By Madeline Boast, MSc, PAS

Horses rely on sweat to cool themselves, and when sweating they are losing both water and electrolytes. If you have ever observed a horse sweating, you might have noticed a white residue left on their coat, which consists of electrolytes they have lost via sweat. These electrolytes must be replenished for the horse to maintain their hydration, fluid balance in the body, and nerve and muscle function.

Electrolytes are minerals that, when combined with water, will break up into electrically-charged cations and anions (positively and negatively charged ions, respectively). The analysis of equine sweat is Na + K + Ca + Mg = Cl + P + S (sodium + potassium + calcium + magnesium = chloride + phosphorus + sulfur). Horses lose chloride, sodium, and potassium in the largest quantities, with calcium, magnesium, phosphorus, and sulfur lost in smaller amounts.

Research has shown a multitude of benefits from effective electrolyte supplementation, such as delaying the onset of fatigue, preventing dehydration, and improving performance. This article will dive into the importance of optimal supplementation, and how to better support your horse’s health and performance with electrolytes.

Not All Sweat is the Same 

Equine sweat is hypertonic to plasma, meaning that the concentration of electrolytes is greater in the sweat than in the plasma. In humans, it is the opposite — our sweat is more dilute and hypotonic to plasma. This means that the electrolyte content in our sweat is less than in our plasma.

As equine sweat is more concentrated with electrolytes, horses need more electrolytes provided after their work to adequately replenish the body. In fact, it is estimated that horses can sweat up to 10-15 litres per hour! That is a lot of both water and electrolytes to replenish.

The Dangers of Dehydration 

Dehydration is serious and can be deadly for horses. Symptoms of dehydration can range from mild fatigue and weakness to more serious symptoms such as colic or tying up. When horses sweat without proper electrolyte replenishment, the sodium content of their blood will become lower, which can lead to a reduced desire to drink. 

There are numerous ways to monitor your horse’s hydration status, but most owners tend to be familiar with the skin-pinch test and capillary refill time (looking at the colour of the gums). When using the skin-pinch test, the pinched skin should retract to normal within seconds of being released. Checking the colour of the gums and capillary refill time is another way to monitor hydration status. If you are unfamiliar with these methods, ask your veterinarian for instructions.

The goal of electrolyte supplementation is to replace the electrolytes and water lost through sweating to prevent unnecessary fatigue and dehydration. Providing the horse with maintenance sodium and chloride daily and adding an electrolyte product to the program when they are sweating is critical to preventing dehydration.

Related: How Important are Electrolytes in a Horse's Winter Diet?

When to Use Electrolytes 

Before adding an electrolyte supplement to your horse’s diet, first ensure their baseline nutrient requirements are met. Many owners rely on a salt lick or block to meet their horse’s sodium and chloride requirements. Although offering horses free-choice access to a salt block is positive, it should not be relied on. The Nutrient Requirements of Horses, 2007 states requirements for both sodium and chloride. For example, a 500-kg horse in moderate exercise requires 17.8 grams of sodium and 53.3 grams of chloride daily. It is highly unlikely that any horse would be consuming that quantity from a salt block.

If free-choice salt is relied on to meet baseline requirements, loose salt is recommended. This is because horses often struggle to consume adequate amounts from a block, but when provided in a loose form they can more easily consume larger quantities. 

The general guideline is to provide the average-sized horse (500 kg) with 28 g (1 oz) of loose salt per day; this equals about two tablespoons. Then, whenever the horse is producing sweat, whether it is a hot, humid day, or a cold winter day, electrolytes should be added in addition to their maintenance salt intake.

The amount of electrolyte required to replenish losses directly relates to the amount the horse is sweating. The more they sweat, the more electrolytes are required. Demands for electrolytes are highest when the weather is hot and humid, but relying on the heat stress score is recommended when deciding on supplementation amounts.

The heat stress score (Figure 1: Heat-Stress Chart) is a combination of the relative humidity and the environmental temperature. As the score increases, it becomes more difficult for the horse to thermoregulate.

Related: Understanding Electrolyte Loss in Active Horses 

Sweat residue, which is electrolytes lost from sweat after a race. Photo: Alamy/Alan Crowhurst

Choosing an Electrolyte Supplement  

When electrolytes are provided at recommended times and in ideal amounts, research has shown that the onset of fatigue can be delayed by over 22 percent. Additionally, there is a reduction in muscle cramping, and the horse’s ability to “bounce back” from the work is improved. However, to see these incredible benefits, the electrolyte used must be well-balanced, provided in adequate amounts and at ideal times.

First, select an electrolyte supplement. When looking at the nutrients in the product, the amount of sodium and potassium combined should almost equal the amount of chloride. Note that chloride should be the highest as it is the mineral lost in the greatest quantities.

Refer to the equation of minerals lost in equine sweat at the beginning of the article. When adding an electrolyte, we are trying to replenish what is lost in the sweat; therefore, what is in the product should closely resemble what is lost in the sweat. Additionally, salt (sodium chloride) should not be more than 70 percent of the product. If the product contains more than 70 percent salt, it is not considered a performance electrolyte.

Another consideration is the quality of ingredients used in the product. Citrate, chloride, amino acid chelate, and yeast chelate minerals are preferable to carbonate and oxide-based minerals because they offer greater bioavailability.

A final consideration is to ensure that the horse likes the product. Palatability is critical. Sugar, often dextrose, is beneficial for absorption and palatability; however, it should not be the first ingredient, and if your horse has metabolic health conditions, opting for a product that uses a different flavouring, such as fenugreek, is recommended.

When selecting an electrolyte product for your horse, take an extra few minutes to compare the guaranteed analysis and ingredient list, and then try it out to be sure your horse likes it.

How to Administer 

Once you have selected a well-balanced electrolyte product, it should then be supplemented as a liquid solution for best results. This is recommended as electrolytes need to be dissolved in water prior to being absorbed into the bloodstream of the horse.

When you provide the horse with an electrolyte, they must drink the necessary amount of water to ensure it is absorbed into the body and able to do its job. If the horse does not drink adequate water afterwards, the electrolyte can actually dehydrate the horse before it helps with the process of rehydration. For example, a paste electrolyte will lie in the digestive tract of the horse until it can draw adequate water from the body to break down the electrolytes and allow for absorption. Because electrolytes need to be dissolved in solution prior to absorption it is recommended that we supplement them in solution to speed up the absorption process.

There are a few different ways to supplement an electrolyte product in a solution. It can be mixed with water and provided as a drink, or it can be mixed with water and poured over the horse’s concentrate mixture or a soaked fibre source such as hay cubes or hay pellets. The faster the electrolyte is absorbed from the stomach and intestine into the bloodstream, the faster those cations and anions can be delivered to the muscles.

If adding the electrolyte to your horse’s water source, be sure to always offer free-choice access to plain water.

Related: Heat Stress: Keeping Your Dogs Safe

When to Administer 

As previously mentioned, whenever a horse is sweating, electrolytes should be provided to assist in replenishing what has been lost in the sweat. However, there is also benefit to preloading electrolytes in some cases. If your horse is going to be trailering in warm weather, providing electrolytes one or two hours before the beginning of the trip is recommended.

Additionally, if you know that your horse is going to be working hard in a high heat index, feeding electrolytes two hours before the start of that work is beneficial in reducing their risk of dehydration.

When a horse has exercised and produced sweat, it is best to provide the electrolyte solution within 45 minutes of the cessation of work. For horses competing in endurance events or any other prolonged workload, providing electrolytes in adequate amounts of water throughout the work, such as at vet checks, is recommended when dehydration is a serious concern.

If your horse is at a multi-day competition and working at a high level on consecutive days, get them used to having electrolytes immediately after the cessation of work (within 45 minutes) to better support their ability to recover and return to performance at a high level the next day. Being precise with electrolyte supplementation can make a positive difference on exercise recovery and performance.

Amount to Administer 

The amount of electrolyte supplement necessary to replenish losses is directly related to how much the horse was sweating. Although it is not practical to measure exactly how much sweat your horse has lost after an exercise session, use their work level, average heart rate, and the heat stress score to guide your decision (see Figure 1 - Heat Stress Chart). 

Note the environmental temperature in Celcius (left side) or Fahrenheit (right side), and relative humidity (along the bottom). Find the environmental temperature, then follow along until you reach the day’s relative humidity to find the heat stress score. Chart: Worldwide BioMedEx Inc. and Equine Guelph

A horse coming off pasture on a hot and humid day and producing sweat will not require the same amount of electrolyte supplementation as an endurance horse doing 100 miles in hot weather. (see Figure 2 - Supplement Recommendations).

If you are unsure of the amount of electrolyte your horse should be provided based on their unique workload, consult a qualified equine nutritionist.

Related: Your Horse: Heat Stress and Hydration

Additional Tips 

Your horse should become accustomed to their electrolyte before travelling or attending an event. Begin introducing it at home, where stress levels are lower and the surroundings are familiar. Once you’ve selected a well-balanced electrolyte product, continue using the same one both at home and at events to maintain consistency once your horse is comfortable with it.

When introducing the electrolyte as a solution, start with a dilute solution and work up to the recommended concentration of about one ounce per one litre of water. If your horse does not like the product, do not hesitate to try a different one.

For some horses, adding the electrolyte to their daily concentrate meal or their water is not recommended because if they do not find it palatable, their water or feed intake could decrease. In these cases, an electrolyte might be offered on a free-choice basis; however, this is not the best way to ensure rehydration as it can be difficult to track the intake of the product.

Take Home Message 

Dehydration is a serious concern for all horses and can be detrimental to health and performance. Understanding how to prevent dehydration with effective electrolyte supplementation strategies is an important consideration in the management of our horses. 

Remember that choosing a well-balanced product and then administering the correct amount in solution is often the best way to both prevent dehydration and rehydrate your horse after losses through sweat. If you have questions about optimal electrolyte supplementation for your horse, consult a qualified equine nutritionist.

Related: Equine Nutrition Myth Busters

Related: How to Test Your Horse for Dehydration

Related: Thumps in Horses

More by Madeline Boast

Main Photo: Alamy/Sorge