Educate me? If you're going to be riding outside all winter long, why is your horse blanketed now? I would think that being blanketed would prevent her from growing a good hair coat and THAT would be very useful in keeping her warm. Do you show? It's been into the 40's and blowing down here and none of our horses are shivering....but then we don't blanket unless it's going to be extremely cold and wet at the same time.
Just responding about my particular horse.
My horse doesn't grow much of a winter coat. I don't know why, but he doesn't. He has about a 1/2 inch of fur, maybe a little more, and that is positively FUZZY for him. He tends to grow just enough hair to take a long time to dry. I haven't clipped him this year. When it drops into the 40's, my horse is uncomfortable. I visited him earlier to bring him inside for the hurricane and it was very rainy, about 51 degrees. He was under shelter, dry under his rain sheet, and he was *shivering*. He doesn't eat nearly as much as normal when he's cold (side note: Why don't the shelters at any barn I've been at have hay feeders? The hay and the shelter are always separate!) He's also a hair overweight (I figure in the winter a little too fat is better than a little too skinny). I brought him in, warmed him up and left him inside a much happier horse - bundled up in a thicker blanket with a neck.
He loves his blankie.
I don't consider him to be "normal" in that regard - he has no tolerance for cold and really hates the wet. He normally lives outside 24/7, but the barn is new (new as in just finished a few days ago with lots of finishing touches still going on), and he was jus so much happier inside with his blankie and hay net that I let him be.
Why he doesn't grow much winter coat, I have no idea. I tried waiting longer this year to start regularly sheeting him and he did not grow any more fur than he did last year. Maybe it's his breed/breeding (SE Arabian), maybe it's something else. I blanket him because he's a happier horse with the blankets on and I really don't mind providing them. If he was comfortable without, I'd leave him nekkid.