For a horse, dehydration can negatively affect both health and performance. Dr. David Marlin provides data on the link between hydration and forage.
Dr. David Marlin, Scientist and author
The water is really amazing
It is the only substance on Earth’s surface that exists in solid, liquid and gaseous states. About 97 percent of the water on earth is found in the oceans (326,000,000 cubic miles) and only 0.3 percent is accessible for drinking; about 1/10th of the water that is not from the sea. Pure water is sometimes referred to as a universal solvent because it dissolves more elements than any other substance. Pure water is also colorless, odorless and tasteless.
The water we drink today is the same water that existed on the planet when life began, and it is the same water that dinosaurs drank. Water is continuously recycled. We all know how important water is for survival. Without water, we can survive about 3 days, although some people have managed to last up to 10 days. We can go much longer without food; anywhere from 2 to 4 weeks. If a person, during exercise, lost 5 percent of his or her body weight in the form of sweat, he or she would be in trouble. For a horse, this amount of dehydration is well tolerated. Horses seem to be particularly resistant to dehydration, most likely because of their caudal gut that retains a lot of water.
When water is lost due to sweating, it is replaced by water taken from the caudal intestine. This is one reason why the sounds produced by the stomach are reduced in endurance horses. The downside of this is that if the intestine has less water than normal, its proper function is affected, increasing the risk of impaction colic.
Dehydration can negatively affect both health and performance, which begin to be affected with a loss of about 5 percent (25 kg in a 500 kg horse). Interestingly, a decrease in body weight of up to 3% is associated with improved performance. Dehydration can worsen respiratory diseases such as IAD, particularly in horses with chronic conditions such as equine asthma. As if this were not enough, dehydration leads to thickening of mucus and a reduction in airway clearance.
During transport on longer trips (over 10 hours), dehydration also increases the risk of “transport fever” (pneumonia). This is because the elevated head position and reduced air quality, combined with mucus thickening and immune suppression, leads to the rapid growth of pathogenic (disease-causing) bacteria that normally live in horses’ airways.
A 500 kg horse contains about 300 liters of water. Of these, 100 liters are outside the cells of the body and 200 liters are inside the cells. Of the 100 liters outside the cells, 50 liters are in the gastrointestinal tract, 40 liters are in the circulation, and the remaining 10 liters are between cells or in the lymphatic system.
Surprisingly, the lungs are composed of 90% water, the blood contains 80%, and the brain about 70%. Bones also contain 40% water! The horse incorporates water by drinking, but also through water stored in feed and forage; at the same time, water is lost in urine and feces, breath and sweat. The amount of water that can be lost in feces is often underestimated; in fact, feces are usually composed of 80-90% water. Some water is lost through the skin every day, even when the horse is not sweating. In conclusion, 45% of the water lost each day will be in feces, 30% in sweat, breath and skin, and only 25% in urine.
Over time, water intake and loss are balanced to maintain normal hydration. The body’s hydration status is monitored by detecting the concentration of sodium in the blood. The body then, to correct dehydration, stimulates drinking through thirst, while increasing urine production if there is too much water.
Several factors influence the amount of water a horse drinks each day. These include forage and feed type, protein intake, exercise, transport, stress, thermal environment, water quality, electrolyte intake, reproductive status, lactation, diarrhea, health, and behavior. Some drugs can increase water intake, including diuretics (drugs that increase urine production, e.g., furosemide) and glucocorticoids (e.g., prednisone). A normal average water intake would be about 5 percent of body weight per day – 25 liters for a 500 kg horse.
The amount of water a horse drinks is strongly influenced by the type of forage given. A horse grazing 24/7 could receive 50 liters of water from grass, because grass is low in dry matter and rich in water, and horses can thus drink very little water from buckets or troughs. The same horse eating hay or hay-silo might get only 20 liters of water, since hay has a higher dry matter content than pasture grass, and so it is natural to expect a higher water intake. Finally, if we are feeding only hay, then the horse may get only 5 liters of water per day from the hay: in this case, we would expect to find them drinking 20-30 liters of water from the trough.
Climate also has a strong impact on water consumption. Taking into account differences in diet, a horse working under light conditions in a cool climate may drink 20 liters per day, while the same horse working hard in a hot climate may reach 50 liters per day. Adding electrolytes to horses’ diets also increases water intake, and these are often used for horses are prone to impact colic to ensure they are always hydrated. Studies have shown that water consumption varies depending on how the water is presented to the horse, finding lower water consumption in small bowls than in buckets or troughs.
Temperature can affect water consumption especially in cold weather. Ponies living outside at about -5°C drink 40% less cold water at ~1-2°C than water at 19°C. However, when inside at 15-29°C, the ponies drank similar amounts of cold and warm (23°C) water (Kristula and McDonnell, 1994; McDonnell and Kristula, 1996). The answer following the exercise seems to be different. In the first 5 minutes after exercise, the horses drank an average of 10 liters of water at 10°C, 12 liters at 20°C or 10 liters at 30°C. Between 20 and 60 minutes of recovery the horses drank: 10°C – 5 liters; 20°C 8 liters; 30°C – 7 liters. During the entire hour after exercise, the horses drank the most water at 20°C (20 liters) compared to 30°C (16 liters) or 10°C 15 liters (Butudom et al. 2004).
Soaked or steamed hay?
Soaking and steam purification of hay are good ways to increase water content, help improve hygienic quality, and compensate for dehydration. Of course, steam purification has the advantage of being much faster and maintaining the nutritional quality of the hay. The amount of water absorbed will depend on the water content and maturity of the forage being soaked or steam Steamed. Earing et al. (2013) found that steam purification increased the water content of a mixed alfalfa and nettle hay from 8 percent to 23 percent (a nearly 3-fold increase). In a 2-hour feeding period, the horses also ate 4 times more Steamed hay than the same soaked hay. In contrast, soaking for 15 to 60 minutes increases the water content from 9% to about 17-21%.
It may seem that some horses may drink more than expected relative to their diet, exercise level and climate, due to a number of conditions. One of the first signs of Cushings (PPID) in horses is an increase in water intake. If you notice that your horse is drinking more than normal and if there have been no changes in workload, weather, or diet, then it would be advisable to contact your veterinarian. As in people, increased drinking and urination can be a sign of renal failure, but this is rare in horses. Finally, some horses may drink excessively due to boredom; this is referred to as psychogenic polydipsia. Horses with this condition are relatively easy to diagnose and will have a low plasma sodium concentration combined with a very pale urine (low urine specific gravity).
Key Points
- Normal healthy horses have a wide range of normal water intake.
- Water consumption is influenced by forage type, feed type, protein intake, exercise, transport, stress, thermal environment, water quality, electrolyte intake, reproductive status, lactation, diarrhea, health, medication, and behavior.
- Do not restrict water intake unless advised by your veterinarian.
- Horses can be allowed to drink up to the time of exercise and immediately after finishing exercise without any side effects.
- Steamed hay is not only a good way to improve hygienic quality and palatability, but also increases water intake.
