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This is a controversial question, as I am told horses are designed to thermoregulate in the wild (which does make sense).

However my horse is getting old (20) shivering and she's hard to keep weight on and I'm concerned she needs a rug.

This is a photo of her for interest.

enter image description here

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In the wild horses do a number of things to combat the cold.

  1. They will find shelter - horses will naturally seek out sheltered areas in which to graze or stand out of the rain

This is possible when horses have unlimited acres to wander and gallop across. Many domesticated horses are confined to smaller paddocks which don't have the same aspects of shelter that are provided in nature.

  1. They move - exercise keeps them warmer

This is possible when a horse is well fed and has plenty of space to wander. As for point 1, many horses are confined to small spaces. Increase in feed and access to adlib pasture or hay can help keep a horses energy levels up required to combat the cold.

  1. They shiver - the muscle contraction burns energy to keep them warm, much like exercising

This works well for a 'good doer' or horses that have put on weight over the spring and summer. For horses that are battling, often these will be older horses, the weight loss incurred from shivering can leave a horse too thin.

From HOW DOES THE HORSE PROTECT HIMSELF FROM THE COLD?

The shivers are a reflex mechanism of muscle contraction that produce heat from the degradation of sugars and fatty acids (multiplication of the metabolism by 4 or 5). This moderate mechanism can be maintained for a long time, unlike intense (rather anaerobic) muscular exercise which also allows a rapid rise in body temperature, but produces lactic acid responsible for muscle fatigue is highly energy consuming (multiplication of the metabolism by 25).

From the RSPCA:

In cold wet weather a good quality and well fitting rug can help the horse to maintain condition, as a cold wet horse will burn a lot of energy keeping warm. Keep in mind though that if your horse is young and healthy but tends to get fat, rugs will actually help him or her to maintain that fat. In a natural situation excess body fat is burned off through the winter.

  1. Their fur stands up on end - Piloerection - much like our goose bumps, the horses' fur will stand up to create a barrier between the outer cold air and the horse's skin

This works when a horse is unrugged - a rug will smooth down the horses hair, making it unable to do this. This can result in a horse being colder without a rug, if the rug is not well fitted, or not water proof (in the wet) to name two possibilities.

A lightweight blanket will simply press the hair down and eliminate the coat's ability to hold and heat air, without adding any warmth.

From the RSPCA:

The coat of an unrugged horse stands up in cold weather to trap air and warm the horse. If you decide to rug you have to compensate for this mechanism as a rug will stop the hair from being able to do its job. In some circumstances a rugged horse is actually colder than an unrugged horse if it is a badly fitting thin rug that flattens the hair and reduces the movement of the horse without providing any real warmth.

This will also work if the horse is not wet. If a horse is not groomed, natural oils will build up in the horses coat, making the outer coat water resistant. Shampooing and brushing washes and brushes out the natural oil and mud that a horse uses to help maintain a barrier against the elements. A horse with a wet coat to the skin can become cold in the colder seasons.

.../ horses can make their hair stand-up, which is called piloerection (think of goose bumps), which acts to increase their hair depth and traps air next to their bodies creating an insulating layer
.../...
They then rely on the oils in their coat to prevent their skin from getting wet, which is why you should not bathe a horse that lives out in the winter or use a body brush which drags the oils through the coat, as they need the oils to stay near their skin to act as a protective barrier

  1. They can stand together in a herd - the collective bodies of a herd will help keep individual horses warm

Horses are often kept in small numbers, preventing the benefit of larger herd numbers.

  1. Vasoconstriction to the feet - to reduce blood flow and loss of heat and, in extremes, frost bite.

The vasoconstriction is a constriction of the blood vessels of the limbs extremities which limit the loss of heat. The blood flows back to the central organs (the phenomenon induces white fingertips in cold temperature) and reduces the temperature of the extremities up to 1.7 ° C, avoiding tissue damage when horses have their feet in the snow.

so the decision whether to rug or not will depend on where your horse is living, it's body score, age, health and many of the factors above. Overheating a horse is bot good for a horse, rugged horses need to be checked regularly, preferably twice a day, to check that they are not over heated or that the rugs are still in tact.

My rugged older girl, no longer shivering and in a better position to cope with the increasing cold.

enter image description here

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