How does she look? If she looks thin, add more. If she looks heavy, feed less.
I would be leery of keeping a Miniature Horse on enough pasture to require no hay, myself. I have found that they do best in a dry lot, fed hay, offered limited pasture. They can get to heavy quickly and be at risk for laminitis. I also do not see any reason to feed alfalfa. For what its worth, our seven minis are on about 1/2 bale a day of a very grassy (Timothy) hay. About 30 pounds of hay between the lot of them, and it is the dead of winter, several feet of snow and cold. They each get 1 cup of a low sugar pelleted feed, also.
Be careful with the spring grass!