on-one,â€ said Catherine Whitehouse, M.S., a nutrition advisor for Kentucky Equine Research (KER).
â€œAlthough some owners might seek an advantage in competition, I feel the majority of requests come from people that wish to enjoy their horses, many likely young and inexperienced, without the quirks and behaviors associated with anxiety,â€ she continued.
Researchers at the University of Guelph in Ontario, Canada, set out to compare the effects of acepromazine, a common neuroleptic agent that produces marked sedation, with a magnesium-based antianxiety formulation on heart rate and cortisol levels as horses engage in everyday tasks. Six horses were randomly assigned to one of three treatment groups: (1) a single oral dose of a paste containing guar gum (a thickening agent), water, thiamine, magnesium citrate, and glycerin; (2) a single dose of acepromazine given under the tongue; and (3) a control paste consisting of water and guar gum.
On test days, horses were challenged with the following tasks starting, in order, 30 minutes after treatment: walking onto and standing on an electronic scale; loading onto a two-horse trailer; riding in a trailer for 28 km (about 25 minutes); standing for jugular venipuncture; and walking into a stall containing an unfamiliar object. Stress response with respect to peak and average heart rate, time to complete tasks, and plasma cortisol were measured.
The researchers found that all of the tasks asked of the horses caused a stress response, as observed by elevated heart rates and cortisol levels. Of the tasks presented, the highest peak and average heart rates were recorded for stepping onto and standing on the electronic scale, loading into a trailer, and riding in a trailer, indicating these were the most stressful undertakings. Acepromazine and the supplemental paste were successful at lowering the average heart rate during one task, stepping onto and standing on the scale, though subsequent tasks were not affected by either treatment.
â€œAcepromazine is a well-known sedative, but it cannot be used anytime near competition, as it is usually regarded as a prohibited substance. Additionally, there are some side effects associated with acepromazine administration, including irregular heartbeat, penile prolapse, and hypotension, so itâ€™s understandable that horse owners are looking for safe alternatives to pharmaceuticals,â€ said Whitehouse. â€œWhile this study yielded some interesting results, there is still more room for research in this field.â€
In sum, acepromazine and the supplemental paste were successful at moderating average heart rate before onset of stress.
KER has performed extensive research on heart rate variability of horses consuming different energy sources and on heart-rate tracking with specific attention to conditioning goals. KER ClockIt is a modern way to track fitness in equine athletes.
Pearson, W., and J. MacNicol. 2017. Acute effects of a single-dose nutritional product on stress response and task completion in horses. Journal of Equine Veterinary Science 51:86-91.