Horses wear coats (called blankets or rugs) for several reasons: to replace insulation lost when their winter hair is clipped, to protect vulnerable horses from extreme cold, and to shield against insects and sun damage in warmer months. A healthy, unclipped horse with access to shelter often doesn’t need a blanket at all, which surprises many people. Understanding when a coat helps and when it actually hurts is one of the most practical things a horse owner can learn.
How Horses Stay Warm Naturally
Horses are remarkably well-equipped for cold weather. A healthy, acclimatized adult horse doesn’t start losing body heat faster than it can produce it until the temperature drops to around 5°F (-15°C). That threshold, called the lower critical temperature, is far colder than most people expect. Yearlings handle cold nearly as well, with a threshold around 12°F (-11°C) when fed freely. Newborn foals are the exception, needing warmth below about 68°F (20°C).
The secret is their winter coat. Each hair follicle has a tiny muscle controlled by the nervous system. When the temperature drops, those muscles pull the hairs upright in a process called piloerection, puffing the coat out to trap a layer of insulating air close to the body. It works exactly like a down jacket: the loft, not the hair itself, provides the warmth. This is why a horse’s coat looks fluffy on a cold morning.
This system has one critical weakness. When the coat gets wet or caked in mud, the hairs can’t stand up and the trapped air disappears. As little as a tenth of an inch of rain can flatten the coat enough to cause cold stress, raising the horse’s effective comfort threshold by 10 to 15 degrees. Wind compounds the problem by stripping heat away faster. So a dry, calm 20°F day is far more comfortable for an unclipped horse than a wet, windy 35°F day.
Why Clipped Horses Need Blankets
Horses that work or compete during winter present a dilemma. A thick winter coat traps air beautifully at rest, but during exercise it becomes a liability. Sweat soaks into the dense hair instead of evaporating off the skin, leaving the horse wet and slow to cool down. Research published in Acta Veterinaria Scandinavica found that unclipped horses had visibly wet coats after exercise, while clipped horses showed no surface moisture because sweat evaporated directly from the skin.
Clipping the winter coat solves the exercise problem but removes the horse’s natural insulation. A clipped horse in cold weather is essentially standing outside in a t-shirt. These horses need blankets whenever they’re not working, with heavier blankets as temperatures drop. A common guideline: clipped horses wear a lightweight blanket (under 100 grams of fill) in the 40-50°F range, a medium-weight blanket (150-250 grams) from 30-40°F, and a heavy blanket (300+ grams) below 30°F.
When Unclipped Horses Benefit From a Coat
Even horses with full winter coats sometimes need help. The most common scenarios involve weather, age, or health.
- Wet and windy conditions without shelter. If a horse is turned out with no run-in shed and the weather turns rainy or the wind chill drops below 5°F, a waterproof blanket replaces the insulation their soaked coat can no longer provide. Horses with access to a shelter conserve up to 20% more body heat than those fully exposed, which can make the difference between needing a blanket and not.
- Older or thin horses. Senior horses and those with low body condition have less fat insulation and a harder time generating heat. Horses recovering from illness face the same challenge, burning energy to heal while also trying to stay warm.
- Caloric demands. For every 1°F drop below about 45°F (the practical critical temperature when factoring in coat condition and weather), a horse needs roughly 1% more calories just to maintain body heat. On a day when the wind chill is 25°F, that’s a 20% increase in energy needs. A blanket can offset some of that metabolic cost, which matters for hard-keeping horses or when hay is expensive.
Unclipped horses with full coats generally need lighter blankets at lower temperatures than their clipped counterparts: a lightweight sheet from 30-40°F, medium weight from 20-30°F, and heavy weight below 20°F.
Summer Coats: Flies, Sun, and Skin Protection
Not all horse coats are about warmth. Lightweight mesh sheets, often called fly sheets, serve an entirely different purpose during warmer months.
Biting flies and midges are more than an annoyance. Their saliva can trigger allergic reactions, and one condition called sweet itch, caused by Culicoides midges, leads to severe itching, hair loss, and sometimes permanent scarring. A fly sheet creates a physical barrier that keeps these insects off the horse’s skin entirely. It also protects open wounds from blowflies and screwworm flies, which lay eggs directly in injured tissue. The hatching larvae feed on the wound and can be fatal if untreated.
Fly sheets also function like sunscreen. Dark-coated horses are prone to coat bleaching from UV exposure, while horses with pink skin or light coloring can develop sunburn and even skin cancers. Some horses also experience photosensitivity, a sun-triggered skin reaction linked to chemical imbalances in the body. A UV-blocking sheet addresses all of these problems without chemicals.
Risks of Over-Blanketing
Putting a blanket on a horse that doesn’t need one can cause real problems. The most immediate risk is overheating. A blanketed horse that gets too warm will sweat under the fabric, but the blanket traps moisture against the skin and blocks the evaporative cooling that would normally bring the temperature back down. Signs of heat stress include sweating under the blanket, lethargy, flared nostrils, and a rectal temperature above 102°F.
There’s also a subtler issue. Blanketing a horse with a good natural winter coat physically compresses the hair, preventing piloerection. The coat can’t puff up to trap air, so the blanket actually overrides the horse’s own thermoregulation system. If the blanket isn’t warm enough for the conditions, the horse ends up worse off than it would have been with no blanket at all. Moisture trapped under a poorly ventilated blanket can also promote bacterial skin infections.
The general principle is straightforward: if a horse is unclipped, healthy, at a good weight, and has access to shelter, it likely doesn’t need a blanket until conditions get genuinely harsh. When you do blanket, match the weight of the blanket to the actual temperature and check underneath regularly for sweating.