Vit A is a fat soluble vitamin. Excess is stored in the liver. Pasture and fresh hay have an abundant supply of Vit A. As the grass in the pasture dies and hay ages in the barn, the level of Vit A declines. At first, your horse will pull reserves from the liver but overtime that supply is depleted. By about Christmas, you probably start to see the beginnings of RR and runny eyes that are some of the signs of vitamin A deficiencies. If that's the origin of his problems, it's a very simple thing to deal with. Just get on it by Thanksgiving and once a month hit him with the A until the first green grass starts poking through.