Keeping Your Horse Hydrated | Equine Science Matters™

We all know the importance of keeping our horses hydrated, and electrolytes play a key role in that. As well as making sure horses stay well hydrated, and all of the health implications of that, the electrolyte minerals themselves also play vital roles in muscle contraction, blood pH balance and creating nerve impulses.  

When horses sweat heavily, they lose the full spectrum of electrolyte minerals. Sodium, chloride and potassium are lost in the greatest quantities, with ratios around 56% chloride, 27% sodium and 15% potassium, along with calcium and magnesium to a slightly lesser degree.  

• Chloride is a negatively charged mineral that works closely with sodium and potassium. It helps keep the body’s acid levels balanced. If chloride levels drop too low, it can affect blood pH, which can interfere with normal body functions. 

• Sodium helps control how much fluid is in the body by influencing water balance in the blood. It also plays a key role in sending electrical signals, which are needed for the brain and muscles to work properly. Sodium works together with potassium in a system that keeps sodium mostly outside cells and potassium inside them. 

• Potassium helps keep cells firm and functioning properly. It is also important for nerve signals and muscle movement.  

• Calcium is essential for muscle contractions, including the heartbeat. It also helps build strong bones, supports the nervous system, and is needed for blood clotting. 

• Magnesium is involved in many processes in the body—over 300 in total. It helps muscles relax and contract normally and supports healthy nerve function. 

A heavily sweating, exercising horse can lose 10-15 litres of sweat per hour, and as sweat contains proportionally more electrolyte minerals than blood, they can quickly become deficient in these even more quickly than just water. 

The loss of these minerals during sweating lowers the concentration, or osmolality, of the blood. Because higher sodium levels in the blood are the main trigger for thirst, this drop in osmolality can reduce the horse’s natural urge to drink. The old saying ‘you can lead a horse to water, but you can’t make it drink’ is certainly true for a horse that has been sweating heavily! However, A loss of just 2% body water may have negative effects on performance, and further losses can dramatically impair the health of the horse: 

• Lack of thermoregulation, leading to overheating 

• Muscle fatigue 

• Sporadic Exertional Rhabdomyolysis 

• Synchronous diaphragmatic flutter (thumps).  This is usually due to low blood calcium, but it can also be caused by low levels of the other electrolytes.  When an imbalance of electrolytes occurs, the electrical signal that causes the heart to contract is also passed through the phrenic nerve, which signals the diaphragm to contract (usually this makes the horse breathe).  If you look at the horse, you can see the contractions of the diaphragm with every heartbeat.  

For horses in hard work or in warm climates who sweat regularly, adding electrolytes to the feed can ensure the levels of these minerals remain at adequate levels in the blood to stimulate thirst, thereby making sure the horse is prepared in advance for the water and electrolyte mineral loses that will occur during exercise, and also help to replenish those minerals and encourage thirst over the ~24 hours post exercise. 

Adding electrolyte minerals to a horse’s water after exercise helps rehydrate the body more effectively than plain water alone. Electrolytes, especially sodium, play an important role in water absorption. Sodium is actively transported across the gut wall, and as it moves, it effectively “pulls” water with it from the gut (lumen) into the bloodstream. This allows fluid to be absorbed more quickly than if water were moving by osmosis alone, or if only small amounts of sodium were present. 

When a horse drinks large amounts of plain water, it can dilute the blood (lower its osmotic concentration). Before this water has time to move into the muscles and other dehydrated tissues, the kidneys may detect the excess fluid and remove some of it as urine. This reduces how much of that water is actually used for rehydration. 

In contrast, an electrolyte solution has a similar osmotic concentration to blood. Because of this, it is absorbed efficiently and is less likely to be rapidly excreted by the kidneys. This gives the fluid more time to move from the bloodstream into dehydrated muscles and tissues, improving overall rehydration. 

If you are taking your horse to an event where it will sweat heavily, it is important to introduce electrolyte water at home beforehand. Start with a low concentration and gradually increase it so the horse becomes accustomed to the taste. Sudden changes in flavour can reduce how willing the horse is to drink, especially when it most needs to rehydrate. If necessary, small amounts of apple juice, blackcurrant squash (such as Ribena), or molasses can be added to improve palatability.