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If your horse doesn't exhibit any of the above, chances are it's your standard (albeit uncomfortable) case of sunburn. Make sure to remove your horse from exposure to the sunlight during the recuperation period and provide him plenty of roughage and water.
Although there isn't any magical formula that can instantly repair the damage to sunburnt skin, there are two supplements that have proven effective in lessening discomfort and speeding up the recovery.
The first is Vitamin E in an oil/liquid form. Rubbing Vitamin E over the burnt skin helps soothe it and keep it moist. Generally Vitamin E oil is very sticky (much like honey), so you may wish to dilute it a bit with some mineral oil.
Another supplement that appears to soothe sunburn is apple cider vinegar. Cider vinegar is as close to a legitimate equine "cure-all" as there is, assisting with internal issues such as digestion as well as external issues such as burns. In the case of sunburn, apply some cider vinegar to the damaged areas to provide your horse a bit of relief.
Other than those two supplements, it's pretty much a waiting game. Observe your horse carefully for signs of trouble that extend beyond the skin itself, keep him as comfortable as possible, and be sure to provide ample protection before your horse is released again into the pasture.
Summer Isn't Your Only Enemy…
While it's true that summer is the most dangerous season when it comes to equine sunburns, don't make the mistake of underestimating the potency of the sun during the spring and fall months either; the sun is perfectly capable of burning an exposed horse during those periods too. Some regions suffer from hotter weather and nastier sun exposure than others, but even relatively "cool" regions such as the New England area, those Indian summers can pack quite a punch.
With proper vigilance and care, you can help keep your horse safe and comfortable no matter what the time of year or your location.