HORSE CARE - DEHYDRATION IN THE PERFORMANCE OF THE HORSE
Heather Smith Thomas
June 3, 1988
Published in The Chronicle of the Horse
An athlete sweats when he exerts. Working
muscles produce heat as one of the by-products of energy production. Sweating
is one way to “keep the engine cool” and make sure the horse’s temperature
stays within healthy limits. In order to restore fluid lost through sweating,
the horse must increase his water intake to combat hydration – loss of fluids
Symptoms of dehydration appear when the loss
becomes great enough to interfere with normal body function. Water is one of
the most important ingredients of the body. A horse is actually 60 percent
water. Water is crucial to digestion, and makes up most of the bloodstream and
other body fluids – his brain is 85% water, muscles 75%. Even the bones are
About 25% of the horse’s body weight makes up
the flexible fluid in his body – fluid that can be shuttled around to
compensate for losses. A horse can lose about five percent of his body weight
in fluids (50 pounds or six gallons) before he shows much sign of dehydration.
Some horses will lose up to 10 % on a hard endurance ride, which is enough
fluid loss to severely reduce muscle function and cause other problems as
The body tries to keep a constant water level
and a stable temperature. Normal temperature ranges from 90 degrees to 100.5
degrees and rises with exertion. After hard work, it’s perfectly normal for a
horse to have a temperature of 102 degrees. But if it climbs much higher, the
body will start to show stress. The horse usually sweats in an attempt to cool
himself. When faced with the choice of overheating or dehydrating, the body
chooses to dehydrate, using up water to keep the engine cool. A temperature
that stays over 103 degrees or keeps on climbing puts the horse in serious
Evaporation from sweat cools the body. As the
liquid evaporates, it draws dome surface heat with it, cooling the horse. But
there is a limit to the horse’s water reserves, and as he begins to run out of
body fluid, his temperature climbs. A dehydrated horse without sufficient
fluid to stay cool, may suffer 3 degrees increase in temperature per hour of
When the body has plenty of water, excess
fluids and salts are excreted as urine. But when the body is short on water,
it conserves fluid. The horse urinates less often and the urine is scant, dark
Sweating can use up as much as four gallons per
hour when the horse works hard in hot weather. If air is dry, sweat evaporates
quickly and cools the horse. But if the air is humid, sweat won’t evaporate;
it covers the horse and runs off in streams. He stays hot and clammy and
sweats even more in an attempt to cool himself.
A horse that is not fit will sweat more
profusely than a well-conditioned horse and will also lose more electrolytes
(mineral salts) and proteins through his sweating. The soft horse has sticky,
smelly sweat that lathers easily because it contains more waste products.
Electrolytes are the same elements found in
seawater – sodium, chlorine, potassium, calcium, phosphorus and magnesium.
These minerals made life possible for the first sea animals.
These important minerals are all well supplied
in the horse’s diet except for salt, which must be supplemented. However,
prolonged sweating can deplete his reserves. The horse that dies of exhaustion
actually ran out of fluids and electrolytes.
The horse has several sources of water
reserves. About 18 to 20 gallons is stored in the gut, half of which can be
rapidly absorbed by the bloodstream, if necessary. (Blood loses water first
during sweating.) When the bloodstream becomes short of fluid, it is replaced
by water from between the body cells, which in turn is replaced by water from
within the cells. Cell fluids are eventually replaced by water from the
digestive tract. But if the horse hasn’t had enough to water to drink, cell
water is not replaced, and the functions begin to suffer.
As the horse’s temperature climbs, overheated
blood goes to the small capillaries at the skin surface, where the sweat
glands open up and release fluid from the blood. Water can leave the blood
swiftly as sweat, but is replaced more slowly. Heavy sweating uses water
faster then drinking replaces it.
You usually can’t tell a horse is dehydrated
until he has lost at least four gallons of fluid (or about 32 pounds of body
weight.) By then his performance is affected. He needs to slow down. The
dehydrated horse must have his water replaced (he needs to drink), needs to
stop exercising and stay in a cool place or have his body sponged with cool
water until he recovers his internal water balance and his temperature returns
SIGNS TO LOOK FOR
There are several ways to tell if a horse is
dehydrated. If you pull out a pinch of skin on the horse’s neck or shoulder,
and it springs right back into place, he’s not very dehydrated. If he is
moderately dehydrated, the skin will stay elevated a few seconds after you
pull it out.
Some people estimate that fluid loss is about
one percent of body weight for each second it takes the skin to return to
normal position. Since about five percent dehydration occurs before any change
is noticeable, if the skin takes on e second to spring back, the horse has a
fluid loss of about six percent of his body weight. The more dehydrated, the
longer the skin will stay elevated.
Another sign of dehydration is dry, red mucous
membranes inside the nose and mouth, and dry gums. Color of the membrane under
the horse’s eyelid is normally pinkish to pink-yellow, but becomes more red
with stress. Brick red is serious. If stressed even more, the horse’s membrane
color turns blue from lack of oxygen.
Capillary refill time is also a good clue. If
you press your finger into the horse’s gum just above his front teeth, that
spot will turn white since you have pressed out the blood. In a normal horse,
as soon as you remove your finger the blood will come back. Normal color will
return within one to four seconds. But in a stressed and dehydrated horse, the
spot will stay pale and bloodless longer. The more dehydrated the horse, the
slower the capillary refill time.
The eyes of a dehydrated horse will seem
shrunken and dull, with eyelids wrinkled. The cornea is normally moist and
clear, but in the dehydrated horse it may look glazed because it is drying
As the horse dehydrates, his sweat becomes
thicker and more lathered, like that of a soft horse. Temperature rises, heart
rates increases and these rates don’t return to normal quickly when the horse
stops exerting. They stay elevated or rise even more. A big respiratory rate
is one way the horse cools off, through the lungs, but if it does not drop
soon it may mean the horse is dehydrated.
Other signs of dehydration are shallow panting,
muscle tremors and weakness, weak pulse, depressed attitude. The horse’s brain
is sensitive to fluid and electrolyte imbalances, and when dehydrated he may
experience mental changes. Dehydration can also trigger muscle cramps (typing
up or colic) since fluid and electrolytes necessary for proper muscle function
The stressed and dehydrated horse may not feel
like eating. If he has some appetite, it’s a good sign. He’ll be more
interested in green grass than in hay or grain. Grass may be 50 to 90 percent
water, and it is the best possible feed for a dehydrated horse. Dry hay is
only five to eight percent water and can be pretty hard to eat. Grain is more
apt to mess up the digestive system of a stressed horse than hay or grass.
Research with endurance horses has shown they
do best with all the good quality roughage they can eat, and not much grain.
The roughage in the digestive tract helps it act as a big vat to hold
moisture, and the horse doesn’t dehydrate as quickly on a long ride.
To combat dehydration, condition slowly and
gradually, and carefully prepare for the work the horse will be asked to do.
Electrolyte replacement is sometimes beneficial during or after a strenuous
long ride, but is of little value if given before the ride (and can actually
be harmful). The horse can’t store extra minerals; he must flush them out of
his system and uses up extra water in doing it.
But when he is stressed and dehydrated,
replacement of these crucial minerals can be helpful. Remember that you don’t
have to replace everything he loses during a hard ride and that salt and
electrolytes should never be given without adequate fluid at the same time.
Severe and dangerous dehydration can best be
reversed by giving electrolyte fluid intravenously. Fluid by stomach tube or
into the rectum (where it is readily absorbed) can be helpful, but a severely
dehydrated horse will need lots of intravenous fluid, and veterinary
attention. Moderate dehydration can be reversed by giving the horse all the
water he will drink, and allowing him some free-choice salt or electrolyte
powder with his feed or dissolved in his water.
Give the horse a chance to fill up on water
before a long ride, and let him drink at every opportunity. Some horsemen are
afraid of watering a hot horse, but most horses are better off if allowed to
drink whenever they wish. A horse won’t have trouble handling water, if he is
going to continue exercising after drinking. The danger in watering a hot
horse is when he is allowed to tank up on very cold water and then must stand
idle. In this situation he may colic or get muscle cramps as blood leaves his
tired muscles to rush to the stomach to warm the cold water.
If the day is hot and humid, periodically
sponge the horse with water to help cool him and conserve his own body fluids.
He won’t have to sweat so much. If he’s sweating and over-heated, keep him
wet. If the day is hot and his temperature is over 103 degrees, use cold water
on head, neck, and insides of his legs, to bring down his temperature. But
under most conditions, cool or lukewarm water is better. Don’t wet his whole
body. Applying cold water all over him (especially over heavy muscles of
hindquarters, shoulders and back) can cause muscle cramps. Cold water also
chills the skin and may stop his sweating reflex, interfering with the body’s
own cooling system.
If the air is very dry or windy, causing water
to evaporate quickly, the horse will cool out rapidly on his own with just a
normal rubdown; don’t use cold water. On cool or windy days you may even need
to walk or blanket him after strenuous exercise so he won’t cool too fast and
Anyone who competes in strenuous athletic
events or uses a horse hard should be familiar with his horse’s abilities,
recovery rates, conditioning and body needs, and aware of the signs and
effects of dehydration – and how to prevent excessive fluid loss. Some degree
of dehydration is inevitable with strenuous performance, learning to cope with
it will keep your horse healthy.