How do horses adapt to the cold ?

Thermal comfort zone is the temperature range within which the body does not have to provide additional energy to maintain internal body heat. In humans, this zone is very small and is around 25°C.

The lower critical temperature is the temperature below which the horse needs help to fight against cold by providing him with additionnal energy by offering him a shelter. This threshold is -15°C for adults, 0°C for foals and only 22°C for newborn foals.
[1]From raw energy to metabolized energy

• Gross energy = energy supplied by the feed (this is indicated in the tables)
• Digestible energy = gross energy - energy excreted in the feces
• Metabolisable energy = digestible energy - energy produced during digestion or in the urine = net maintenance energy (heat production, movement) + net work energy + net production energy (milk, meat, gestation)

In a few minutes, for a fast adaptation to cold.

Shivering or intense muscular exercise will produce heat.

• Shivering is a reflex mechanism of muscle contractions (aerobic) which will produce heat from breakdown of sugar and fatty acids (metabolism multiplied by 4 or 5). This moderate mechanism can be maintained for a long time, unlike intense muscular exercise (rather anaerobic) which also allows a rapid rise in body temperature. It produces energy from the incomplete breakdown of glycogen, as well as lactic acid responsible  for muscle tiredness (metabolism multiplied by 25).
• Piloerection or bristling of the hairs increases the insulating effect of the hair by increasing its thickness by up to 30%; it will thus be able to contain more air.
• Vasoconstriction or constriction of the blood vessels in the extremities of the  limbs limits heat loss. Blood flows back to central organs (this is what makes our fingertips white when it's cold) and helps reduce the temperature of extremities to 1.7 ° C, preventing tissue damage when horses have their feet in the snow.

Mobilization of the neuro-endocrine system allows a release of glycogen which will serve as "fuel".

Over a few days to several months

For a more gradual adaptation, several factors can be observed :

Behavior changes

Horses living outside in cold weather will reduce their activity in order to spend less energy. In case of heavy rain, activities such as staying still, resting or lying down are reduced. In case of cold wind, horses will expose their hindquarters and not their head, which would be related to the possibility of shivering.

Physiological changes

Horses gradually acclimatized to cold see the density of their hair increase and their fatty tissue is distributed differently: instead of accumulating on certain parts of the body, it is distributed over the entire surface.

Changes in energy consumption

A sudden decrease in ambient temperature triggers a more important consumption of food in horses which increases their thermogenesis.

Over thousands of years…

Adaptation of breeds during evolution has allowed certain breeds to acquire morphological, anatomical and physiological characteristics favoring production and limiting heat loss, depending on :

• Morphology

Morphology determines a surface ratio (of the skin in contact with the outside) / volume (of the body). The larger the animal, the smaller the ratio. As the heat production is proportional to the volume and the loss proportional to the area, larger animals are favored, as are those whose morphology is closest to a sphere. In this configuration, short stocky breeds have therefore  an advantage over more streamlined silhouettes(draught horses versus thoroughbred horses); the same is true for youngsters compared to the older horse.

• Thermal insulation capacity

Vasoconstriction of the peripheral vessels reorienting blood circulation towards central vessels, causing a drop in the  T ° of the skin to the T°of ambient air. This increases the effectiveness of insulation provided by the subcutaneous adipose tissue (which is distributed over the surface of the entire body), the skin and the waterproof coat. The high efficiency of this mechanism is obvious when one can see the backs of horses covered with snow which does not melt. Therefore, some breeds with a very dense winter coat are particularly adapted to cold environments, unlike others very close to thouroughbred breeds whose coat grows little in winter. Likewise, horses with thicker skin and a layer of fat distributed all over the body will benefit from a better “insulation system”. Donkey's hair, which is not waterproof and thin, makes it particularly vulnerable to cold and humid temperatures. It is also possible that the color of the coat has an effect, with black or bay coats absorbing heat better than light coats.

• Proportion of slow or fast muscle fibers?

It's a hypothesis, but it's tempting. Horses with more slow muscle fibers (aerobic), able to contract themselves for a long time with moderate intensity would be better adapted to cold. Indeed these horses spend most of the time moving slowly while eating. They sleep upright, which allows locomotor or postural muscles to produce heat. They lay down very little, often in the middle of the day, when it is not as cold.

Thus it is possible to identify characteritics of horses who are more adapted to cold.

Horses more adapted to cold are heavier and more compact, with shorter extremities, long hair, especially feathering, thick skin where the blood vessels are not apparent, and subcutaneous fatty tissue distributed over the entire surface of the body, a capacity for efforts of moderate intensity but over a long period, by mobilizing fat reserves without production of lactic acid.

By allowing a gradual adaptation

It is recommended to let horses gradually adapt to the natural drop in temperatures, leaving them outside in the fall. Thus, horses need 10 to 20 days to adapt to a drop in temperature of approximatively 15 ° C, whether it is to go from 20 ° C to 5 ° C or from 0 ° C to -15. ° C.
For horses who are more sensitive to sudden temperature variations, it will be therefore be necessary to take precautions at the time of a transition, either for turning your hore out in the middle of winter, or for bringing him back into a heated stable.

By adapting the diet

Maintenance energy consumption for adult horses should be increased by 2.5% per degree Celsius below -15 ° C. For horses on maintenance diets, the easiest way is to provide them with good quality hay. Most horses eat 2-2.5% of their weight in hay per day. A 600kg horse will eat between 12 and 15kg of hay per day. At -40 ° C, he will therefore need 4.5 to 5 kg more hay.

It is necessary to use good quality hay, that is to say easily digestible and with high energy value. Indeed, hay harvested late, very rich in fibers non digestible will not provide enough energy and requires higher consumption of energy for its digestion. In addition, if horses do not drink enough water, this poorly digestible hay can cause impaction colic.
It is also possible to add concentrates to the food, but it’s generally  not necessary if hay is of sufficient quality.

IIt is possible to raise a growing foal outside in cold weather, if their diet is adapted accordingly. An experiment carried out in Canada on newly- weaned foals, kept in paddocks with a windbreak fence over a  6 month period with temperatures going from +10°C to -30°C, began to gain less weight from -11°C, suggesting that energy could be redristibuted towards tissue maintenance. However, the growth of the skeleton was not affected.

In these extreme conditions, it is important to pay attention to horses that do not feed enough, loose weight and reduce their tolerance to cold. This can be the case with dominated horses who may not have access to food.

Salt and minerals must also be provided, as well as a good quantity of clean water.

By offering them a shelter

Observations of feral or Prewalski horses have shown that they search for dry areas in case of wet cold, particularly in the case of heavy rain and wind. Likewise, when horses have shelters available, they will use them more in these difficult climatic conditions. In general, foals use more shelters than adults. Horses kept in a shelter can retain up to 20% more body heat, especially if they can lie down.

Shelter can be natural (hedges, trees) or artificial, the main thing is that horses can have access to it when ever they want. In very cold regions, shelters can be set up behind snow piles, headlands or ravines. For horses living in groups, the shelter should be big enough so that all the  horses can lie down in complete safety whilst respecting the hierarchy. A three side shelter represents a good protection against wind and snow. Ideal dimensions are a depth of 8 meters and a surface of 7.5 to 9m² per horse with adequate bedding, preferably straw, especially for young horses.

French regulations do not require the  presence of a shelter as such. It requires that, when it’s possible or necessary, horses must be protected from bad weather and predators.

What should one do with clipped horses ?

For working horses, absence of hair facilitates perspiration and speeds up drying of the horse, preventing him from catching cold. Obviously, it significantly modifies the thermal comfort of the horse, minimum critical temperature then being closer to 5°C than -15°C. It is then necessary to put a rug (to be chosen according to temperature) to protect them from cold and rain and to adapt the ration to provide the horse with the necessary energy.