Getting Horses Loaded! The Smart Way

Getting Horses Loaded! The Smart Way

Supplementation of fluid and electrolytes for the exercising equine.

Gayle L. Ecker and Michael I. Lindinger

Leave a horse without food, and it may survive for many days, even weeks, before the situation is critical. Leave a horse without water and death may occur in as few as 3 to 5 days. That water is important to any living creature is a ridiculous understatement; virtually every bodily process, digestion, nerve and muscle function, all require water for proper functioning. A horse requires about 20 - 30 L of water a day for normal maintenance requirements, but hard work or prolonged exposure to heat can push this requirement as high as 80 L. When horses exercise, large amounts of heat are generated by the exercising muscle, which must be dissipated through sweating or the body temperature will rise to levels which will damage the proteins in the body. The fluid loss due to sweating from an endurance ride alone could be 40 L or more, depending on the weather and the intensity of the ride. Electrolytes, or ions, are also lost in the sweat.

For the average 500 kg horse, the total amount of water in the body is about 300 L, about 60% of the total body weight. About 200 L of this water is maintained inside the cells and is called the intracellular fluid (ICF) The rest (about 100 L) is distributed throughout the rest of the body and is called the extracellular fluid (ECF) because it is outside the cells. Of the ECF, about 20 L of water is in the plasma portion of the blood. The gastrointestinal tract may hold about 6 to 10% and the rest is found in tissues between the cells and in the lymphatic system, about 8 to 10% of the total. When water is lost from the body, it is lost from the ECF, however the water will move from the ICF and the tissues to the ECF according to osmotic forces. Continued water loss from the ICF will have detrimental effects on cell function, therefore the water must be replaced.

Many riders have experienced the frustration when faced with the truism of "You can lead a horse to water, but you can't make him drink." It would seem logical that the thirst response is mediated by the total body water, however, this is not exactly the case. Under normal circumstances, water would be lost from the body and this would cause the plasma sodium (Na+) concentration to increase. It is this increase which causes the thirst response. During periods of heavy sweating, water and salt are being lost, therefore the NA+ concentration may not increase despite the high water loss. Looked at from this way, it is not surprising that horses undergoing prolonged sweating may show no interest in water even though there is clinical evidence of dehydration, including skin tenting, slowed jugular refill, and pale, dry mucous membranes. With exercise, the concurrent loss of potassium (K+) through sweating also reduces the intracellular K+ of the thirst sensitive cells, partially offsetting the lack on increased plasma NA+ concentration.

Research on animals have shown they willingly allow dehydration of about 10% plasma volume before thirst and drinking are stimulated, despite access to water. In humans, thirst is affected by many other factors such as training or psychological factors, but even so, humans may not consume enough water to compensate for the losses. The human athlete learns to drink during training in order to avoid the fluid loss that precedes the onset of thirst. Therefore, in horses too, it is advisable that they be trained to drink during the training sessions and not just after the workout. During the endurance competitions, it is better to take extra time to encourage the horse to drink in the early part of the ride, as you will have a stronger horse at the finish. Our research, and that of others, has shown that the majority of fluid and ion losses occur during the first half of the endurance ride when horses tend to exercise harder but are not yet drinking. Horses that are hot and breathing hard are not likely to drink, so good strategy for an endurance ride would be to slow down before reaching the water source, or give the horse time to catch its breath and have a drink before going on. With experience, many seasoned endurance horses begin to drink earlier in the race, as they "seem to know" that fluid losses will occur.

Monitoring body weight during an endurance ride gives a good indication of total fluid loss, with 1 kg of weight loss equaling approximately 1 L of water. This method does not take into account of weight loss due to the breakdown of stored fuels like glycogen and fats, although much of the metabolic water is bound to fat and glycogen. It does not allow correction for sweat that may be trapped in the hair, however with the summer hair coat, we have found this to amount to much less than 1% of total body weight even if the horse is dripping wet. (I have not found that weight tapes are accurate for determining weight loss due to heavy sweating. If there is a scale available, riders would be well-advised to use them to help access sweat losses due to the event.)

If the weight drops by 3% or more (approximately 15 L of fluid for the average horse), this is likely to affect performance. Weight losses of 8 to 10% (about 36 to 50 L of fluid) are considered dangerous and even life-threatening. Heat-induced injuries are not only life-threatening but can cause long lasting or even permanent performance decrements. Body weight losses of 4 to 6% were quite common for most rides, however, heat or intensity (speed or muddy terrain) would increase those losses to 6% and up to 10% for some horses at Ontario endurance rides.

As water is lost from the blood, the blood thickens or has increased viscosity which impairs tissue perfusion (blood flow carrying oxygen to the tissues). The water loss means the total blood volume is reduced, and this forces the heart to beat faster, to pump a smaller amount of blood to meet the demands of the muscles and organs. This means there is less blood available to go to the skin for sweating. In extreme cases of dehydration, sweating may stop and heat stroke is imminent.

During the research conducted on fluid and electrolyte losses, we measured plasma proteins (PP), which gives an indication of dehydration (as water is lost, the concentration increases). During endurance rides, it was not uncommon to see PP rise from a normal resting value between 62 to 70 g/L to values of 70 to 85 at the midpoint of the ride corresponding to a minimum fluid loss of 3 - 5 L. Many horses would stay the same or go higher to the end of the ride but generally, for more seasoned horses, the PP would decrease by the end of the ride as they started to drink more. We would typically see PP of 80, and as high as 91 for rides in the heat or strenuous rides in the mud (moderate to serious dehydration). It is worth noting that the PP concentration did not always correlate to the skin pinch, especially if the fluid loss occurred at a fast rate. Thin-skinned horses tended to show the dehydration more than did horses with thick skin or higher levels of fat. This underscores the importance of monitoring the mucous membranes, jugular and capillary refill time, gut sounds and heart rate recovery in the field to determine the "fit-to-continue" criteria or metabolic fitness of the horse. This monitoring system can and should be learned by horse owners in order to do your own assessment and help you decide if electrolyte supplementation is necessary (More on this later).

In the intensely exercising horse, sweat rates may reach 10 to 15 L per hour. However, sweat losses at lower rates over a prolonged duration can also result in significant fluid losses. Prolonged exposure to heat, such as hauling a horse in a trailer without adequate ventilation, in a hot barn, or while outside with little escape from the sun, can cause the horse to suffer heat stress.

Most of you know that ordinary table salt is made up of sodium (NA) and chloride (Cl) which when expressed in chemical terms is NaCl, meaning that one sodium is bound to one chloride to make up the salt. A salt that will separate, or dissociate, in water is called an electrolyte. When the salt dissociates, each part of the salt will carry a charge and the charged particle is called an ion. When an ion carries a positive charge, such as sodium (NA+), it is called a cation, whereas, if the ion carries a negative charge, such as chloride (CL), it is called an anion. The balance of positive and negative charges is essential for virtually all processes in the body, such as fluid balance and thirst response, muscle and nerve function.

The cell membrane is responsible for controlling the electrolyte or ion balance between the ECF and the ICF. The membrane is selectively permeable, which means that some things can pass in or out, others cannot. The ECF contains high amounts of NA+, CL but low amounts of K+, whereas the ICF contains high amounts of K+, but only small amounts of NA+ and CL The balance of ions and the concentration of each is controlled by regulatory mechanisms, which include hormones and ion transport pumps such as the muscle NA+/K+ pump. This pump uses energy to transport three NA+ out of the cell for every two K+ it pumps into the cell. In this way, muscle cell K+ is kept high, NA+ low and the electrical charge stays balanced for maintaining the membrane potential, an electrically negative state which allows the muscles and nerves to function.

If a horse did not sweat to lose heat when exercising the body temperature could rise as much as 2 degree F each hour, with only a short time till the body temperature was at a critical level (42 degrees C). When horses sweat during exercise, they are not only losing water but electrolytes as well. Researchers have collected samples of equine sweat and analyzed the ion content. Their studies suggest that equine sweat contains higher amounts of ions than human sweat (it is hypertonic to the blood). With each litre of sweat lost, the horse could also lose approximately 100 mmol/L of NA+, 130 mmol/L of CL, and about 30 mmil/L of K+. Calcium, magnesium and zinc are lost in small amounts.

Electrolytes and What They Do

Sodium (NA+)
The soft tissue on the body contains about 12,000 to 14,000 mmoles of NA+. Sodium plays a vital role in maintaining the fluid balance between the various compartments. Increasing the NA+ concentration on one side of the membrane causes the water to follow. For example, with sweating during exercise, plasma NA+ usually increases, which results in a flow of water from cells to the plasma. There is also an efficient system within the kidneys to retrieve much of the NA+ before it is lost in the urine, so that the loss of NA+ is reduced, which helps maintain proper volumes of water in the ECF and ICF.

When NA+ is lost from the body, this affects the balance of ions on both sides of the cell membrane and hence, water distribution is altered and nerve and muscle function is affected. In heavily sweating horses therefore, the plasma NA+ may stay the same or increase. On order to calculate total body losses of NA+ one must correct for the water loss using plasma protein (PP) concentrations or direct measures of plasma volume. A reservoir of NA+ is available in the gut, which the horse may access during times when NA+ is in short supply. As mentioned earlier, the level of NA+ also affects the thirst response.

In a typical endurance ride in Ontario, losses of ECF were 4 - 10 L and NA+ were 800 to 1000 mmoles with typical fluid/electrolyte replacement.

Potassium (K+)
The total body soft tissue store of K+ is about 275 to 300 mmoles. The highest concentration of K+ is found inside the cells and is maintained at this level by the NA+/K+ pump which regulates the NA+, exchanging NA+ for K+. The level of K+ inside and outside the muscle and nerve cell helps to regulate how "excitable" the cell is, that is how easy it is to stimulate the cell. If the muscle K+ falls, the muscles can become weak and "twitchy" and there may be reduced bowel motility (i.e., reduced gut sounds). The vasodilatation of the capillary beds in the muscle is influenced by increasing levels of ECF K+ during exercise, and low plasma levels may not favor vasodilatation, thus the muscle many not receive as high a blood flow; cramping of the muscle may result in cases of severely low plasma K+ (below 2.5 mM). Reduced levels of K+ are also associated with fatigue. Since K+ will move from cells to plasma during exercise, the loss of K+ in the sweat may not be seen as a decrease in plasma K+, and the plasma K+ concentration may be within normal values. Even correcting for the water deficit, the loss of K+ will be underestimated unless changes in muscle ICF volume and K+ concentration can be measured, but these changes are difficult to quantify under field conditions. Planned research on the treadmill under controlled conditions will compartmentalize the loss of ICF K+ versus ECF K+.

Chloride (CL)
The ECF contains about 80% of the soft tissue component of CL, or about 9,000 to 10,500 mmoles. Plasma and cellular CL is not controlled by hormones, but rather follows the NA+ movement and reabsorption As CL is lost from the body through sweat, the kidneys reabsorb bicarbonate (HCO3-) to maintain ionic balance and this contributes to the development of alkaloses in exercising endurance horses. Because the cellular reserve of CL in the body is low, decreased plasma CL concentrations usually reflect a total body loss. A typical horse may lose 800 to 1000 mmoles of CL during a race depending on the speed, temperature and intensity.

Calcium (Ca++) and Magnesium (Mg++)
Most horse owners are aware that calcium is required in the diet for growth and maintenance of the bones, but less are aware of the important role calcium plays in muscle contraction and nerve function. Calcium is present in both the active or ionic form but more than 99% of total cellular Ca++ is bound to proteins. The plasma levels of Ca++ are maintained by the interaction of several hormones. When plasma Ca++ levels decrease, these hormones cause the reabsorption of Ca++ by the kidney, absorption from the gastrointestinal tract and from the bone itself. While this response is slow, it may however, contribute to maintaining plasma Ca++ levels in long endurance races of 75 to 100 miles, but not in events of shorter duration. Typical losses in an Ontario ride have been found to be 25 to 37 mmoles. Decreases in muscle and plasma Ca++ contribute to neural and muscle problems such as cramping and thumps.

Magnesium is also lost in the sweat, and is an important mediator for cellular function. It has been quantified in the sweat but less is known about total body losses due to exercise. Mg++ was not included in the present study on fluid and electrolyte losses.

Replacing The Losses

Ion losses through sweat can generally be readily replenished by a good quality diet for sedentary horses or horses undergoing low level, or short-term work. However, horses in training (and therefore sweating) several days a week, and are regularly involved in intense or prolonged activity during competition, may become chronically deficient in electrolytes, because there may not be enough in the feed to replace those losses. Horses involved in long-term, higher intensity work may lose electrolytes at a rapid rate (10 to 15 L of sweat per hour) and the sudden decrease in electrolytes can cause the muscle problems and heat injuries mentioned earlier.

Ways in which chronic and acute losses of electrolytes and fluids may be minimized or eliminated are listed below.

1. During the week, make sure the horse has a high quality diet and add electrolytes (see recipe below) only if the horse has been working at intensities and durations causing moderate to high sweat losses. Lower losses are easily replenished by normal amounts of these ions in the feed and hay. Always make sure that the horse has access to a clean salt block. It may be advisable to break the block up into smaller pieces and place these in the grain bin. Some owners have a small container fixed to the stall to hold free-running salt. Excess electrolytes during the week will not be stored up for the event on the weekend and are not advised. Excess electrolytes will be excreted in the urine.

2. One to two hours before the event (or before a long trailer ride), it may be beneficial to give the horse a dose of electrolytes to build up a reservoir in the gut. Do not give electrolytes to a horse that is already dehydrated. a high concentration in the gut may actually pull water from the blood, increasing the dehydration of the body. Attempt to give about 4 gallons of water before or after the dose, or give the dose in the water if the horse will drink it. Feeding hay 4 -5 hours before the event may also increase the water in the gut which would be available to the horse when dehydration develops.

3. Do not use electrolyte preparations with glucose or other sugars listed as one of the first few ingredients. The high sugar will cause a surge of glucose in the bloodstream which in turn causes a release of insulin. The insulin causes an uptake of glucose from the blood and could cause the horse to be come hypoglycemic while it is exercising. It is important to have a small amount of glucose in the gut as this helps the gastric uptake of NA+ and water. It would be better to give small amounts of glucose frequently than to give large amounts at any time as this would avoid the "peaks and valleys" of blood glucose. Also, avoid high fat supplements as this may slow down the absorption of electrolytes in the gut. Fat supplements are beneficial during training as part of the diet, but should not be given during exercise or during rest breaks.

4. Supplement fluids and ions early in the endurance ride or when you know the horse will be needing electrolytes. You may increase the NA+ level and the horse may start drinking sooner as a result. If the weather is hot, don't wait until the horse shows signs of dehydration before administering electrolytes. If you wait until the horse "looks" like it needs electrolytes, it will have some level of dehydration by that time and less blood is available to the gut for normal function. Also, if an electrolyte mix is given without an adequate amount of water (about 4 gallons), the high concentration of ions may actually pull water into the gut, and this takes more water out of the blood, further dehydrating the horse.

5. For horses experiencing high sweat losses, do not use an electrolyte supplement containing bicarbonate. Generally, due to the sweat loss the horse is already alkalotic, and the bicarbonate would increase this.

6. Once the event is over, do not stop giving electrolytes. The horse may still be sweating for some time after the event to reduce body temperature. It may be more important to get the electrolytes into the horse after the event. If a horse consumes water only, this can actually "dilute" the blood, further lowering the plasma concentration. If the horse is already dehydrated and low in K+ due to sweat losses, then this condition can be worsened by feeding the horse large amounts of hay. The hay requires a lot of water for digestive enzymes and will pull water and K+ into the gut, worsening the effects of dehydration. Allow the horse to drink water with ions till satisfied before providing hay and feed, i.e., replace fluids and ions first, then provide the feed.

7. Ensure that you are helping the horse to cool itself. The blood flow is shunted away from the gut to the muscles and skin when the horse is overheated in order to assist with heat loss through sweating. Monitor the rectal temperature of the horse and the vasodilatation of the skin to ensure the horse is cooling out. Once the horse is cooled out, there will be more blood available to establish normal blood flow to the gut. The intake of electrolytes assists in the restoration of cell and plasma volume, and this aids in cooling and reduces work of the heart by keeping blood volume high.

8. Monitor the clinical parameters yourself to help determine if electrolyte solutions are necessary. Ask your veterinarian to teach you to perform skin pinch, capillary and jugular refill and mucus membrane testing as well as listening for gut sounds. These will help you to determine if the horse needs more electrolytes and fluid. If these parameters are not showing an improvement about half an hour after the horse has had fluid and electrolytes, then a second dose of electrolytes may be necessary as long as the horse has had a good drink of water.

9. Encourage higher fluid intake by making up a "slush" or sloppy mash of the feed mixed in with lots of water, and perhaps pieces of apples and carrots. You may want to experiment with different flavors such as apple or orange to see if the horse likes it.

10. A noted endurance veterinarian, Dr. Kerry Ridgway, advises that a mix of 2 parts table salt, 2 parts Lite salt (half and half NaCl and KCl) plus one part of dolomite (calcium and magnesium) given in doses of 1 to 2 tablespoons (a 35 mm film container contains 2 tablespoons) will closely approximate the sweat losses and is much cheaper than many commercial mixes.

11. Frequent small doses of supplements are preferable to one or two large doses. Make sure the horse has access to water and possibly some electrolytes at least every two hours. The electrolyte powder can be mixed up with water and some apple sauce and squirted into the horse's mouth (open the side of the lip and squirt it to the back of the mouth with the head held up) using a 60 cc syringe with the nib cut off. Or mix the electrolytes into a sloppy mash which you are using to feed the horse. If a slurry or mash high in electrolytes is used, ensure that as much water is consumed as possible in the one hour preceding and following.

12. If your horse refuses to drink even though you have given it lots of time, you might try putting a tablespoon of salt into the tongue or into the lip of the horse. The salty taste sometimes causes the horse to start drinking.

If you are new to events which might require supplementation of electrolytes, discuss the needs with your veterinarian and with other riders who have been competing successfully. Their routines will not be exactly what your horse needs, but it gives you a place to start and by trial and error, you will learn to adjust the dosages to suit the needs of your horse.

Future Studies

The next phase of the research of fluids and electrolytes will be looking at losses of electrolytes under controlled conditions in order to quantify the amounts lost at different speeds, intensities and conditions within different body compartments. This will give baseline data to support the observations collected over the last two years in the field. The treadmill research will be extended to look at the absorption of electrolyte mixes from the gut under different conditions; this research will then be applied to field conditions for further testing.

With all the recent interest in electrolytes, it may be worth emphasizing that the best electrolyte supplement in the world will only assist in preventing performance problems. It does not take the place of proper conditioning and diet, smart riding and pit-crewing, and aftercare of the horse!

The contents this article are copyrighted but may be copied,
on condition that the Equine Research Centre be
acknowledged for the use of its information.

The Equine Research Centre
University of Guelph
Guelph, ON N1G 2W1

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