Water: The Overlooked Nutrient
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This article describes the nescessity of fresh water in the
horses diet
Water: The Overlooked
Nutrient
The most important nutrient in the horse's diet is one that is
rarely added to feeds: water. Although it is often overlooked
in discussions involving equine nutrition, water could be
considered the first limiting nutrient of all horses, as they
cannot survive for as many days without water as they can
without feed.
The amount of water required by the horse is determined by the
magnitude of water losses from its body. These losses occur
through feces, urine, respiratory gases, and sweat and, in the
case of lactating mares, milk.
These losses are affected by the amount, type, and quality of
the feed consumed, environmental conditions and the health,
physiological state, and physical activity of the horse. Horses
will generally consume as much water as they need if given
access to a palatable water source.
Horses at rest in a moderate climate will generally consume
between three and seven liters of water per 220 lb (100 kg) of
bodyweight. This translates to around 4-9 gallons for an
1,100-lb (500-kg) horse.
Diet plays a major role in determining voluntary water intake
and requirements. As a general rule, water intake is
proportional to dry matter intake, but the composition and
digestibility of the diet can alter this relationship
substantially.
Horses consuming all-hay diets drink more than horses fed a
large amount of concentrate coupled with hay or a complete
pelleted diet. In a study conducted by Kentucky Equine
Research, horses fed all-forage diets ate 19% more dry matter
to provide a similar caloric intake to those fed a mixed diet,
and they drank 26% more water.
In another study, horses fed about 13 lb (5.8 kg) of a hay-only
diet drank 40 lb (17.8 kg) water compared to 22 lb (10.1 kg)
water consumed by horses fed 4 lb (1.8 kg) grain plus 3 lb (1.3
kg) hay, which was partly because of lower dry matter intake,
as well as different dietary composition.
Fiber intake affects water requirements for two reasons. First,
when horses consume forage, water shifts from the interstitial
space into the gut. This results in a decrease in plasma volume
that triggers a thirst response. A meal of 5 lb (2.27 kg) of
hay resulted in elevated total plasma protein (an indirect
measure of decreased plasma volume) and greater water intake
than when a similar-sized grain meal was fed.
Second, plant fiber has a greater water-holding capacity and is
less digestible than other components of the diet, resulting in
greater fecal water losses.
The physical form of forages also affects fecal water losses.
In a digestibility study, horses fed alfalfa hay in a long-stem
form had significantly wetter feces (81.5% versus 75.2%) than
when an identical amount of the same forage was fed in a
pelleted form.
Diet can also affect urinary water loss. High salt intake
increases urine output and stimulates thirst. Also, protein
intake above the horse's requirement increases water intake and
urinary output as the horse voids excess nitrogen via urine.
Besides increasing water requirements, this creates an
environmental hazard for stalled horses since the nitrogen in
urine is broken down into ammonia in the horse's bedding.
Body condition can affect water intake as well. Because fat is
low in water content compared to lean body tissue, obese
animals typically require less water than animals maintained at
a more optimal body condition.
Ambient temperature influences water intake. Horses typically
drink less water in cold weather. Water consumption of
weanlings exposed to cold temperatures, -8°C and -17°C, was
compared to that of weanlings housed in temperatures of 8°C and
above. Water intake was up to 14% lower in weanlings maintained
in cold temperatures.
Heat and humidity increase water requirements, especially in
exercising horses. In one trial, daily water intake increased
79% when horses transitioned from a thermoneutral environment
(20°C and 45-50% relative humidity) to a hot, humid one
(33-35°C and 80-85% relative humidity).
In addition to temperature and humidity, other factors impact
water intake of equine athletes, primarily duration and
intensity of work as well as acclimation of the horse to the
environment. Depending on the conditions in which a horse is
exercised, total water intake by an 1,100-lb (500-kg) horse
could reach over 90 liters per day.
Aside from equine athletes, lactating mares drink more water
than other horses. Increased intake is likely due to a
combination of factors, principally the fluid losses associated
with milk secretion and the increased consumption of feed to
support milk production. Though other factors, such as diet
composition and ambient temperature, will play a role in volume
of water consumed, 1,100-lb (500-kg) lactating mares may drink
up to 75 liters per day.
Certain illnesses predispose a horse to increased water
consumption. A horse with diarrhea but with normal appetite and
thirst might have incredible fecal water losses. The horse
compensates for fluid losses by boosting water intake and
decreasing urine output. Without resolution of the diarrhea,
however, dehydration is likely.
Polydipsia, or excessive drinking, especially when combined
with other signs such as hirsutism, might be indicative of
equine Cushing's disease or renal insufficiency.
Although often neglected in nutrition discussions, water is
vital for the health and well-being of all horses. As such,
horses should have access to fresh water at all
times.
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