When the heat is on

We take a look at heat, dehydration and electrolyte use in horses

At this time of year, most of us are enjoying (or enduring) the hot summer weather. There have already been some days with very high temperatures in parts of New Zealand and there may well be more to come. 

It is also a very busy time of year for a lot of our competition disciplines, peak trekking time and coming up to the start of hunt season, so a lot of horses and ponies are working extra hard. 

It’s very important to be aware of the effect of high temperatures on horses and ponies, particularly when they are having to work hard. During exercise, the horse’s body generates heat. We can all relate to this feeling, as it is same with us when we exercise (even if it is unplanned exercise such as running around trying to catch an uncooperative horse or pony!). 

Of course, the harder we exercise, the hotter we become. Humans and horses are unique in the animal kingdom in that they produce large amounts of sweat over their bodies to cool down. It’s the sweat evaporating that carries the heat away from the body.

There are, however, differences between human and horse sweat. Horse sweat contains a lot more electrolytes, or minerals (eg. sodium, chloride, potassium, calcium, magnesium). Horse sweat is especially high in sodium, chloride and potassium, and the amount of these electrolytes in the horse’s sweat is high compared to the level in the blood. This means if the horse sweats a lot it is losing electrolytes quite quickly, which can cause problems as will be discussed later. 

We humans lose comparatively lower levels of electrolytes in our sweat. Most of our sweat is is simply fluid, which is why we are advised to just drink plain water to rehydrate unless we are exercising particularly long or hard. 

Interestingly, horse sweat also contains a high concentration of a protein called latherin. It is well named, as it is this latherin that produces the lather seen on horses when they build up a good sweat.

Preventing problems in the heat

To prevent problems; we can look at four main areas – the environment, exercise, water and electrolytes. 

Environment: Make sure your horse has adequate shade when outside. Sunblock (masks or cream) may be needed on some horses’ pink areas, such as around the nose. If horses are kept indoors, it should be a pleasant temperature and be well ventilated. Open doors and windows to get a light breeze and/or use fans. Big fans are great to have and can be used to cool horses down after exercise too. Remember, heat rises and so vents in the roof will let out warm air while cooler air will be drawn in from open windows and doors.

Exercise: Use the cooler times of the day to exercise your horse or pony when possible (early morning or late evening) and/or use a well-shaded area. This won’t be possible all the time or during events, but hopefully will be possible for most of the time, and will help prevent problems happening. It may take a bit more planning, but it will be worth it for both of your comfort levels and for your level of performance. 

If you have to work your horse while it is hot, keep your session short and monitor your horse carefully, making sure you give plenty of walking breaks to allow recovery if he or she is breathing rapidly or sweating excessively. Allow your horse to drink some water in these breaks if they want to, although remember the horse’s stomach isn’t very big, so allowing them to gulp down a whole bucket in one go is probably not a good idea if you still have work to do.

After exercise, make sure you have an adequate active cool-down period of walking before stopping work. Then, hose off well with cold water; you can keep hosing continually if you have a plentiful supply, or, if you have a big fan stand them in front of it to increase evaporation of the water (which as we discussed earlier is one of the main ways heat is taken from the body to cool it down). Again, allow your horse or pony to drink a moderate amount at a time, then offer again.

Hosing off well with cold water after exercise is important (image: iStock)

In a critical situation, isopropyl alcohol is used in place of water as this evaporates very quickly in front of the fan, cooling the horse down more rapidly. This should not be necessary in a usual situation, and is very drying on the skin, so is not recommended as a routine.  

Water: Ensure your horse always has access to water. At home, in the paddock or stable, ensure the water is clean and fresh and make sure some horses aren’t been kept away from the trough or bucket by other horses. 

Away from home, some horses are much more selective than others and will only drink water if it tastes like what they are used to. Some will absolutely refuse to drink any ‘foreign’ water. If it is your first time away from home with a particular horse, it can be a good idea to take own water with you if feasible, or alternatively, habituate your horse to drink water at home with a small amount of flavouring added (eg. molasses, apple sauce). You don’t need to add much, but these flavours can mask the taste of different water when used when you are away.  Always offer plain water too. In the equine hospital, we once had some horses with three different buckets of water in their stable: plain water, electrolyte water and molasses water and still they wouldn’t drink until the owner brought their ‘home’ water in, which they gulped down! 

If your horse is refusing different-tasting water, you can make up a feed with the addition of a lot of water, and this will help get some fluids into them. 

The day-to-day maintenance water requirements of a 500kg horse is around 24-25 litres, although a variable amount of this will be taken in as part of their diet (there is more water in grass compared to hay, for example). Once they start exercising and sweating, horses lose more water, which needs to be replaced, and so this amount increases.

Electrolytes: Supplementation with sodium, potassium, chloride (and in some cases calcium and magnesium) can be important in heavily sweating horses. When a horse starts to get low in electrolytes it becomes fatigued, can start to show muscle weakness and the drive to drink water when it is dehydrated is actually reduced, so it can become rapidly dehydrated too. 

Electrolytes can be given to horse and ponies in several ways. 

  • They can be added to a feed using either a commercial electrolyte mix or a homemade mix (there are several recipes).
  • They can be given in drinking water.  
  • They can be given in the form of a tube of paste. 
  • For routine supplementation they can have access to salt blocks, but extra supplementation in one of the above ways may be needed after heavy work.

It is important that your horse drinks (or takes on liquid from a feed) as well as taking the electrolytes. Usually, the electrolytes will cause a thirst and make the horse want to drink (which is why bars and restaurants offer salted peanuts to encourage us humans to buy more drinks!). When choosing which to use, look for one with the electrolytes as the main ingredients rather than sugar.

Preloading a horse with extra electrolytes a few days before an event where the horse is expected to sweat a lot is not of benefit, as the excess is just passed in the urine. Ensuring that the usual diet is high in fibre (while still meeting the energy needs) will encourage a steady intake of water and electrolytes. Having salt blocks in the paddock or stable allows free choice access to electrolytes, or you could ask an equine nutritionist or your veterinarian to look at your horse’s diet and workload and advise you on a regular electrolyte supplement.

More serious problems

Serious veterinary problems that can occur due to heat, dehydration and loss of electrolytes include:

  • Exhausted horse syndrome. This is a serious combination of dehydration, loss of electrolytes, lack of energy, fatigue and shock, and needs early recognition and treatment. It is seen sometimes during endurance rides in hot environments, and needs urgent veterinary treatment
  • Synchronous diaphragmatic flutter. Again, this is most commonly seen in endurance horses performing in hot humid weather, but can be seen in other situations. It is seen as an obvious twitch in the flank at the same time as the heart beat, and is due to changes in the electrolytes in the blood, particularly low calcium levels. It is not life-threatening in itself, but changes in electrolyte levels need to be corrected as they can cause other problems.
  • Heat stress. This can occur if insufficiently-trained horses are exercised for a long time in hot humid weather or at high intensity. They become depressed and weak, and can go on to collapse and even die. Ideally, this should be prevented by stopping a horse if it doesn’t appear to be coping in the heat but, if it does happen, then rapid intensive veterinary treatment is required.
  • Anhidrosis. This occurs more in very hot humid countries, and means that the horse can no longer sweat when it needs to. Of course, this is a big problem and most horses with this condition have to move to a cooler climate or be housed in a climate-controlled stable.


Here in New Zealand, we don’t have to worry about the heat all year round but, when we do get very hot days, we need to prepare our horses to cope with it. Make sure their environment is suitable, plan to exercise in the cooler parts of the day or modify your training, provide plenty of water and good quality fibre with added electrolytes when needed, and monitor your horse as you ride. By doing this the more serious complications of heat should be avoided; however, if you do have any concerns about your horse or pony in the heat, please call your vet sooner rather than later as early treatment is always best.

  • This article was first published in the March 2019 issue of NZ Horse & Pony