Dehydration is a serious problem in competing and working horses, especially during hot weather. Horses are very sweaty animals and can quickly become dehydrated.

Horses competing in endurance racing or engaging in sustained heavy exercise are at the highest risk of dehydration.

Dehydration occurs when your horse loses excessive fluids from sweat, urine, feces, and respiration. If your horse does not drink enough to offset the fluid loss, she can become dehydrated.

The most common signs of dehydration are poor performance, loss of skin elasticity, weakness and increased respiratory rate. Dehydration also increases the risk of heat stroke, tying-up and slow exercise recovery.

Knowing the signs of dehydration and how to help keep your horse hydrated is important for every horse owner.

Dehydration in Horses

Horses become dehydrated when fluid loss is not offset by adequate water consumption. Water is one of the most important nutrients for the horse and makes up 61 – 72% of a mature horse’s body weight.

To maintain appropriate hydration status, the average non-working horse must drink at least 6.6 gallons (25 litres) of water daily. [1]

If your horse is exercising or in a hot climate, the amount of water they need to drink increases. [1]

Exercising horses can lose +10 litres of moisture per hour through sweating. Even in moderate environmental conditions, dehydration can occur after 3 hours of exercise. [3]

Field studies of endurance horses show that fluid losses are greatest by mid-ride. Horses can lose 20-25 litres total after an 80-kilometre ride. [4]

During heavy, prolonged exercise on a hot day, a horse’s water needs can increase by 300%. [2] Offer your horse water multiple times during long, intense workouts or when a horse is being exercised during hot weather.

Illness

Illness or fever can also cause dehydration if a horse isn’t drinking sufficiently.

Additionally, horses with severe and acute gastrointestinal infection are at risk for dehydration. Severe diarrhea caused by colitis or copious gastric reflux caused by enteritis can result in life-threatening fluid losses.

A horse with chronic loose manure can have four times greater fecal output than a healthy horse. To compensate for increased fluid losses, they would need to double water intake. [1]

Electrolyte Imbalance

Sweating also causes the loss of electrolytes including sodium, potassium, and chloride. [14]

These electrolyte minerals are needed to maintain proper fluid balance, nerve and muscle function, as well as acid/base balance in the horse’s body.

Low electrolyte levels can actually suppress your horse’s thirst response and worsen dehydration, especially with continued exercise and sweat loss. [17]

Feed your horse 1 – 2 ounces of salt every day to promote water intake, and use an electrolyte supplement in hot weather or after heavy work.

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Top 10 Signs of Dehydration in Horses

Every person who cares for or owns a horse should be able to recognize the early warning signs of dehydration.

Early detection can help you ensure you can rehydrate your horse quickly and avoid heat stroke or other complications.

1) Loss of Skin Elasticity

The skin pinch test can show loss of skin elasticity. Though not completely foolproof, it is one of the most common ways to check for dehydration in horses.

To perform this test, use your thumb and forefinger to gently pinch the skin of your horse’s neck, just above the point of the shoulder.

In normal circumstances, the skin should snap back quickly (1-2 seconds), but dehydrated skin will form a wrinkle or tent and take longer to disappear.

Older horses may have a longer-lasting skin compared to younger horses. The anatomical position of the skin pinch test as well as coat moisture can also affect the duration of tenting.

To gauge results, it’s important to know what is normal for your horse. Practice the pinch test several times on different days to set a baseline for your horse. [5]

2) Slow Capillary Refill Time

Dehydration can also be determined through the gums and capillary refill time (CRT). Lift your horse’s upper lip, and press your thumb on the upper gum for one to two seconds to blanch the surface.

A well-hydrated horse will have pink and moist gums. The colour should return in less than 2 seconds after you press your thumb into the gum.

A dehydrated horse will have dry or tacky mucous membranes. Additionally, their CRT will be prolonged, meaning it takes longer than 2 seconds for color to return to the blanched area. [6]

A failure for colour to return quickly in the gums can also mean that your horse is going into shock. Call your veterinarian right away if this occurs.

3) Rapid Changes in Body Weight

Changes in body weight can also be used to determine fluid balance during exercise. Dehydration is commonly described based on the percentage of body weight lost. [4]

Body weight is typically monitored in endurance races as well as in pulling competitions. If horses lose too much body weight, she can be eliminated from the competition.

Dehydration greater than 15% can be fatal. This would be equivalent to a weight loss of 95 kg for a 500 kg horse. [1]

4) Weariness or Weakness

A horse that is visibly lethargic, tired, or weak may also be dehydrated. It is best to allow a tired horse to rest and offer water and electrolytes.

Other illnesses or issues can also cause weariness and/or weakness. Monitor your horse to determine whether veterinary attention is required.

5) Stiffness

Horses that are stiff during stretching or exercise may also be dehydrated. If you regularly stretch your horse and know how flexible she is, changes in flexibility can indicate dehydration.

6) Dark Urine

A dehydrated horse may either not urinate often or produce urine that is dark (brownish) and more pungent than normal.

7) High Resting Heart Rate

For most adult horses, a normal resting heart rate is 25 to 40 beats per minute. A heart rate above 60 beats per minute could indicate dehydration.

8) Increased Respiratory Rate

An increased respiratory rate that does not return to normal during a cool-down routine is another sign that your horse may be experiencing heat stress and dehydration.

9) Tying Up

Though some horses are prone to tying up due to metabolic conditions such as Recurrent Exertional Rhabdomyolsis or PSSM, other horses may only experience tying up after strenuous exercise when dehydration is present.

Signs of tying up include:

  • Sweating
  • Stiffness
  • Reluctance to move
  • Dark-colored urine
  • Muscle twitching in the flank region

10) Altered Blood Work

Certain blood tests can also reveal dehydration in your horse. Examples of laboratory tests that can indicate dehydration include packed cell volume (PCV), serum total protein (TP) and osmolality in peripheral venous blood. [3]

Assessing the Level of Dehydration

Horses often show several of the above signs when they are dehydrated. The following table shows the level of dehydration associated with results on tests of skin tenting and capillary refill time (CRT): [7]

Dehydration Skin Tent CRT Other Signs
5-7% 2-3s >2s Mucous membranes may be normal
8-10% 6-10s >2-4s Dry/tacky mucous membranes
Possible signs of early shock
10%-12% Remains tented >5s Cold extremities
Other signs of shock
12-15% Shock, death

 

Signs of shock from severe dehydration include:

  • Irregular heartbeat
  • Weak pulse
  • Poor jugular refill
  • Cold extremities
  • Weakness, incoordination

How to Treat Dehydration in Horses

Most horses have experienced dehydration at some point in their life. This problem is usually self-corrected through free choice access to clean, palatable water.

In moderately exercising horses, changes in hydration status, mucous membranes, and heart rate usually return to within or near the reference range by 30 minutes post-exercise. Other measures in blood tests such as electrolytes and minerals also return to normal in this time frame. [8]

However, some horses will not rehydrate on their own during or after exercise despite extensive fluid losses. [1] In cases such as this, veterinary intervention is needed.

If dehydration is mild to moderate, offering water and electrolytes free choice is recommended. When offering your horse electrolytes in water, always offer plain water as well as some horses may not want to drink electrolytes.

Fluid & Electrolyte Replenishment

The following recommendations for fluid and electrolyte replenishment are provided to horses performing in different endurance­ related events: [4]

  • In most instances, both fluid and electrolytes should be supple­mented.
  • Replacement of fluid and electrolytes should be based on sweat losses occurring per hour of exercise. In most cases, this is equivalent to 2 to 5 litres per hour of exercise, but can be higher (10 to 15 litres) if exercise is performed in hot and humid environments.
  • Alterna­tively, the level of dehydration can be assessed clinically by blood test and treated accordingly.
  • Isotonic electrolyte solutions should be used to promote absorption and rehydration without causing plasma fluid and electrolyte distur­bances. A rough estimate of an isotonic solution is 1 tablespoon of potassium and sodium chloride and 1 tablespoon of table salt dissolved in 4 litres of water.
  • Horses should be encouraged to drink water containing electrolytes, as this is the ideal means for rehydration. If that is not possible, the administration of fluids via a nasogastric tube or intravenously (IV) by a veterinarian is the only way to reestablish hydration.

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How to Prevent Dehydration in Horses

It’s always better to avoid dehydration in your horse than to try and resolve this condition after signs of dehydration set in.

You can help to promote hydration with some of the following strategies:

1) Provide fresh, clean water

Always provide fresh, clean water for horses being exercised heavily or for long periods during hot or humid weather. This is also important for horses being transported long distances, especially in warm weather.

Horses exercising for prolonged periods, particularly in the heat, should receive fluid at least every hour. They should not go beyond two hours without fluid supplementation.

2) Encourage water intake

There are several ways to encourage your horse to drink more water, including adding flavours to the water, adjusting water temperature and providing wet feed or hay.

Flavours can include apple juice, Gatorade, electrolytes or sugar. Your horse’s metabolic health should be taken into account when deciding on a flavour additive. For example, horses with equine metabolic syndrome that are not exercising should not be given added sugars.

In cold weather, horses will drink more if the water temperature is lukewarm (approximately 20oC / 68o) compared to cold water. [9]

3) Feed salt

Hay and grass typically do not provide enough sodium or chloride to meet the typical horse’s nutritional requirements.

Adding salt to the diet and providing access to free-choice, loose salt will ensure your horse is meeting their sodium requirement. Salt also encourages water intake by triggering a thirst response.

Horses need approximately 1 – 2 ounces of salt per day in moderate climates and up to 4 ounces (7 tablespoons) a day in hot, humid climates. It is virtually impossible to give an adult horse too much salt as they will urinate out excess amounts of sodium.

4) Choose appropriate electrolyte supplements

When using electrolyte supplements, read labels carefully and take note of the amount and ratio of sodium, chloride, and potassium provided.

Electrolyte supplements should also contain calcium and magnesium to help prevent muscle cramping and thumps. Some high-quality electrolyte supplements will also contain added nutrients to support exercise recovery, including Vitamin E and Vitamin C.

Dextrose (sugar) is a beneficial component of electrolyte supplements. It enhances the absorption of water and electrolytes from the gut and also helps replenish glycogen stores after exercise. [10]

Electrolyte supplements should be given:

  • In hot, humid weather
  • Before and after transport [13]
  • Before, during and after long competitions
  • During illness as directed by your vet

5) Modify your horse’s exercise program

Your horse’s fitness level also impacts dehydration. Well-conditioned horses have a stronger heart and a larger volume of oxygen delivered to cells in the body. They may not sweat as much as an unconditioned horse, resulting in less fluid loss.

Asking a horse to perform a task above his level of fitness, especially during warm weather, increases the chance of dehydration.

During extreme summer weather, early morning is the best time of day for exercise. Riders should also decrease the duration and level of performance during hot weather.

Horses should be cooled after exercise by hosing or sponging with cold water and should always have free access to water in stalls or pastures.

Recent research suggests the most efficient cooling method is continuous showering with tap water. [11]

6) Feed fibre sources with high water-holding capacity

Your horse’s diet can also help to prevent dehydration. The large intestine serves as a fluid reservoir, which can be used during exercise.

The amount of fluid retained in the large intestine is affected by diet composition. Giving your horse feeds or concentrates that have a high water-holding capacity can help prevent dehydration.

Beet pulp, ground flax and dried carrots are fibrous feeds with a high water-holding capacity, whereas cereal grains have a low water-holding capacity. [12]

7) Feed oil as a source of calories

The digestion of feeds generates heat in your horse’s gut. Excessive heat production from digestion increases the amount of heat that needs to be dissipated out of the body to maintain a stable core temperature.

For exercising horses, limiting the amount of heat generated through digestion helps to reduce fatigue and the risk of dehydration.

Fats are metabolized more efficiently and generate less heat during digestion compared to fibre fermentation. Fat is efficiently absorbed in the small intestine and does not require microbial fermentation. [15]

Horses can be adapted to high-fat diets of up to 8% fat. Feeding good fat sources is especially beneficial for performance horses to provide calorie-dense “cool energy” and reduce bulk/weight in the gut.

8) Limit dietary protein oversupply

Dietary protein is critical for many functions of the body, including supporting muscle development and repair.

However, excess protein cannot be stored in the body and must be broken down and excreted in urine as urea. Horses that eat too much protein need more water to make urine and are more likely to become dehydrated.

Research shows that increasing the protein content of the diet from 10 to 15% of dry matter increases water requirements by 5% to eliminate the additional urea. [16]

Exercising horses and horses in hot climates should meet their protein requirements without significantly exceeding them. Choosing grass hay over legume hay and reducing additional protein sources may be necessary.

Summary

Dehydration is a common issue for exercising horses, especially in hot climates. If not properly managed with fluid replacement and cooling, dehydration can escalate quickly to heat stress or heat stroke.

Consult with your veterinarian and equine nutritionist to optimize your horse’s management and feeding program to reduce the risk of dehydration. You can submit your horse’s diet for a free diet evaluation by our equine nutritionists.

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References

  1. Cymbaluk, N.F. Water. Equine Applied and Clinical Nutrition. 2003.
  2. NRC, 2007 Nutrient Requirements of Horses, Chapter 7: Water and Water Quality. NRC. 2007.
  3. Pritchard, J.C. et al. Validity of a behavioural measure of heat stress and a skin tent test for dehydration in working horses and donkeys. Equine Vet J. 2006. View Summary
  4. Sosa Leon, L.A. Treatment of Exercise Induced Dehydration. Vet Clin North Am Equine Pract. 1998. View Summary
  5. Pritchard, J.C. et al. Validity of indicators of dehydration in working horses: A longitudinal study of changes in skin tent duration, mucous membrane dryness and drinking behaviour. Equine Vet J. 2008. View Summary
  6. Nyman, S. et al. Water intake and fluid shifts in horses: effects of hydration status during two exercise tests. Equine Vet J. 2002. View Summary
  7. Kentucky Equine Research. Dehydration: It Happens to Horses Too. 2011.
  8. Zobba, R. et al. Physical, Hematological, and Biochemical Responses to Acute Intense Exercise in Polo Horses. J Equine Vet Sci. 2011.
  9. Kristula, M.A. and McDonnell, S.M. Drinking Water Temperature Affects Consumption of Water During Cold Weather in Ponies. Appl Anim Behav Sci. 1994.
  10. Lindinger, M.I. and Ecker, G.L. Gastric emptying, intestinal absorption of electrolytes and exercise performance in electrolyte-supplemented horses. Exp Physiol. 2013. View Summary
  11. Takahaski, Y. et al. A Comparison of Five Cooling Methods in Hot and Humid Environments in Thoroughbred Horses. J Equine Vet Sci. 2020. View Summary
  12. Humer, E. et al. Characterizing the Moisture Expansion of Common Single and Mixed Equine Feeds by Their Water-Holding Capacity and Nutrient Composition. J Equine Vet Sci. 2018.
  13. Bollinger, L. et al. Age and Hydration of Competing Horses Influence the Outcome of Elite 160 km Endurance Rides. Front Vet Sci. 2021. View Summary
  14. Hodgson, D.R. et al. Thermoregulation in the Horse in Response to Exercise. British Vet J. 1994. View Summary
  15. Kronfeld, D.S. Dietary fat affects heat production and other variables of equine performance, under hot and humid conditions. Equine Vet J. 1996.View Summary
  16. Harris, P. Feeding management of elite endurance horses. Vet Clin North Am Equine Pract. 2009. View Summary
  17. McCutcheon, L.J. et al. Equine sweating responses to submaximal exercise during 21 days of heat acclimation. J Appl Physiol. 1999 View Summary