By Dr David Marlin

As we head into the colder winter months, most of us are reaching for our body warmers and jackets when we go to tend to our horses.   Does this mean it is time to start dragging out the rugs and blankets from where we stored them last spring?   We should not necessarily judge what clothing our horses need based on how we feel, for two very good reasons.

The first, of course is that horses have fur coats.   The second is that due to its size, the horse does not lose heat as rapidly as we do.   So at the same temperature the horse will actually feel warmer than we do.

Horses are very adaptable to different climates and are found in some of the hottest and coldest places on the earth.   Thriving in temperatures ranging from -40°c to 60°c.   Being warm-blooded, horses will work to keep their core temperature as close to 38°c as possible.   However, in very cold climates the extremities of the body, such as the feet, may fall as low as 5-10°c.   Or reach as high as 60°c when standing on hot sand.

Warm Vs Cold Blooded:

Cold blooded animals such as reptiles are hot when their surroundings are hot and cold when they are cold- frequently hibernating during the winter months.

The horse, work to maintain their body temperature at 38°c, irrespective of what the environmental conditions are.   This is known as thermoregulation.   The benefit is that it can be active all year round, with the activity remaining unchanged.   The disadvantage is that it must take in a lot more energy to generate the heat to maintain the body’s temperature, but it must use energy to cool itself down when the weather is hot.   The fuel used to heat the body is energy stored from the food the horse eats.   The heat is generated by the breakdown of food inside the cells or by fermentation of the fibre in the hindgut.

What determines how cold a horse feels?

In simple terms, the horse will feel either hot or cold when the air temperature falls below 0°c (the lower critical temperature) or rises above 25°c (the upper critical temperature) this is known as the horses thermoneutral zone, and it is easy for the horse to control the body temperature by simply opening or closing the blood vessels in the skin, to lose or retain heat.   Outside of these temperature ranges means the horse must use other means to keep warm or cool.

At temperatures above 25°c the horse may increase its respiratory rate and it may begin to sweat.   In addition the behaviour may also change, by seeking shade and water.   When temperatures drop below 0°c the heat needs to be retained.   The metabolic rate will be increased (effectively turning up the central heating by using more fuel) seeking shelter from the elements and allowing limb temperature to drop to a lower range in winter by reducing blood flow, shivering comes when the animal is really cold.   The range between summer and winter may change once they become acclimatised.

There are many factors that may affect how much energy a horse requires to keep warm.   The actual air temperature being the most obvious one.   The colder the air temperature the greater the difference between the horses surface temperature.   Add in wind and rain and heat loss is increased.

Size and age also have a relevant influence; in general larger animals usually have the advantage in cold climates.   As with people, aged horses and ponies are likely to cope less well in the cold.   The young and older horses often have less body fat and will lose heat more rapidly.   Elderly horses may also have reduced digestive efficiency, contributing to their risk of succumbing to the cold.

Heat production is greater on a high fibre diet, compared with starch or oil based diets.   Horses living in cold climates will cope well in the cold if given unlimited access to good quality forage.   Additional calories may only be required if the temperature drops below 0°c for several weeks, or if the horse was in poor condition to begin with.

The horse keeps warm by trapping air between the hairs.   This coat becomes less effective when the coat becomes wet and causes the hair to collapse.   Obviously a clipped horse would be much more vulnerable to the elements.

Studies have shown that even in severely cold weather, mature horses are able to maintain body heat if given unlimited forage and shelter.   An interesting consideration is what your shelter is made of.   Stone shelters will draw heat from horses by a process known as radiation.  So while the horse may not need a rug in a wooden stable, the radiant heat loss in a stone stable may mean the same horse would benefit from a rug.

Horses that are in work are often rugged for convenience, so they stay clean and dry.   We clip to avoid overheating and sweating into a long winter coat, which would then result in the horse being chilled.    However the risks of over rugging include; rubbing, especially if the horse sweats.   Skin infections caused by the skin becoming hyper-hydrated (the effect you get when you have been in the bath) leaving the skin prone to infection.   Rugging and stabling prevent sunlight reaching the skin, which affects the uptake of vitamin D.   Vitamin D is involved in the regulation of calcium and phosphorus.   Horses will use a considerable amount of energy to keep warm.   Over rugging will result in fat deposited and the horse gaining weight.

What Thickness of rug should I use?

A horses winter coat has been estimated to have a tog rating of 1-2.   If we consider that most summer duvets will have a tog of 4.5, then we have already increased the horses insulation by 2-3times.   Some heavy weight rugs may have a tog of 15, which would only likely be needed in a sick newborn foal or a very thin older horse, in extremely cold weather.

How to decide when to rug

-Don’t rug your horse based on how cold you feel

-Old, young or clipped horses will need rugs first

-Most horses (unless clipped) will not need a rug until temperatures drop below 5-10°c overnight

-ideally start with thinner rugs, and move to thicker, once it gets colder

-After a month of cold weather, the horse may well be able to move to a thinner rug after acclimatising.

-Feeling your horses face, legs or ears is a poor indication of how warm your horse is.   The withers is a better indication.

-Remember wind and Rain will induce the greatest heat loss.