Water and Electrolyte
Balance in the Exercising Horse
The following paper
was presented to the Horse Industry Breeders Conference: Sponsered
by the Horse Section of Alberta Agriculture.
Last Revised/Reviewed May 8, 1996
Laurie Lawrence, Ph.D.
Department of Animal Sciences, University
of Kentucky, Lexington, KY
Exercise
generates a great deal of heat that must either be dissipated
from the body or stored. The dissipation of heat is very important,
because if a large amount of heat is stored, body temperature
will rise to dangerous levels. In the horse, the processes that
contribute to the dissipation of heat during exercise include
radiation, convection, conduction and evaporation. Of these,
evaporation may be the most important; particularly when horses
are exercised in warm conditions. To facilitate evaporative cooling,
horses sweat. Sweating is certainly desirable as a means to maintain
body temperature, but high sweating rates result in high rates
of water and electrolyte loss. The following discussion will
address the effects of water and electrolyte losses on exercising
horses as well as some strategies for helping horses maintain
fluid and electrolyte balance during competition.
Fluid Losses
In a practical setting, it is
difficult to accurately measure the total amount of sweat that
a horse loses during an exercise bout. However an estimate of
sweat losses can be obtained by weighing horses before exercise
and after exercise.
We have recently been involved
in weighing horses before and after workouts, races or other
events to estimate sweat losses. At this time we have not weighed
enough horses in different environmental conditions to have solid
estimates of sweat losses in all situations but some examples
of weight losses that we have observed in field conditions are
shown below.
Range of weight losses when horses
were weighed before and after various events**
|
|
Range of Weight Loss
(lb) |
Standardbred Harness Horses
(before and after a 1 mile race) |
12 to 33 |
Field Hunters
(before and after 3 hours of fox hunting) |
24 to 100 |
Thoroughbred Horses in Race Training
(before and after galloping or breezing) |
10 to 16 |
Endurance horses
(from the night before a race to the end of 54 miles) |
22 to 88 |
** These values may not apply
to all horses performing these types of events
Almost without exception, the
owners, trainers or riders/drivers of the horses are amazed at
the weight differences of their horses before and after work.
Although there are often fecal losses during exercise, weight
loss during most types of exercise is generally thought to be
about 90% water, so clearly water losses during some activities
are quite large.
Where does the water in sweat
come from? The horse's body is about 65% water. Most of the water
is contained in cells (intracellular water) but some is outside
of individual cells (extracellular water). Blood plasma, which
constitutes a large portion of the horse's blood volume, is an
important component of the extracellular water pool. When horses
(or humans) sweat, some of the water in sweat is obtained from
the plasma volume. Consequently, if sweat losses are large, the
plasma volume may decrease. A reduction in plasma volume (and
thus total blood volume) may affect the ability of the horse
to maintain adequate blood flow to muscles during work. Progressive
dehydration may also result in a reduction in sweating rate and
thus an increase in body temperature.
Electrolyte Losses
Horse sweat contains many things
besides water. Horse sweat contains calcium, magnesium, some
trace minerals and protein. However, most notably, equine sweat
is relatively high in sodium, chloride and potassium. When horses
lose large volumes of sweat, they lose considerable quantities
of these electrolytes. Although we have not measured the amount
of electrolytes actually lost during various events, it is possible
to roughly estimate electrolyte losses if the approximate sodium,
potassium and chloride content of sweat is known. Many researchers
have measured the amount of the various electrolytes in sweat.
Chloride is present in the highest concentration, but sodium
and potassium concentrations are also reasonably high. The following
table provides estimates of the sodium, chloride, and potassium
losses that might be experienced by horses in various activities,
given the weight losses listed earlier.
Range of Estimated Electrolyte
Losses of Horses Performing Different Activities**
|
|
Sodium
(g) |
Potassium
(g) |
Chloride
(g) |
Standardbred
(during a race) |
16-46 |
6-17 |
31-88 |
Field Hunters
(3 h foxhunt) |
33-148 |
12-51 |
63-284 |
Thoroughbred
(during a work) |
16~23 |
6-8 |
31-44 |
Endurance Horse
(54 mile ride) |
33-132 |
12-48 |
63-252 |
** These values are estimated
from weight loss during exercise and approximate composition
of equine sweat. Many factors could cause the actual values for
a particular horse to be higher or lower.
It is apparent that heavily sweating
horses experience large electrolyte losses as well as large fluid
losses. Electrolytes have many functions including maintenance
of acid-base balance in the body fluids and nerve and muscle
function. Large losses of electrolytes can result in several
neuromuscular and systemic disturbances including muscle cramping,
tieing up, synchronous diaphragmatic flutter (thumps) and systemic
alkalosis. Horses with large electrolyte losses may also have
reduced sweating rates and therefore a reduced ability to manage
body temperature. Electrolyte concentrations in the blood may
play a role in the horse's thirst response or desire to drink.
Ironically, when horses sweat a lot and lose a considerable amount
of sodium, their thirst response may be depressed and the horse
will not drink adequate fluids to maintain a desirable state
of hydration.
Maintaining Fluid Balance
During Work
When humans work for long periods
in hot environments, frequent fluid ingestion is recommended
as a means of preventing dehydration. For example, an intake
of 200 to 300 ml every 2 to 3 km has been suggested for humans
running in the heat. A similar suggestion might be applied to
horses except for the all too common "you can lead a horse
to water, but you can't make it drink" scenario. Some horses
will willingly drink during endurance rides or other long term
activities; but many horses do not exhibit a strong thirst response.
There are a number of reasons that horses may not drink during
an event, such as excitement or fatigue, or because the water
is not palatable to the horse. A poor thirst response may also
be attributed to the loss of sodium that accompanies water loss
in sweating horses. In humans, the sodium content of sweat is
quite low, so sweating results in a proportionally greater water
loss than sodium loss. The loss of water without a proportional
loss in sodium results in an increased concentration of sodium
the plasma. Importantly, the increased concentration of sodium
in plasma may be a signal for the thirst response in humans.
In horses, sweat is quite high in electrolytes and as a result
sodium is lost in a proportional amount to water. Therefore,
in the heavily sweating horse, plasma sodium concentrations may
not increase and the "signal" for drinking is not produced.
To increase the likelihood that
a horse will drink during an event, the horse should be allowed
to drink during training bouts. Adapting the horse to flavored
water for several days before an event and then flavoring the
water available at an event may encourage the horse to drink
water that is otherwise unpalatable. Allowing the horse access
to forage may also increase water consumption. Some horses may
drink more if a bucket is held for them; other horses may be
stimulated to drink if they see another horse drinking.
We have been researching another
way to assist horses in maintaining fluid balance during exercise.
In particular, we have been interested in how the pre-event feeding
practices can affect the ability of horses to deal with fluid
and electrolyte losses during exercise. We hypothesized that
diets that maximize water intake and retention would be best
for horses that are going to compete in events where fluid and
electrolyte losses may impair performance. Nutritionists have
long observed that water intake is correlated with dry feed intake;
that is, the more dry feed a horse eats, the more water it consumes.
The table below shows the water intake of horses (from one meal
to the next) eating two different diets -that contained about
the same number of calories, but different amounts of total feed.
Effect of amount of feed intake
on water intake in horses
|
Total kg feed
(evening meal) |
Water intake (L)
(over-night) |
diet C
(2.1 kg hay + 2.5 kg grain) |
4.6 |
11.0 |
diet R
(6.1 kg hay + 1 kg grain) |
7.1 |
16.4 |
From; Danielsen et al, 1995
Meeting Electrolyte Needs
Electrolyte availability can
become a problem when the rate of loss exceeds the rate of replacement.
If a horse is not involved in regular strenuous exercise and
has access to good hay or pasture and a salt block, it will probably
receive adequate amounts of the major electrolytes: sodium, chloride
and potassium. When horses engage in regular exercise, it is
generally believed that they will increase their salt intake
to compensate for their increased losses. However, during an
endurance ride or when a horse is worked very hard several days
in a row, intake of electrolytes from a salt block and the regular
diet may not be able to match the losses in sweat. Thus, in these
situations, a horse may benefit from electrolyte supplementation.
When considering the use of an
electrolyte supplement, it should be remembered that horses do
not "store" sodium, potassium or chloride from one
day to the next. Therefore, a high level of daily electrolyte
supplementation is necessary only when horses sustain high sweat
losses every day. For horses that are not sweating very heavily
every day, adding high levels of electrolytes daily will probably
only increase water intake, urine losses and the owner's sweat
losses when cleaning the stall!
While high rates of electrolyte
supplementation are probably not necessary on a daily basis,
relatively high levels of electrolyte administration may be appropriate
during an event. It may be advisable to provide the electrolytes
as a paste or in feed, rather than in the water to eliminate
any possible negative effects on water intake. Electrolyte administration
will be most effective if the horse is actively drinking, and
should be given only under the supervision of a veterinarian
if the horse is already very dehydrated or experiencing metabolic
problems. While administration of electrolytes without water
can create problems, administration of water without electrolytes
may also be detrimental to dehydrated horses. The veterinarian
will be most qualified to determine what combination of fluid
and electrolyte is most beneficial for a very dehydrated or exhausted
horse.
Many electrolyte products are
commercially available and recipes for home made mixes can be
found in textbooks or magazine articles. The most appropriate
way to chose a product is to compare the electrolyte content
of the supplement to the electrolyte content of sweat. If economy
is a concern, the supplement should be evaluated on the basis
of $/units of actual electrolytes, not $/unit of supplement,
since some supplements may contain "filler" ingredients.
Some "electrolyte" supplements actually contain very
low levels of electrolytes, or are designed. for horses that
have electrolyte disturbances from disease, rather than exercise.
There are lots of opinions about
appropriate administration schedules and dose rates. As noted
above, electrolytes should not be given to a horse that is already
dehydrated, except under the supervision of a veterinarian. However,
small amounts of electrolytes can be safely given to most horses
before they reach a critical depletion point. The ideal situation
would be to make enough electrolytes available to simultaneously
match losses, but this would be hard to do in real events. Reasonable
strategies for horses that are competing in situations that will
result in large sweat losses include the following:
Give small doses of electrolytes
at rest periods, before ,the horse gets dehydrated. A reasonable dose might contain around
3-7 g sodium, 6-15 g chloride, 1-3 g potassium, .4-.8 g calcium
and .1-.3 g magnesium. Another way that riders judge the appropriate
dose amount is to estimate losses and then administer an amount
of electrolyte supplement during the event that replaces 1/3
to 1/2 of the total losses.
Make sure water is available
and encourage the horse to drink.
If possible, keep track of about how much water the horse is
consuming.
Like water, electrolytes can
be retained in and then absorbed from the large intestine, so
adding some electrolytes to the diet just prior to the event
may be helpful.
Once the event is over, monitor
the horse carefully. Although it is not always convenient,
it may be best to wait several hours after finishing a long
ride or event before transporting the horse home. Transportation
can be a dehydrating experience on its own, and transporting
an already dehydrated horse may increase the potential for more
serious problems such as colic.
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