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Summer Heat: Help Your Horse Cope and Recover

December 2017 by David Marlin, PhD,

With another hot Summer ahead in the Southern Hemisphere and last year’s record temperatures, it’s important to make sure you’re up to date on the latest, evidence-based advice on prevention of dehydration, heat stress and heat exhaustion.

Exercise physiologist and scientific consultant Dr David Marlin was involved in extensive research on equine thermoregulation, transport, air-conditioning and cooling in preparation that took place before the Olympic Games of Atlanta and Beijing. He shares his expert advice, so you can help your horse cope and recover in the heat.

In general terms, when it’s warm, it’s better to be small and, when it’s cold, it’s better to be large. It’s better to be a Polar Bear in Winter than a small Mouse, and in Summer, it’s better to be the Mouse. 

Yet, horses are a little different in that they can cope well in the Siberian Winter and the heat of the desert. They’re large, so that gives them the advantage in Winter as the ratio between skin surface area and weight is low (around 1m2 for every 100kg, compared with 1m2 for every 40kg for a human), meaning heat is lost slowly. 

When it comes to hot weather, horses should be at a disadvantage because of their size (large animals lose heat slowly), but horses have two unique adaptations that allow them to cope. 

Firstly, they can actually tolerate much higher body temperature than we can. After exercising, a rectal temperature of 41°C for a horse - whilst elevated - does not present much of a health risk, but for a human, this would be a serious cause for concern. 

The other advantage the horse has is being able to sweat faster than any other animal. A square cm of horse skin can produce sweat around three times as fast as a square cm of human skin. The only risk in relying on sweating to keep cool is that it becomes less effective the higher the humidity. Sweat cools the skin down, and, in turn, the blood flowing through it, by evaporation. 

In hot, dry air, the sweat evaporates very quickly but, as the humidity increases, the speed at which sweat evaporates becomes less effective. When the air is saturated with moisture (100% humidity), sweat does not evaporate at all. Fortunately, in the United Kingdom, we rarely, if ever, experience such conditions, but that is not the same for Australia and other parts of the world.

Sweating and dehydration

One of the risks of being able to sweat at high rates is that horses are at risk of dehydration. As a result, dehydration can increase the risk of certain health problems, such as colic and respiratory disease. If there is less water in the body, then food material in the gastrointestinal tract becomes firmer and moves more slowly through the intestines, increasing the risk of impaction colic. 

With dehydration, the mucus in the airways of the lungs becomes thicker and moves more slowly, leading to greater accumulation of allergens, and even bacteria or viruses. This may lead to inflammation or infection. And, with increased sweating, there is increased loss of electrolytes. Horse sweat contains around 11g of electrolytes per litre and is much more concentrated than human sweat. 

Over a period of weeks and months, this can lead to electrolyte depletion or imbalance (depending on what is being provided by the diet as horses cannot make electrolytes, but must get them from food) and an increased risk of problems, such as reduced performance, tying-up (exertional rhabdomyolysis) and thumps (synchronous diaphragmatic flutter); the latter is most common in endurance horses, but does occur in racehorses and eventing horses.

Getting acclimatised to heat

Long periods of warm weather present less of a health risk to horses than sudden changes. For example, a sudden increase in temperature the week before Badminton Horse Trials in the United Kingdom has caused some problems for horses on the cross-country course in past years. The event is held during Spring when its, generally, around 10°C but has been known to suddenly shoot up to 25°C. 

The reason for this is the horses are simply not used to or not ‘acclimatised’ to the heat. Horses, like people, can acclimatise to heat - either by living in a warmer climate, or living and exercising in it. The benefit of living in a warm climate is, however, perhaps only 10-20% of the benefit that comes from living and exercising in the heat. 

The process of heat acclimatisation (if a horse is suddenly taken from a cool climate to a warm one) takes place in around 2-3 weeks if exercise is carried out each day. One of the risks for horses to fail to cope with warm weather is where training is done in the Summer in the cooler parts of the day (i.e. early morning and evening), but the horse competes or races during the hottest part of the day. 

If you want to compete in the heat, then you do need to train in the heat to, at least, maintain performance and, at best, reduce the risk of any heat-related illness.

Problems with hot weather

We have already mentioned hot weather carries a risk of horses becoming dehydrated. Horses will sweat more and, of all the electrolytes, its sodium (from ordinary salt) that is likely to be limiting as forages and feeds are naturally low in sodium, but high in potassium. Providing a salt block is a good thing to do, but controlled studies show the majority of horses do not balance their sodium needs correctly from access to salt blocks alone. 

A better way is to add some table salt to the diet. As a general guide, ½-1 25ml scoop per day for horses that are not in work, 1-2 25ml scoops per day for horses in medium work and 2-3 25ml scoops per day for horses in hard work.

Water intake may increase significantly in hot weather, so it’s important to supply at least two 15 litre buckets and check them at least twice daily. If it’s very hot during the day, your horse may be better off stabled for all or the hottest part of the day, and turned out morning and evening, or overnight. However, this is only if the conditions and temperature inside the stable or shelter are better than under the shade of a tree in the horse’s pasture. 

The orientation, type of construction materials, ventilation and insulation properties of the building will have a dramatic effect on a stable or shelter’s inside temperature, and this has to be taken into account. In some cases, it can also be easier to reduce irritation from flies and other biting insects inside (e.g. spraying the wood around doors and windows with insect repellent).

Hot weather can lead to feed going off quicker than normal. This is especially true for feeds that contain oil. Heat causes oils to degrade (oxidise) more quickly. This can lead to your horse refusing to eat. Heat will also degrade the vitamin content of feeds and supplements. Another problem with hot weather is it increases how much energy horses use, even at rest to try and control their body temperature (thermoregulation), and so, horses may lose some weight in hot weather.

A horse’s capacity for exercise may also be reduced in hot weather and they may tire earlier than expected when training or competing. Large horses (e.g. dressage and show jumping horses), heavier breeds and overweight horses are at greater risk of heat-related problems in hot weather, especially if they are training or competing. Hot weather and calm days are also often associated with a decrease in air quality and an increase in levels of pollutants. This can present a challenge to horses with chronic respiratory disease, particularly recurrent airway obstruction (RAO) now known as equine asthma.

Horses with pink areas of skin, especially on the face, may be prone to sunburn, so use a good factor 50 SPF sunblock and/or a flymask to reduce the risk of sunburn. Remember, anything black absorbs more heat and heats up more than anything white.

What to look out for when training or competing in hot weather

Signs your horse may be suffering from the heat include:

  • Lethargy and being unsteady, especially when pulling up after exercise (ataxia). 
  • Blowing (deep and moderately fast breathing) excessively for a prolonged time after exercise. 
  • Panting (faster shallow breathing)
  • Nostril flaring
  • Feeling very hot to touch
  • Increased rectal temperature
  • Very prominent blood vessels visible on the skin. 
  • Decreased appetite and thirst
  • Dark urine
  • Reduced urination
  • Reduced performance
  • Dark mucous membranes
  • Muscle spasms
  • Thumps (synchronous diaphragmatic flutter) 

(See the article on the Horses and People website: to learn more about thumps.)

  • Abnormal (irregular) heart rhythm
  • Slow recovery after exercise

This is often referred to as heat exhaustion but, if it’s not managed properly and quickly, it can progress to heat stroke. This may include ataxia and/or collapse.

If you are at all concerned your horse may have severe heat stroke, then it’s important you seek vetetinary advice as soon as possible.

Severe heat stroke or heat exhaustion can lead to renal failure, colic, myopathy (muscle damage), laminitis, liver failure and may be fatal if not treated promptly. 

If you think your horse may be suffering heat-related illness, move them into the shade and start to cool them by pouring large amounts of water all over the body. If a hose is available, then use that. If ice is available, then use that to cool the water further. Do not worry about scraping the water off, just apply more cool water. 

If your horse has developed heat exhaustion or heat stroke, you may need to cool continuously for 10-15 minutes before you start to see an effect. You are unlikely to do any harm and your horse is at much greater risk from not being cooled. If shade is available nearby and your horse is steady on their feet, move into the shade whilst continuing to cool.

How to help your horse cope during hot weather

Clipping you horse is an obvious step. Keeping your horse sheltered in a cool stable during the hottest part of the day and turning out overnight may be an option if your stables are well insulated and ventilated. 

If you will not be competing or racing in the heat, then riding early morning or late evening will reduce the risks of heat-related illness. 

If you are transporting your horse, leaving very early or very late not only avoids the heat of the day, but also the traffic. Whilst you are moving, the ventilation will be better; the last thing you want is to be stuck in traffic on a hot day. 

When training or competing, offer water immediately after exercising as this is the time when a horse’s thirst is strongest. Try to avoid ice cold water, but don’t restrict intake. It does not cause colic in healthy horses. If you are competing, then leave water in the stable right up until the time you are going to tack-up. If you have warmed-up, then there is no harm in washing your horse down and allowing them to drink before competing. 

Feeding electrolytes daily will help keep your horse hydrated and reduce the risk of tying-up, colic and respiratory disease. If you have to compete in the heat of the day, then train at least 3-4 days a week in the heat. Remember, even if your horse is ‘acclimatised’ to the heat, they will not be able to perform at the same level as in cooler weather.  


Hot weather can present a challenge to horses, especially if they are competing, are old or overweight, or have existing health problems. Sensible management in hot weather can help reduce the risk of heat-related problems. 

Horses can acclimatise to perform in the heat, but only if they are exercised in the heat. When acclimatised, horses will be at less risk of heat-related illness, but exercise capacity will still be reduced, compared with capacity for exercise in cooler weather. 

Learning to identify the signs of heat-related illness, knowing how to cool horses effectively and when to call for veterinary help can save lives.