I'd have your vet test her for IR and Cushings, just to be on the safe side. If she does have one of those, there are supplements that can help manage her weight.
I would not give her much alfalfa. Alfalfa is high in protein and digestible energy. My boy is on cheap, meduim to low quality mixed grass hay, and he still gets fat, lol. If I give him good hay, he blows up fatter than your girl! I'd rather give a lower quality hay (supplemented with a little alfalfa and vitamins) and more of it, than have to restrict the hay even more and give good hay. I feed my boy 25-35 lbs of hay a day and he's 14.2h and 1,000 lbs.