Study shows horses are more susceptible to heat than humans

Horses feel the heat more than we do so take the time to cool them down.

I’ve always thought it ironic that humans pile their horses with blankets in the winter (although most horses are perfectly comfortable in colder weather) but aren’t concerned about heat in the summer. Freedom (and Kroni before him) always preferred cold to heat. They’d stand outside in the worst nor’easter with nose numbing temperatures yet stand inside all day to escape the summer heat.

We’ve had some hot weather here in Boston this June. Lots of days in the mid 80s with major humidity. Some mornings when I’ve gone to feed Freedom already had the “it’s too hot” look on his face. He just looks irritable. He doesn’t want to go outside on the grass pasture but prefers to stand in his stall out of the direct sun.

Now a study performed at the University of Guelph (Ontario, Canada) has confirmed that horses heat up 10 times faster than humans and are more susceptible to the negative effects of heat stress. So if we’re hot, they’re bothered.

When the Rider is Hot, the Horse is Hotter, was published on the University’s Website. Author Teresa Pittman interviews Professor Michael Lindinger, an animal and exercise physiologist about his study. Lindinger became interested on the effect of heat on horses when he was a lead researcher on the Canadian team that contributed information on the response of the horse to heat and humidity for the Atlantic Summer Olympics.

“It only takes 17 minutes of moderate intensity exercise in hot, humid weather to raise a horse’s temperature to dangerous levels. That’s three to 10 times faster than in humans. Horses feel the heat much worse than we do.”

And the effects can be serious. If a horse’s body temperature shoots up from the normal 37 to 38 C to 41 C, temperatures within working muscles may be as high as 43 C, a temperature at which proteins in muscle begin to denature (cook). Horses suffering excessive heat stress may experience hypotension, colic and renal failure.

Horses are more susceptible to heat for several reasons, explains Lindinger. First, they are larger and have a higher percentage of active muscle than people do during exercise. When muscles are being used, they produce a lot of heat.

Horses also rely to a significant extent on sweating to cool them off. They can sweat 15 to 20 litres (4 – 5 gallons) per hour in cool, dry conditions and up to 30 litres (8 gallons)per hour in hot, humid conditions, but only 25 to 30 per cent of the sweat produced is effective in cooling the horse by evaporation.

“Because so much more sweat is produced than can be evaporated, the rest just drips off the horse’s body,” says Lindinger. “By comparison, up to 50 per cent of the sweat people produce is evaporated from our bodies during exercise and helps to cool us.”

The salts in horse sweat are also four times as concentrated as in human sweat. Lindinger refers to a photograph of an area where endurance horses had been standing while their sweaty bodies were repeatedly scraped and cooled with water. As the liquids evaporated from the ground, the soil surface was left white because of the salt in the horses’ sweat.

“Those salts have to be replaced,” he says. “Just giving the horse water will not rehydrate a dehydrated horse. When horses drink plain water, it dilutes their body fluids, and their bodies respond by trying to get rid of more water and more electrolytes.”

So, how can you protect your horse from the harmful effects of excessive heat?

Keep your horse hydrated. Encourage your horse to drink by making sure cool, clean water is always available. One of the reasons I feed soaked beet pulp is that it’s a good way to add more water to my horse’s diet. I even add water now when I’m only feeding pellets. Freedom seems to like his soup!

Add electrolytes or salt to your horse’s water or feed to help replace sweat losses. If you are adding it to water, start with small amounts to get your horse used to the taste and gradually increase it. I add salt year round to Freedom’s grain and also have a salt block available.

Make sure your horse is acclimatised to the heat before you work him hard or compete. For example, if you know you’re going to be competing mid-day, make sure you spend enough time training your horse in similar conditions — don’t ride in the early mornings or evenings when it’s cooler just because you’ll be more comfortable.

Most horses like being hosed off.

After working your horse the best way to cool him down is to rinse the horse’s body repeatedly with cold water and scrape off the excess water. Don’t worry about the water being cold, it will not cause him any harm, it will simply cool him down faster!

“You can cool the horse two degrees in 10 minutes this way: pour on the water, scrape it off, pour on more, and just keep repeating it,” says Lindinger. “The scraping part is important because otherwise the water will be trapped in the horse’s hair and will quickly warm up. By scraping and pouring on fresh, cold water you keep the cooling process going.”

Offer your horses water and let them drink as much as they want. It’s an old wives’ tale that horses shouldn’t drink when they’re hot and won’t cause your horse to colic or tie up.

Let your horse drink -- it will not cause them to colic or tie up.

Keep your horse walking if he’s really hot. Walking promotes the flow of blood beneath the skin and the movement of air increases evaporation. You can even add some rubbing alcohol to improve evaporation.

I know Freedom always appreciates being hosed down after a ride. Especially if I also let him drink from the hose.

9 thoughts on “Study shows horses are more susceptible to heat than humans

  1. Brilliant post. I was taught how to cool down a horse in the humid VA summer in just this way. I will repost if you don’t mind.

  2. This is such a good point! I do think we over-rug horses in the winter. Like you, my guys have always used the walk-in stalls to stay out of the heat more than to avoid the cold. Thanks for the post, I have worried about my young gelding during a couple of very humid day lessons. He seems particularly susceptible to humidity. I’ll use your cool down ideas.

  3. Thank you so very much for posting this. I have MS and heat sickness is a major problem for me. This post has left me wondering if the horses I ride get as heat sick as I do!
    I think that horses bred for hotter climates (Arab, Barb, Mustang) tend to do a bit better in the heat, but as far as I know there is no competition level breed that descends from horses bred in hot and humid climates.
    In the hot & very humid Southeast (USA) I watered 3 times a day so they would have cool, fresh water, and I always put salt in their feed. This seemed to help. Now I don’t have horses, but I have to ride at 8 AM and it is still too humid for me, so I mostly walk and do a little trot (1/4 way around the ring at a trot and I’m done for.)
    May every horseperson in the world read this post.

  4. Thank you so much for the excellent info. I was taught to hose a horse down, not scrape, and let them “evaporate” as air-conditioning. I’m horrified I’ve been making Tiny HOTTER.

    You have earned much thanks from me, Tiny, and other horses I ride who will now cool down much more efficiently! Bless you.

  5. I agree with the general point that humans (at least, thin and fit humans) tolerate heat better than horses, however I think this article oversimplifies in important ways. In my experience, the body type and lineage of the horse, and where they spend their first few years, have huge impact on their ability to tolerate heat and cold. For example, I have a mare and a gelding that travel with me between Vermont and Florida. The mare is mostly TB: lean, thin-skinned, vascularized. We both get cold easily, and we both tolerate heat well. The gelding is a baroque cross, he is stockier and tends to be fatter. The heat is tougher on him and his aerobic capacity is intrinsically lower – it takes twice the time to get him ready to gallop an XC course as it does the mare. In fact, if we stay in Florida past June he develops anhydrosis. Yet I have seen the mare start shivering in temps as warm as 58 degrees, simply by getting wetted by a passing storm with an 8 kt breeze. In the winter, she comes over for her blanket and hood. The gelding turns away when he doesn’t want his. So it’s individual. Lastly, the early studies that concluded that horses are comfortable and energy neutral down to very cold temperatures were disproved by later studies. On average, they are comfortable at temps approximately 10 degrees colder than what we find comfortable. But, as mentioned above, that average sums up a large range.

  6. my 1 year old gelding is sweating for no reason at all. Could you put any light on to why this is happening please i have lost a 5year old to grass sickness back in august maybe i am just paranoid now but i dont know why he is sweating so much any help or suggestions would be reallt appreciated many thanks
    kellie hall

    1. Kellie, Is his winter coat growing in? Even in the 50’s my horse is sweating due to the new heavier coat. I guess I’d want to know what your temperatures and humidity levels are, and what kind of coat/weight your gelding is carrying.

      Localized sweating (sweating only in one place, or unevenly) can be a sign of pain. Does he have “high water” marks? Salt rings from previously dried sweat? Where are they located? I’d definitely take his temperature to check for illness, and do a skin dehydration test.

      It may be within the realm of normal given the time of year, but if he is sweating continually, or unevenly, that is cause for concern. Especially if he foundered (I’m not sure what grass sickness is.) In those instances, it’s wisest to call the vet asap. Good luck!

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