The other day, someone asked me if her horse should be blanketed in the winter or not. My friend has her horse on "inside board" at the barn. The horse is turned out in the pasture during the day but is brought inside to a stall at night. She has a standing order to have the horse blanketed if the temperature is 10 degrees or colder (which in Pequot Lakes it frequently is!).
Is it really necessary to blanket a horse that's outside during the day? Personally, I think not. I have the same situation with my horse at the stable – in at night and out during the day – and I haven’t ever blanketed her. She does just fine, even in the coldest weather, since she has a nice, thick coat and she's definitely not thin.
Of course, if I have been riding her hard in the indoor arena and she is hot and sweaty, I won’t turn her out into the pasture like that. At that point she goes into her stall until she has a chance to dry off and cool down.
That being said, if a horse is too thin, old or ill and has trouble staying warm, then I would recommend having a blanket put on him. Another reason for blanketing would be if you are trying to keep a horse from getting a thick coat to make it easier to shed out early in the spring – mainly for showing. But once you start doing this, you have to continue to blanket all through winter, because your horse won't have had a chance to get a proper winter coat. You also need to realize that when he’s had a blanket on continuously, his coat will get a little compacted and he will lose the insulating factor of having the hair fluffed up and holding air in among the hair.
In my experience, unless you have a horse as stated above – old, thin or ill – blanketing your horse in winter is more to make you feel better than the horse!