Most grass has a lot of sugar in it, especially during spring and early summer. Muzzle or dry lot are your best options for keeping the weight off your horses. Just like in humans, if ‘calories in’ equals more than ‘calories out’, they will gain weight. And calories from sugar have added health side-effects. ALL of my equines (2 horses, 2 ponies) get muzzled during the day and put on a dry lot at night during spring and early summer. My older mare, with equine metabolic syndrome, is muzzled March-December. Just like with humans, there is no “magic diet pill.” I know it can be hard to muzzle them because it can make you feel guilty, but after putting a horse down due to severe founder, I’ve come to realize that I have to do what is best for the horse, not what makes me feel better. Good luck!