ADehydration can be a life-threatening condition, so you are wise to want to keep it at bay. Horses sweat to keep cool and can lose large amounts of fluid as sweat. Even when working at only a moderate level, horses can lose around 5 liters (more than a gallon) of sweat per hour!
Equine sweat contains significant amounts of electrolytes, and the electrolyte concentration in sweat mirrors or is slightly more concentrated than equine blood (around 6 grams of chloride, 3.5 grams of sodium, and 1.2 grams of potassium). Therefore, in those 5 liters of fluid, about 50 grams of body salts are lost. Of course, not all horses sweat that much but no matter how much sweat is generated, chloride is lost in the greatest amounts followed by sodium and potassium. While hays and feed do contain these electrolytes, they are not present in amounts that can replace such losses. As such, in many cases, your horse will need an electrolyte supplement.
Adequate sodium keeps horses drinking the same way eating salty chips make us thirsty. If blood sodium levels drop due to losses in sweat, horses can lose their desire to drink. (Hence, after work you can lead a horse to water but you can’t make him drink.) Avoid getting to that stage through daily salt supplementation to provide maintenance sodium and chloride, as well as an electrolyte to replace sweat losses.
Giving your horse 1 tablespoon of table salt per 500 pounds of body weight each day will help ensure your horse is covering daily maintenance sodium needs. Many people reply on a salt block for this purpose. While I believe horses should always have access to salt either as a block or in loose form, many do not use blocks consistently. For this reason, I prefer to give the maintenance dose each day in a form that’s easy for me to tell whether it’s been consumed.
On days when your horse is sweating, you will need to add a well-balanced electrolyte in addition to the salt. Avoid electrolytes in which dextrose or glucose (i.e., sugar) are the first ingredient; while these will likely be highly palatable, they might not contain adequate concentrations of the necessary minerals. The electrolyte’s purpose is to replace sweat losses so it is important that whichever product you choose contains adequate amounts of chloride, sodium, and potassium. Many do contain these key electrolytes but not in great enough concentrations to have a major impact. Find an electrolyte that delivers at least 12 grams of chloride, about 6 grams of sodium, and 4 grams of potassium. Some products might also provide small amounts of calcium and magnesium, as these are also lost in sweat, but in relatively small amounts.
Feed the electrolyte in addition to the salt. Typically, the product will give several feeding levels depending on level of sweating, and you should feed according to the instructions for your horse’s body weight and work level.
If you can tell a horse is dehydrated using the skin test or capillary refill test, that means he’s already 5-6% dehydrated. I have read that for every percent dehydration you lose 4% performance ability. So, beyond health risks, even a mildly dehydrated horse will not be able to give its best performance and likely accounts for reduced performance over the length of a competition.
Horses also need water to replace muscle stores of glycogen, the storage form of carbohydrates. So, with mild dehydration, it becomes harder for them to replenish energy stores, which leads to earlier onset of fatigue and decreased performance.
Don’t forget: Do not just rely on salt and electrolytes during the competition season. Provide salt year-round, and add electrolytes anytime your horse has obviously been sweating. If you can keep your horse’s electrolyte levels up during training you will not need to increase them during competition. Keep in mind that horses can lose a considerable amount of sweat during transportation, so be sure to supplement accordingly during travel to and from events.
When it comes to dehydration, an ounce of prevention is definitely worth a pound of cure!