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 in equine thermoregulation, transport, air-conditioning and cooling in preparation for the Olympic Games of Atlanta and Beijing. He is currently consulting for the FEI on climate and event management in the lead up to the Tokyo 2020 Olympics. 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.
Fast Fact: Cold water on a hot horse does not cause constriction of blood vessels in a way that prevents the horse cooling down.
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.
Fast Fact: Hosing down with cold water (0oC to 5oC) does not cause muscle cramps or tying-up
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.
Fast Fact: When cooling your horse with water, you should NOT concentrate on large veins, arteries or muscle groups. This is much less effective than continually pouring cold water over the whole body until the horse cools down. In very hot horses, after intense exercise, this can take 10 minutes or more of continuous cooling.
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.
Fast Fact: You do NOT need to scrape off water. If the water sits there, it will evaporate and contribute to cooling the horse. Especially when dealing with a severely hyperthermic horse (very hot to touch, blowing hard and unsteady on their feet), you will cool them more effectively if you continue to apply cold water, rather than stopping to scrape.
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) read more about it here.
- 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.
Did you know? We developed cooling systems for the 2008 Beijing Olympics. They consisted of hand-held spray heads that pumped ice water from reservoirs. No scraping! The horses were sprayed continually until they started to cool down, then they were taken for a short walk. This study shows that cooling takes place effectively even when horses are not scraped. And that the skin continues to cool in between applications of cold water (blue blocks)
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.
Top Tips for Riders and Trainers:
1. Working and competing in the heat
Accept your horse will not be able to do the same amount of exercise in the heat as they would in cooler weather. Your horse will produce more adrenaline in hot weather and use up muscle energy stores (glycogen) more quickly. Dehydration also increases adrenaline, which further compounds the problem. Therefore, your horse will tire earlier. You simply can’t expect to compete as hard as you would in cooler weather.
Horses need 2-3 weeks of regular exercising in the heat to acclimatise. Beware in the first three to five days, their ability to deal with heat and exercise gets worse before they start to improve.Competing whilst not acclimatised, such as during a sudden heat wave, is a major health risk.
3. Travelling on hot days
Leave as early or as late as possible (after sunset) as it will be cooler and, generally, there will be less traffic. Horses can lose 3-5kgs per hour in warm weather, as a result becoming considerably dehydrated after a 4-5 hour journey in the heat.
4. Never withhold water
Your horse may drink considerably more in hot weather. If you rely on buckets for watering your horse (e.g. at shows) add an extra bucket. Allow your horse water right up until the time you are going to compete. You can also allow them to drink after warming-up and before competing. After exercise, they will have a strong urge to drink - let them drink! There are many myths about water and exercise. Cold water does not cause problems! Large volumes of water do distend the stomach, but that is also the mechanism by which the stomach knows to empty and allow the water through the small intestine.
In hot weather, soft tissues (muscles, tendon, ligaments, etc.) need less time to warm-up, so aim to reduce your warm-up time by 50%. When you are standing around waiting to compete, find shade to stand under, and cool your horse with water and ice. Reducing your horse’s body temperature does not counteract the physiological effects of warming-up. There is also no reason why you cannot go into competition with a horse that has been wetted. Covering the horse with water means he doesn’t have to use as much of his own sweat to cool down during the competition.
6. Cooling down
The most effective way to cool a horse is pouring cold water (15oC or less and ideally 5oC) all over the body’s surface, particularly in front of and behind the saddle if you are riding, or all over if you have finished and removed the gear. If ice is available, use it to cool down the water. It doesn’t matter how you get the water on! Buckets and hoses are best. Sponges are okay.
Remember, warm water is no good for cooling down horses. In northern parts of Australia, it is common for tap/town water to be above 20oC, and may not be as effective in cooling a hot horse.
Electrolytes Do’s and Dont’s
- DO provide the correct amount of a balanced electrolyte regularly to your horse (e.g. 50g split between two feeds daily).
- You can provide electrolytes dry in the feed, in a paste or in water. There is no difference in speed of uptake between electrolytes in water or dry electrolytes given in feed.
- If you provide electrolytes in water, give your horse the option to drink plain water as well. (e.g. offer the electrolyte water first and, if refused, offer the plain water). Aim for a ratio of 5-6g per litre of water. Studies have shown decreased acceptance when the ratio increases above this - meaning more horses will refuse to drink water with 9-10g electrolytes per litre).
- DO NOT try to suddenly load in large amounts of electrolytes (e.g. 100-200g) if you haven’t been feeding them on a regular basis. It will not replace any bodily deficits, and may cause gastrointestinal upset and feed refusal. Instead of loading electrolytes before or on the day of competition; start feeding up to 50g split between two meals per day a couple of weeks earlier.