Blanketing a wet horse will increase the chances of developing rain rot, but it's better to deal with [potential] rain rot later than to deal with a colicky horse that got too cold.
Weather for Blanketing Horses
Most horses are very comfortable in brisk (but above freezing) temperatures as long as they are dry. If you've got precipitation such as rain, even a drizzle, or snow that could melt on their warm backs and they don't have any way to avoid getting wet, consider a blanket.
Sprinkle a layer of hay or straw over the horse's back, and then apply the blanket. Straw is preferable to hay because its shafts are hollow and will hold air (and it's a lot cheaper!) but either one will work fine. Thatching creates air pockets over the horse's back, which will allow water to safely evaporate.
Blankets tend to compress a coat's layers, which compromises their insulating properties. Horses that do not live in extremely cold environments – meaning routinely colder than 10°F – will do well without a blanket, provided they are either stalled during the coldest temperatures or have access to a protective shelter.
Wool coolers are the best! Keep your horse out of the wind and drafts as he dries. You might be able to let some sunshine warm him up if there's no breeze.
Coleman said horses' hair coats can effectively protecting them from cold temperatures, but they stand up less to wind and wet conditions. “If a horse's coat gets wet in rain or snow, it can dramatically chill them,” he said. “You may need to bring them inside a barn to dry and warm up.
DON'T blanket a wet horse
While it may be tempting to blanket after being exposed to cold rain, horses should not be blanketed when wet. If a blanket becomes soaked, it should also be removed from the horse. This can help prevent rubbing and risk of chill, as they remain wet longer and hair becomes flattened.
While we hate the idea of our horses being cold, it's dangerous to overblanket. If your horse begins to sweat under his blanket, he can overheat or, if wet hair traps the moisture against his body for too long, he could get chilled. Thankfully, it's easy to tell if a horse is too hot under his blanket.
The only essential equipment you'll need is a wool or polar fleece cooler. Both are excellent wicking materials designed to create an air space around your horse. His body heat warms up this air space, which then draws the moisture away from his skin to the outer surface of the blanket.
The clean and rinsed horse should be thoroughly dried, ideally in the sunshine. You can also rub them dry with a towel, walk them around or stand them under heat lamps if you have them. If it is cool, put a cooler rug on to help speed up the drying time.
If your horse has come in from the field muddy and wet, then he will need to dry off before he can be groomed. Brushing a muddy, wet horse just brushes the mud deeper into the coat instead of out of it.
A gentle or even a steady rainfall likely won't jeopardize a horse's health. A cold rainfall would probably call for at least a run-in shed. A chance for severe lightning or winds could be life-threatening.
While rain rot and scratches affect the horse's skin, heavy rain can also impact your horse's hooves. Thrush, hoof cracks, white line disease and hoof abscesses are a few hoof conditions that become prominent in times of wet weather. To reduce occurrence, ensure that your horse can stand in a clean, dry environment.
It's best to wait until the horse is completely dry before putting on a fly sheet. If the horse is sweating or has gotten wet from rain or a bath, it's important to towel dry them thoroughly and allow them to fully air dry before putting on the fly sheet.
The blanket should be checked and removed every couple of days. You'll need to do this to make sure it is still fitting the way it should, and so that you can check the horse's body condition under the blanket to make sure they haven't lost too much weight in the cold weather.
A too-large blanket can shift out of position and slip down around legs, entangling them in the hind-leg straps. If the material does not rip quickly, tendons can become dangerously constricted. Blankets that are too small may cause pressure sores or rubbing injuries.
A blanket will make your horse's hair coat lie flat, thus removing that insulating layer of warm air. If the amount of insulation in the blanket is less than what your horse's natural coat would provide, then he will be cold. Make sure that any blankets used for turnout are waterproof!
If a horse is cold and wet or lacks shelter, dry them off, blanket them, and/or move them to a place of greater warmth or shelter. The shivering should gradually subside. Perform the Whole Horse Exam (WHE), paying particular attention to the presence of fever, or poor appetite.
In cold and 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 the absence of wind and moisture, horses tolerate temperatures at or slightly below 0° F. If horses have access to a shelter, they can tolerate temperatures as low as -40° F. But horses are most comfortable at temperatures between 18° and 59° F, depending on their hair coat.
Leaving water on a horse will not make it hotter: expert busts the scraping myth. An expert in equine thermoregulation is attempting to bust some of the myths spread about cooling horses.
Horses generally don't mind getting a little wet in the rain. However, bad weather can frighten some horses, causing them to not drink enough water. This can lead to colic. Make sure your horse is getting plenty of water during rainy season.
“Application of water without scraping may help decrease the core body temperature in horses more effectively in the early stage of exertional heat illness.”