Firstly, yes, horses do eat at night. They eat & move a lot less, but still do. Horses are 'trickle feeders' built to have small amounts of feed going through their guts near constantly. So whether day or night, don't starve her! If you must dry lot her, ensure she has hay available, pref free choice, in a slow feed net. *If hay is rich, may need to soak & drain it before feeding to leach out some of the sugars.
Secondly, I'd see again if you can adjust or add another strap or some such to the muzzle to prevent her getting it off, as it would be far preferable to locking her up in solitary, if you can get it to work. She shouldn't be in the muzzle full time though, so I'd just put it on for the daylight hours.
Thridly, quit giving her extra high carb/calorie feed if she's already getting too much! Cut the oats!
Fourthly. Exercise. Like us, input needs to equal output, if we are to maintain a healthy weight & not get too thin or fat. If she's getting too many calories, more exercise will help burn some off.
While living with mates in a big paddock is obviously better & motivates more movement than a horse kept alone or confined to a small paddock/yard/stable, if she's only moseying around with her mates, she won't be getting much in the way of aerobic exercise. You can motivate more exercise in the paddock by alterning the environment - making pasture sparse & patchy, so they have to keep moving around, or putting them on a track system(google paddock paradise). But the more low impact exercise the better, so taking her out, either ridden or walking, esp on hills, for an hour or more daily will also help shed some.
Last edited by loosie; 07-23-2015 at 12:11 AM.