What SAcres said. It’s not mean to do what you need to do to limit your horse’s intake of food. While you can limit how much hay they eat without a muzzle (like using a slow feeder net)…you can’t tell the horse, “no, don’t eat all the grass you want even though you’re surrounded by its deliciousness.”
My horse is an easy keeper and out on pasture 24/7. He’s got a grazing muzzle at the ready if necessary and I’ve also had the BO cut back on how much regular food he gets.