I recently had to euthanize a late-20’s Morgan mare who’d experienced years of strange neurological symptoms the vets thought was probably EPM, but she was never sick enough to test (everything else ruled out). Along with that she was morbidly obese year-around, but I did find a few things that helped significantly without taking her off pasture or muzzling.
First make sure your horse is getting a quality source of minerals, as I think they can be sadly lacking or imbalanced. My favorite so far is Hiland’s Big Sky minerals (http://bigskymineral.com/hilands-big-sky-mineral/) and I fed an unusually large Morgan just 2 oz per day (half the recommended). Because of her age and neurological complications I really never had her tested for EMS or IR, but just decided to try the SmartControl IR pellets to see what would happen. Again some improvement but not drastic, but then I ordered the SmartPak Metabo-Lean and saw much better results. She remained overweight, but not morbidly obese and moved a whole lot better!
According to the bare hoof trimming expert I trained with over a decade ago, limiting quality hay, grain and supplements when horses are on unlimited pasture can have “reverse” effects. In their need for whatever nutrient(s) are missing they over-eat to compensate–makes sense to me. I’d definitely try experimenting with her diet and you may find that more of something translates to less fat! Good luck and let us know how it goes…