If you have a horse whose eyes are bigger than its belly then you should try to limit the amount of food it can access. Here at Burnhills we would recommend investing in a grazing muzzle, which would allow your horse to remain in its field in the herd but prevent it from over grazing.
The Shires Grazing muzzle comes in the standard nylon or if your horse is prone to skin rubs then take a look at the Deluxe Grazing Muzzle.