Written by Dr. David Marlin
Water really is amazing. It is the only substance that exists as a solid, liquid and gas at the natural temperatures on the earth’s surface. Around 97% of the water on earth is in the oceans (326,000,000 cubic miles) and only around 0.3% of the total water on earth is accessible for drinking; around 1/10th of the water than is not sea water. Pure water is sometimes referred to as the universal solvent as more things dissolve in water than anything else. Pure water is also colourless, odourless and tasteless. The water that we drink today is the same water that was on the planet when life began and is the same water that the dinosaurs drank. Water is continuously recycled.
We all know how important water is for survival. We can only survive for around 3 days without water, depending on the weather, although people have survived for up to 10 days. We can go much longer without food; anywhere from 2-4 weeks. If a person lost 5% of their bodyweight as sweat during exercise they would be in trouble. For a horse this amount of dehydration is well tolerated. Horses appear to be particularly resistant to dehydration, most likely due to their large hindgut which holds a lot of water. When water is lost from the circulation through sweating, this is replaced by water drawn from the hindgut. This is one of the reasons that gut sounds are reduced in endurance horses. The downside of this is that if the hindgut has less water than normal then its function is affected and this can increase the risk of impaction colic.
Dehydration can negatively affect both health and performance. Performance starts to be affected at a loss of around 5% (25kg in a 500kg horse). Interestingly, decreases in bodyweight of up to 3% are associated with improvements in performance. Dehydration can worsen respiratory disease such as IAD, and particularly in horses with chronic conditions such as equine asthma. Dehydration leads to thickening of mucus and poorer/slower clearance from the airways. Dehydration during transport on longer journeys (over ~10h) also increases the risk of “shipping fever” (pneumonia). This is because the combination of elevated head position, reduced air quality, thickening and therefore slower clearance of mucus and immune suppression leads to rapid growth of pathogenic (disease causing) bacteria that normally live in the horses’ airways.
A 500kg horse contains around 300 litres of water. Of this, around 100 litres is outside the cells of the body and around 200 litres is inside the cells. Of the 100 litres outside the cells, around 50 litres is in the GI tract, 40 litres is in the circulation and the remaining 10 litres is between cells or in the lymphatic system. Perhaps surprisingly the lungs are over 90% water, whilst blood is only around 80% water and the brain around 70%. Even bone contains around 40% water! The horse takes in water by drinking but also through water stored in feed and forages and at the same time water is lost in urine and faeces, breath and sweat. The amount of water that can be lost in faeces is often not appreciated as faeces are usually 80-90% water. Some water is also lost each day through the skin even when the horse is not sweating. This is because the skin is not like plastic and impermeable to water. Around 45% of the water lost each day will be in faeces, around 30% in sweat, respiratory water loss and loss through the skin and only around 25% in urine.
Over time water intake and water loss much balance to maintain normal hydration (sometimes referred to as euhydration). The hydration status of the body is monitored by sensing the concentration of sodium in the blood. The body then either stimulates drinking through thirst to correct dehydration or increases urine production if there is too much water in the body.
Many different things influence how much water a horse drinks each day including: forage type, feed type, protein intake, exercise, transport, stress, thermal environment, quality of water, electrolyte intake, reproductive state, lactation, diarrhoea, health and behaviour. Certain medications can also increase water intake, including diuretics (drugs that increase urine production e.g. furosemide) and glucocorticoids (e.g. prednisone). An average normal water intake would be around 5% of bodyweight per day – 25 litres in a 500kg horse.
The amount of water a horse drinks is very strongly influenced by the type of forage fed. So a horse at pasture 24/7 might be getting 50 litres of water from the grass as grass is low in dry matter and high in water and these horses may drink very little water from buckets or troughs. The same horse eating ad lib haylage might only get 20 litres of water as haylage has a higher dry matter content than pasture and so we would expect to see a greater intake of water. Finally, if we are feeding ad libitum hay then the same horse may only get 5 litres of water a day from the hay and in this case we would expect to find them drinking 20-30 litres of water.
Climate also has a major impact on water consumption. Allowing for differences in diet, a horse in light work in cool weather may drink 20 litres per day whereas the same horse working hard in warm weather may drink 50 litres per day. Adding electrolytes to the horses’ diet will also increase water intake and this is often used for horses that are prone to impaction colic to ensure they are always hydrated.
Some studies have also shown that water consumption varies according to how the water is presented to the horse, with water consumption being less in small bowl automatic waters compared with buckets or troughs.
Water temperature may also influence water consumption, especially in cold weather. Ponies living outside at around -5°C drank 40% less cold water at ~1-2°C than water at 19°C. But when indoors at 15-29°C ponies drank similar amounts of cold and warm (23°C) water (Kristula and McDonnell, 1994; McDonnell and Kristula,1996).
The response following exercise appears to be different. In the first 5 minutes after exercise horses drank and average of 10 litres of water at 10°C, 12 litres at 20°C or 10 litres at 30°C. Between 20 and 60 min of recovery horses drank: 10°C - 5 litres; 20°C 8 litres; 30°C – 7 litres. Over the whole hour after exercise horses drank most water at 20°C (20 litres) compared with 30°C (16 litres) or 10°C 15 litres) (Butudom et al. 2004).
Soaking and steaming hay are good ways to increase the water content and help improve hygienic quality and offset dehydration. Steaming of course has the advantage of being much quicker and maintaining nutritional quality of hay. The amount of water that will be taken up will depend upon the water content and maturity of the forage being soaked or steamed. Earing et al. (2013) found that steaming increased the water content of an alfalfa-orchard grass mixed hay (initial water content 8%) to 23% (an almost 3x increase). In a 2h feeding period the horses also ate 4x as much steamed hay compared with the same hay unsteamed. The same authors using similar hays found that soaking for 15 or 60 min increase the water content from 9% to around 17-21%.