Following is a recap of the seminar ‘Heat stress and cooling down horses with implanted thermal chips’, conducted by Dr. Joe D. Pagan of Kentucky Equine Research during Sport Science Ocala FL 2025.
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Some interesting studies have been done recently into cooling equine athletes post-exercise. Heat stress is on every horseperson’s radar now because of climate change and extended periods of high heat and humidity – and for good reason, as horses produce a lot of heat when they exercise.
During muscle contraction, 80% of the energy horses create gets expended as heat, resulting in them accumulating a lot of heat when they exercise. Horses have several means to expend that heat during exercise: through evaporation from sweating, convection as the wind rushes past them, and conduction from respiration. After exercise, sweating is mainly used for cooling, and walking and fans can expedite that process.
Understanding how much heat horses accumulate during exercise, and how they get rid of, is extremely important in deciding how to best manage performance horses.
“Up until now, making those measurements post-exercise has been pretty tough to do,” said Dr. Joe D. Pagan. “You had to use a lot of reasonably invasive instrumentation to measure muscle and core temperature, so we didn’t really have a lot of information about post-exercise cooling except under some pretty rigid experimental conditions.”
Enter the Bio-Thermo® microchip made by Merck. Originally used strictly for identification, this microchip allows you to measure temperature at the implantation site, quickly and safely.
“The problem is, if you’re going to put a chip in for identification, you’re probably going to put it in the nuchal ligament [top of the neck from the poll to the withers], but that’s not a great place to measure temperature,” noted Dr. Pagan (who was quick to add that he is not involved in any way with Merck). “There’s not that much circulation in the nuchal ligament. But some folks down at the University of Queensland did some studies where they put these chips in different muscle groups in horses and measured temperature changes that occurred both before, during, and after exercise on a treadmill.”
The microchips were placed in the gluteal and pectoral muscles about an inch under the skin, and also in the nuchal ligament, but that location was soon abandoned. The Australian researchers found a high correlation between temperature measured in the two muscle groups and the central venous temperature (temperature in the large veins near the heart).
Dr. Pagan’s team at Kentucky Equine Research thought this could be a great way to explore how horses cool down after exercise, and implanted temperature-sensing microchips in 28 Thoroughbreds located at their facilities in Ocala, Florida, and Lexington, Kentucky. The horses ranged from two-year-olds in training to older horses in their teens, providing the researchers a broad spectrum of data.
The main goal of the study was to understand how various factors like temperature, humidity, fitness, sweating ability, and cooling methods affect how horses accumulate and lose heat during and after exercise. As Dr. Pagan is also a nutritionist, the interest initially stemmed from how nutrition influences heat stress, but the study quickly expanded.
Bio-Thermo® microchips were implanted in the gluteal and pectoral muscles and temperature readings were taken using a wand scanner — the same kind used to scan pets for ID chips. In addition to muscle temperature, heart rate, respiratory rate, and rectal temperature were measured while using both a high-speed treadmill and an actual racetrack.
Horses ran at speeds up to 600 meters per minute (about 10 meters/second) with heart rates exceeding 200 bpm. Muscle temperatures often reached between 104°F and 108°F. Interestingly, muscle temperature didn’t peak at maximum heart rate; it often continued to rise during the cool-down period, showing just how long horses retain heat.
One key insight was that rectal temperature doesn’t reflect real muscle heat accurately right after exercise. It lagged by several degrees and only caught up about 15 minutes later. So, for monitoring heat, muscle temperature is far more reliable.
The KER team also discovered that the way a horse’s temperature drops post-exercise follows what’s called first-order kinetics, which means it cools in a predictable exponential pattern. This allowed them to calculate a “half-life” of cooling — how long it takes for muscle temperature to drop by half. For instance, a horse with a half-life of 30 minutes takes an hour to cool down from a 6°F increase to near-normal. In contrast, a horse with a 10-minute half-life cools down much faster.
Hosing horses with water immediately after exercise was the most effective way to lower both temperature and respiratory rate. “Hosing is a great way to cool a horse,” remarked Dr. Pagan. “That really came through loud and clear. We’ve done lots of different iterations: some were hosed immediately after exercise, then walked. Some were walked after exercise, then hosed, but basically hosing helped horses cool faster and reduced their muscle temperatures significantly more than walking alone.”
They found that sponging with lots of water could be nearly as effective as hosing. However, just hosing the neck didn’t help as much as hosing the entire body, which reduced recovery time by 11 minutes compared to neck-only hosing. Also, scraping off water after hosing didn’t have a noticeable impact on cooling effectiveness.
It wasn’t terribly surprising that individual horses cooled at different rates, but the researchers wanted to understand why. A test was used that measures how well a horse sweats. One horse in the study was anhidrotic (unable to sweat properly). It rated a 4 out of 5 on a sweat-impaired scale, accumulated more heat during exercise and had much higher respiratory rates post-exercise, peaking at 110 breaths per minute. Interestingly, when hosed, even this horse’s respiratory rate dropped quickly, matching the normal horses.
Ultimately, the goal is to build a model that combines environmental data (heat and humidity), plus factors like heart rate, respiratory rate, and fitness, to help owners predict how long their horse needs to recover post-exercise. Dr. Pagan concluded, “I also see a day down the road where the preferential place to put this microchip may not be in the neck anymore, but in the gluteal muscle for measuring both identity and temperature.”
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