July 27, 2023

Hydration In Horses

Water - How Horses Use, Lose and Replenish this Critical Component of their Diet

By Meriel Moore-Colyer

A horse’s body is made up of 60-65% water so it is not surprising that dehydration can negatively affect performance, health and well-being.

Dehydration causes a loss of physical and mental performance, brought on by the decrease in blood volume, increased use of glycogen as fuel for muscles, and impaired ability to maintain a stable internal temperature (aka “thermoregulation”).

Horses can lose up to 5% of their body weight (25 kg in a 500kg horse) without showing any clinical signs of dehydration, although a drop in performance will be evident.

Dehydration in riders is also an issue. Losses of 5% of body weight (3.5kg in a 70kg rider) cause a 30% loss in performance, so ensuring both of you are adequately hydrated is an easy way to maximize performance1.

Water in the Body

Water is essential for many physiological processes. Two thirds of body water is held inside the cells and one third is held in the blood plasma and gastrointestinal fluid. Water maintains cell rigidity and keeps the red blood cell-to-plasma ratio optimal, so nutrient and gaseous transport can take place effectively. 

Water maintains digestive function by carrying enzymes and transporting nutrients in solution. It also plays an essential role in peristalsis – the constriction and relaxation of muscles that helps digesta move through the digestive tract. This enhances the throughput and mixing of feed with microbes to maximize fermentation.

Energy metabolism produces 350-400g of metabolic water/kg dry matter eaten, which can be used for the above processes. To support all bodily functions, your 

horse still needs to consume anything from 5-15 gallons per day, depending on work level and climate.

Hydration and Electrolytes

Hydration status is controlled by hormones which respond to electrolyte levels (Sodium = Na; potassium = K; and chloride = Cl) in the plasma. A horse can lose 10g of electrolytes per litre of sweat, so when work levels are intense (fast or long), attention to water and electrolyte intake is essential. Even short bouts of exercise can drop total body levels of sodium by 8%, potassium by 3% and chloride by 20%.

Electrolytes are required for effective rehydration as they provide the necessary osmolarity (concentration) for water retention in the body in both extra cellular and intracellular compartments. If your horse will drink electrolytes in water (some will not!) then rehydrating using a hypotonic solution of electrolytes will hydrate your horse and enhance muscle glycogen repletion. This occurs because water and glycogen are intrinsically linked, with 3g of water accompanying every gram of muscle and liver glycogen stored.

During periods of work, a daily intake of electrolytes is best. I prefer to give them in the feed as they are generally more readily accepted than when given in a solution. Whichever way you give the electrolytes, always make sure there is clean fresh water available.

To protect plasma volume during acute episodes of dehydration, 67% of the water held in the cells (intracellular fluid) and in the gut (transcellular) can be lost --27% goes from the extra cellular space (between cells) and interstitial space around blood vessels and only 6% from the plasma. This allows horses to move water from one reservoir to another to maintain body functions2.

After significant sweat loss, like after an endurance ride, it can take 5-10 days to fully replace electrolytes. If you have not been feeding electrolytes daily or are particularly concerned that a certain competition, climate conditions or a long journey might have a significant negative impact on your horse, then you can administer a pre-exercise dose 4 hours before activity.

As a rough guide on a per-kg liveweight basis, horses can be given 166mg Cl, 80 mg Na and 16mg K. This can act as a guide to the amount to feed, but generally it is best to use a proprietary blend of electrolytes and follow the instructions on the pack. That ensures the correct proportions of each salt are present.

Hydration and Exercise

It is still quite common for racehorse trainers to restrict water before racing in the belief that “a belly full of water” will negatively impact performance. Others go a step further by giving horses the diuretic Lasix before racing to further reduce body weight and to prevent exercise induced pulmonary hemorrhage (EIPH) aka “bleeding.”

Use of Lasix was universal in the USA, but more recently, its use has been restricted because re-hydration after a dose of Lasix can be challenging. It can cause horses to lose the thirst response, which can necessitate veterinary intervention and aggressive re-hydration therapy.

Lasix is banned in UK and its use globally is hotly debated. The performance limiting effect of dehydration can be significantly greater than the benefits gained from carrying a few less kg, particularly during more extended work.


All working horses sweat, even in winter time, so water intake should be monitored throughout the year. Moderate to hard working horses commonly lose 10 litres/hour or 10% of body weight.

As horses have sweat glands all over their bodies, they are very effective thermoregulators. In many situations, the evaporation of sweat off the coat is so efficient that you may not be aware that your horse is sweating. This is often the case in winter and during travelling.

While I generally advocate measuring things before “wading in” with a cure, in the case of dehydration, I assume that working and travelling horses are losing water and I feed a maintenance dose of electrolytes all year round. I increase this level during periods of intense heat (summer), exercise and travelling. Excess electrolytes that are not utilized are readily excreted in the feces and urine, provided the horse has free access to fresh clean water.

Maintaining Water Intake and Hydration

A good balanced diet with free access to clean fresh water is the main defence against dehydration. Feeding quality fibre helps with hydration status as the hemicellulose and pectins in the fibre hold water in the gut. This water can be mobilised for thermoregulation.

When it comes to mitigating against dehydration and encouraging reluctant drinkers to drink, some owners have tried flavoring the water. You might be one of the lucky owners whose horse likes certain tastes, but recent trials by Filmer (2020) demonstrated that horses preferred plain water to that flavored with banana or cherry.

Similarly, some owners try to increase water intake by wetting the bucket feed and soaking the hay. In a recent study by Glatter5 feeding soaked hay (69% water content), led to a significantly lower water intake of 21.6 litres per day (L/d) compared to those horses consuming steamed hay (32% water content) of 30.3 L/d or dry hay (85% DM) who drank 32.3 L/d.

So, it seems that innate physiological processes of the horse control water intake and we cannot influence this by stimulants like smell, taste or texture. Thus, the old saying of “You can lead a horse to water, but you cannot make it drink’ holds true.”

Measuring Dehydration

If you are concerned that your horse may be chronically dehydrated due to excessive sweating, diarrhea or not drinking enough, then monitoring water intake is a good idea. You will need to take account of water consumed by drinking and eating, so you need to know the dry matter content of the diet.

Work by Pritchard3 has shown that monitoring water intake in terms of number and duration of drinking bouts is the most reliable measurement of hydration status. A rough guide to how much water your horse should be drinking, ranges from 2.5 to 3.5 l/kg of diet dry matter, depending on the type of diet they are eating.

Estimating output can be more difficult. The normal medium and amounts of water excreted are: feces (51%), urine (18%) and insensible losses- sweating, respiration (31%). These proportions can alter quite a bit according to health status, environmental conditions and sweating.

Again, as a rough guide and depending on environmental conditions and work intensity, horses produce 10 to 35ml of sweat/m2 of skin/min, so the total amount of sweat your horse will lose depends on its size.

Diagnosing dehydration has traditionally been done using the pinch test (pinch an inch of skin on the neck and count how many seconds it takes to return to normal) or assessing the dryness of mucous membranes, but Pritchard3 showed that these methods were poorly correlated with plasma osmolarity.

However, other studies4 have shown that horses can lose plasma volume without showing any change in plasma osmolarity, so the current practical tests are a guide only, and not a definitive diagnostic tool.

The best way to avoid dehydration is always ensure you horse has a supply of fresh water and that electrolyte levels in the diet are adequate. The best way to do that is to have all the feed analyzed for mineral content. Be aware that hot weather, exercise and excitement can cause significant losses of fluid and electrolytes.

If you are not routinely feeding the electrolytes, then please start now. Prevention, in the case of dehydration, is an easy and cheap fix.

When thinking about your horse, don’t forget about yourself. Competition conditions in summer, with the nerves and full regalia (inclusive of black jacket!) can significantly increase your susceptibility to dehydration, so buy some electrolyte drinks and keep them close at hand for those days.

Author Meriel Moore-Colyer is an equine science professor at the Royal Agricultural University in the UK.

Studies Referenced

1 Jeukendrup, Asker, and Michael Gleeson. (2015) “Dehydration and Its Effects on Performance.” Humankinetics. N.p., n.d. Web. 29 July

2 Sneddon, J. Van der Walt, J. Mitchell, g. (1993) Effect of dehydration on the volumes of body fluid compartments in horses. Journal of Arid Environments 24. 397-408.

3 cxvJ. C. Prichard, CC Burn, ARS Barr, HR Whay. (2008). Validity of indicators of dehydration in working horses: A longitudinal study of changes in skin tent duration, mucous membrane dryness and drinking behaviour. E.V.J 40(6) 558-564doi: 10.2746/042516408X297462

4 Coenen, M. (2005) Exercise and stress: impact on adaptive processes involving water and electrolytes. Livestock Production Science 92. 131-145

5 Maren Glatter, Mandy Bochnia, Monika Wensch-Dorendorf, Jörg Michael Greef and Annette Zeyner (2021)Feed Intake Parameters of Horses Fed Soaked or Steamed Hay and Hygienic Quality of Hay Stored following Treatment. Animals 11. 2729.


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