We're down south also though. It doesn't get as cold here in the winters as it does in PA. That would add... I used to be of the opinion that some horses just genetically grow longer coats than others... until my friend moved to northern- Michigan and all of her southern-born horses started growing coats 4 inches long! But maybe some just do...? I do know when I have a horse that is overweight during the cold months, he/she does not grow a very thick coat. So I suppose the body fat is partly a contributor.