My 3 year old mare I just got needs to put some weight on. She has very little muscle tone and is boney. They kept her in a muddy pasture and fed her about a 1.5lbs of feed twice a day and a few handfuls of alphalfa twice a day she didn't have any grass to forage on so that's all she ate. right now i'm giving her moorman's patriot pellets, moorman's natural glo nuggets, some alphalfa and she has plenty of coastal hay to eat?

How do I help her build up some muscle?