By Dr. Nadia Cymbaluk, Carberry, Manitoba
Table of Contents:
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Introduction
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Thermoregulation in Cold Weather
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Management of Horses in Cold Weather
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Guidelines for Feeding Horses During Cold Weather
Introduction
The February "miracle" of the Edmonton toddler who "froze" outdoors in
sub-zero temperatures and lived is a solemn reminder of the danger cold
weather poses for homeotherms (warm-blooded animals). Cold weather generally
does not affect horses as suddenly or dramatically, if at all, as was seen
with this infant.
Although horses are very adaptable to cold weather, they must be managed
sensibly during winter. A basic review of energy (heat) exchange in the
horse will help to explain why certain management practices are necessary
during cold weather. The easiest way to understand the impact of cold weather
on a horse is based on the heat (energy) balance equation. Heat or energy
balance is the difference between heat loss and heat gain over time. If
heat gain exceeds heat loss over time, the horse will gain weight. If heat
loss exceeds heat gain, the horse will lose weight. The most important
loss of heat in cold weather is to the environment. Warm objects lose heat
to the environment whenever air temperature is colder than the object.
Horses, whose skin temperature is about 30oC, will lose heat to the air
around them whenever the air temperature is below 30oC. The colder the
air around the horse, the greater the heat loss will be. The main source
of heat gain is the energy (calories which convert to heat) obtained from
feed. Other minor sources of heat gain are muscular activity, the sun and
mechanical heat in barns.
Thermoregulation in
Cold Weather
Horses respond in two ways to cold: acutely (immediately) and chronically
(adaptive or acclimatization). The immediate response of a horse to a sudden
change in temperature is to change its behaviour. Horses will seek shelter
from the cold and wind, or huddle together, to decrease heat loss. Horses
stop foraging and stop moving to conserve energy. Horses in cold, windy
conditions typically stand with their heads away from the wind, their tails
set low and into the wind. Shivering and other voluntary muscular activity
can generate substantial body heat. It is not unusual to see horses running
in cold weather. The muscular contraction involved in physical activity,
such as running, results in heat production. In the dissipation of this
heat, the horse's inner body core (heart, liver, intestine, etc.) is warmed.
Thyroid hormone secretion is increased during sudden cold exposure in adult
horses, but this response is short-lived. Horses will also vasoconstrict
(reduce) blood flow to the extremities, such as the lower legs, the ears
and the muzzle, and their hair will stand up (piloerection) to effectively
increase the haircoat depth.
Horses exposed to constant chronic cold weather acclimatise to the cold.
Typically, horses require 10 21 days to adapt to cold. For example, a
horse kept at 20oC and exposed to temperatures of 5oC will adapt to temperatures
of 5oC over 10 to 21 days. When the temperature drops to 5oC, the horse
will need another 10 21 days to adapt to this increasing cold. This process
continues up to 15oC, which is the lower critical temperature (LCT). Once
the LCT is reached, physiological changes and human intervention, such
as shelter and/or extra feed, are needed to help the horse cope with the
cold. One of the physiological changes is an increase in density and length
of the horse's coat. Body temperature and respiratory rates decrease to
conserve heat/energy. The temperature of the lower legs, ears and muzzle
decrease because blood is shunted from the extremities to reduce surface
heat loss.
The horse's ability to adapt to cold depends on the duration of the
cold weather and on the horse's energy intake. The latter factor, energy
intake, is the most critical in determining how readily a horse develops
a tolerance for cold. Horses lose weight if they do not eat enough energy
to offset the heat loss to the cold surrounding air. Enough feed and good-quality
feed are needed to supply adequate energy intake for the horse. Fat horses
are able to mobilise some of their fat deposits as energy during cold snaps
but enough good feed is the main solution for keeping horses in good condition
through winter. Well-fed horses adapt without problem to cold weather,
whereas unfed horses lose weight and lose cold tolerance. Yearling horses
fed a high quality diet free-choice are able to tolerate temperatures as
low as -11°C with no ill effect. Horses will generally eat to meet
their energy requirements. In cold weather, feeding good quality hay free-choice
is the simplest way to ensure that the horse will meet its energy requirements.
Management of Horses in
Cold Weather
Horse
Various factors affect the ability of a horse to withstand cold. Large
bodied horses, e.g. draft horses, are much more able to withstand cold
because of a lower relative body surface area per unit of weight (area:weight
ratio). Belgians are more cold tolerant than Thoroughbreds. Newborn foals
have very poor cold tolerance. Horses up to one year of age are less cold
tolerant than adult horses. In late pregnancy (9th month and beyond), energy
requirements of mares increase and, consequently, cold tolerance decreases.
However, a winter-adapted, well-fed, pregnant mare is at no higher risk
than a gelding. Poor teeth, parasites and disease also decrease the cold
tolerance of horses.
Shelter
Horses kept in a shelter (shed) can conserve up to 20% more body heat than
horses kept in an exposed area. A three-sided shed is suitable protection
from cold winds and snow. A typical shed should be 8 metres deep and should
provide an area of 7.5 - 9 m2 per horse for lying down. Adequate bedding,
preferably straw, should be provided in sheds, especially for young horses.
Well-bedded bluffs, coulees and treed areas can serve as alternatives for
sheds. Animals that are able to lie down can reduce surface area heat loss
by 20 25%.
Feeding
Dietary energy is the only nutrient that must be increased for horses kept
at temperatures below their LCT. The LCT for growing and adult horses in
Canada is 0°C and -15°C, respectively. Maintenance energy intakes
of adult horses must be increased 2.5% per Celsius degree below temperatures
of 15°C or the equivalent of 2% more feed. Feeding good quality hay
is the easiest and most suitable way to supply additional energy for idle,
adult horses kept outdoors in cold weather. Horses will increase their
voluntary feed intake if permitted. Horses can be fed hay free-choice without
fear of producing laminitis (founder). However, owners who are limit-feeding
their horses should ensure that 2% more feed per degree below -15oC is
available to their horses. Groups of horses housed outdoors should be fed
in a way that reduces competition among horses. Grouped horses typically
have a pecking order for feed and space. Often, timid horses will become
thin, even if plenty of feed is available, because the dominant horses
in the group won't allow them to eat. Any type of feeder, including feeding
on top of clean snowy areas, can be used, as long as enough individual
feeding space (3 15 metres between horses) is available for the horses
in the group. Feeders are preferable since they will reduce feed wastage
by 25%. This is especially true of those with a floor in them. Salt and
mineral should also be provided. Clean fresh water should always be available
to horses. Snow is an unsuitable substitute. A horse would have to consume
ten times its water requirement in snow to meet its needs. The horse would
then need to use energy to raise the temperature of the snow from the outside
ambient temperature to the horse's body temperature of 37o C. This is a
huge energy drain and, when coupled with poorly digestible hay, can lead
to gut impaction.
Winter pastures should not be relied upon to provide the sole source
of nutrients for horses since they are usually poor feed sources for overwintered
horses. Not only is the nutrient quality poor but, in deep snow, the maintenance
energy needs of horses can increase by 40% because the horses have to crater
or paw through the snow to find low quality feed.
The cost of weight loss in the horse is much higher than most people
realise. Thin, malnourished stallions lose their ability to produce sperm.
Thin mares have a very low conception rate. Underfed weanlings can become
permanently stunted. Finally, the extra cost of feed needed to rehabilitate
a thin horse back to normal will equal or exceed the cost of the feed that
should have been given to the horse to maintain its body weight. Feeding
horses is costly; not feeding horses is more costly.
Guidelines for Feeding Horses
During Cold Weather
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The easiest method of feeding horses in cold weather is the simplest: feed
high quality forages free-choice. Most horses will eat 2 to 2.5% of their
body weight in hay per day. Average daily free-choice intakes of hay by
a 600 kg (1320 lb) horse is 12 15 kg (26 33 lbs)/day or about a half
of a 65-lb square bale per horse per day.
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If you are limit-feeding horses outdoors, adult horses being fed at maintenance
will need an additional 2% more feed per degree below the lower critical
temperature (-15oC). At 40oC, the horse will need 4.5 5 kg (10 - 12
lbs) more than it ate at temperatures above 15oC.
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Use highly digestible hays and supply a sufficient quantity of hay. Coarse
overmature hays are low in energy and high in indigestible fibre. In cold
weather, when energy demands are high, overmature hays will not supply
sufficient energy and, furthermore, if fed without sufficient water, can
cause impactions. Alternatively, grain can be added to the diet. Caution
must be used when adding grain to the diets of horses unaccustomed to grain
because founder (laminitis) can occur. All horses fed grain should be gradually
adapted to small amounts of grain over a period of 7-10 days.
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Supplemental vitamins A, D and E may be needed. Appropriate mineral-vitamin
mixes should be chosen. A fortified 2:1 calcium-phosphorus mixture is recommended
for feeding with grass hays and a 1:1 Ca-P mixture is recommended for feeding
with alfalfa hay.
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Provide adequate heated (2-10°C) water, if possible. Water helps maintain
appetite and digestive function. Snow is not a suitable substitute.
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If available, a well-bedded, south or east-facing shed is useful for young
and old horses. Alternatively, provide protection from the wind by providing
bedding areas behind snow fences, in coulees or bluffs, or among trees.
Horses that can lie down will conserve body heat.
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