Hydration & Thermoregulation


Managing hydration and thermoregulation

Managing hydration and thermoregulation in warm environments is crucial to your horse’s health and performance.

Whatever the weather, your horse needs to stay properly hydrated. In the last issue, we discussed the importance of gut fill, the negative effects of heat stress and exercise on hydration, the importance of avoiding heat strain, and strategies for maintaining hydration status. Now let’s look at recovering hydration status and cooling your horse to prevent over-heating, selecting an electrolyte, and approaches for hydration monitoring.

Planning to Prevent Dehydration

A normal horse with low/minimal activity in cool dry conditions loses 20 to 25 liters (5.3 to 6.6 gal) of fl uid (water and electrolytes) per day. On a daily basis, the horse typically consumes this much water, and the electrolytes come from the feed. You do not need to do anything for a horse living like this, even in warm or hot conditions. The challenge occurs with training and competition.

Each hour of training results in sweat losses of water and electrolytes of ten to 20 liters, depending on the intensity of exercise and ambient heat and humidity. This is in addition to the normal 20 to 25 liters of losses occurring per day. When the duration of activity/heat stress is more than one hour a day, the horse should receive supplemental electrolytes to maintain or recover hydration.

Let’s go back to the beginning of this series and consider the horse that did not eat and drink well, and was transported for an hour to a show site. By not having a good breakfast, it is safe to say this horse did not take in at least 8 liters (2.1 gal) of water (volume of the stomach), so gut fi ll could have been down by 8 liters or more. One hour of travel in a hot trailer resulted in a fl uid loss, mostly through sweating, of 15 liters (4 gal). The defi cit at arrival is therefore 23 liters (6 gal) – a horse would have to fully empty his stomach three times in order to gain back this volume. The 15 liters of sweating losses also puts this horse in the clinically dehydrated category because it represents a body fl uid defi cit of 5%. His wellness is already compromised and mental and physical performance will be somewhat impaired. So what to do?

Arrive early. Give your horse a minimum of two hours from arrival until show time.

During this time, do everything you can you restore hydration for your horse. This can include giving him electrolyte solutions before, during and after transport.

Drinking must be encouraged after warm up and between shows and events. Use a system so you know how much your horse has consumed.

Keep your horse in a shaded area with good air circulation.

Do not cover your horse with anything – heat must be allowed to escape and not build up. A blanket should only be used when the horse is already cool and there is a risk of him losing too much heat (i.e. cold conditions). Many people make the mistake of covering a horse when he is still hot/ warm. Think of yourself – do you want to put on a jacket when you are hot and sweaty?

Some traditional thought says that allowing the horse to cool more rapidly may cause muscle cramping or colic. These are myths, refuted by scientific studies. Muscle cramping and colic are caused by dehydration, excessively slow cooling, and inadequate water/electrolyte and feed intake. It is also a myth that allowing your horse to drink when he is hot, right after exercise, will cause colic. A horse will benefit greatly from drinking early, and as much as he wants – just like you would.

Selecting an Electrolyte Supplement

The best products – of which there are very few – have an electrolyte composition that is balanced with respect to how much chloride, sodium, potassium, calcium and magnesium are lost through sweating. Sodium and chloride are required to restore fl uids outside of cells, while potassium with chloride is important for restoring fl uid inside cells. Calcium and magnesium in highly soluble forms are needed, together with sodium, potassium and chloride, to maintain normal muscle and nerve excitability. Have you ever noticed muscle cramping or thumps (synchronous diaphragmatic fl utter) in a horse? These are caused by dehydration and electrolyte imbalance.

The next challenge is getting the powder into solution so horses will drink it. Formulation and taste are very important. Flavoring agents are needed to hide the bitter taste of potassium, calcium and magnesium. Sometimes sucrose and fructose can be used, but only in suffi cient amounts to sweeten the solution. The fructose may also help the gut absorb potassium. The powder should also be 30% to 50% by weight made up of dextrose (D-glucose), which serves as an energy source for intestinal cells and helps take up sodium and water more rapidly. It also helps the solution taste better.

Paste or Powder?

A powdered supplement is recommended, dissolved in water according to label directions. This way, you know how much your horse is getting. When horses are not trained to drink, electrolytes can be injected into the back of the mouth by syringe, using either pastes or slurries. Some of this (sometimes a lot) ends up on the ground. Also, pastes and slurries are very concentrated and unless adequate water is consumed very soon, they can cause other problems. It is extremely diffiult to give enough electrolytes by paste or slurry to replace what is going to be lost, or has already been lost, through sweating.

Some electrolytes balanced with water are better than none at all. Ideally, you want to have enough water and electrolytes in the gut to replace what will be lost through sweating over the next hour of activity. This can, and has, been achieved by many experienced riders and crews. It takes training, time and patience – and the payback is huge. Is it possible to give too many electrolytes? Yes, but only when given as pastes or slurries with not enough water. For each hour of activity, 10 to 20 liters (2.6 to 5.3 gal) of water/electrolyte solution will approach or maintain good hydration.

Monitoring Hydration

Monitoring your horse’s hydration is challenging, and this is also true for veterinarians working in fi eld conditions. Traditional approaches to assessing hydration include the skin pinch test and the capillary refill test, but both are poor indicators of hydration because a normal response often occurs in dehydrated horses.

For the skin pinch test, the skin is grasped firmly between the thumb and forefinger, firmly lifted up, then let go. The “tenting” of the skin should disappear within one second. Tenting for two seconds or more indicates dehydration.

For the capillary refill test, the upper lip is raised and the gum depressed with the soft part of the end of your finger for two seconds. The white spot should return to pink within one second; longer than two seconds indicates dehydration. It is not possible to know how dehydrated the horse is.

Thankfully, technology has brought better hydration monitoring to the horse community. A technique called bioelectrical impedance analysis is used to assess body composition (percent body fat) and hydration in people, ranging from athletes in training to neonates in intensive care wards. This technique has been adapted and validated for use in horses.

At present, there is only one commercially available horse product, the Equistat hydration monitor. To use this device, electrode straps are secured around the forelimb and hindlimb as shown below. The Equistat is connected to the electrode, and readings of impedance are detected by the device at different frequencies of very low input current. The procedure takes less than 20 seconds and the horse feels nothing except the straps. The device displays the size of the intracellular and extracellular fluid volume compartments, the TBW, and sidebar above).

Total body water (TBW) consists of two main compartments– the intracellular compartment (all the fluid inside cells) and the extracellular compartment (all the fluid outside cells, not including the gut.)

Strategies for Keeping your Horse Cool

Hopefully you are convinced that heat is not good for your are often an impediment to the horse’s natural ability to cool himself. Therefore the number one strategy is to let the heat get out of the horse, at least until he is fully cooled.

Keeping him out of sun is important for reducing radiant heat load, especially for dark-colored horses. If shading is sought indoors, there may be a tradeoff with lack of air movement – some wind or breeze is desirable to promote convective heat loss. If the horse can be situated in an area where there is a breeze to help evaporate sweat, this will provide a wonderful cooling advantage.

When the horse is hot and approaching or already in heat strain, aggressive cooling strategies are needed. Researchers from Ohio State University showed that repeated applications of ice-cold water and scraping the warmed fluid from the hair coat is a very effective way to rapidly cool hot horses. There were no muscle or cardiovascular problems and the procedure is considered very safe. When ice water is not available, ground water running out of a hose is a good choice. Ground water has a temperature of about 11°C (51.8°F) and, when coupled with repeated scraping of the water off the coat, can be used as an effective cooling strategy. Scraping the warmed water from the horse’s coat is very important because the water and hair provide an insulating barrier against the removal of heat from his body tissues.

Off to the Show!

Now you’re off to a good start. You have someone knowledgeable and trustworthy to look after your horse’s needs during his competition. He is already accustomed to drinking electrolyte solutions – large buckets are tied to the fence near the gate where they can be easily checked and refilled. The horse is fed early and has adequate time to drink. Your friend loads him onto a light-colored trailer and ensures the dehydration index as well there is hay to eat and electrolyte solution to drink along the way. The trailer door is shut and the windows and vents kept fully open – it is promising to be a warm sunny day. Trailering goes well and the horse backs out nicely. He is a bit wet with sweat and drinks more electrolyte solution. He eats hay while surveying his surroundings with curiosity. You are pleased to see him looking so good – and you know you have done everything to prepare him for the day.


 Dr. Mike Lindinger has  been studying hydration and fluid balance in horses for 20 years. He has published numerous scientific articles and book chapters on this topic. He was involved in the development of Perform’n Win Electrolyte Supplement For Horses, The Equistat Hydration Monitor, and the On To Atlanta Heat Stress Research Studies at the University of Guelph.

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