If you can feed your own grain, aside from the sweet feed they are feeding, I would suggest it. You'll have to work up from half a can once a day to twice a day, eventually over the next 2-3 weeks a can 2-3 times a day. By can, I mean coffee can.
I would buy a 50lb bag of senior feed pellets, a 50lb bag of mare/foal feed, and a 50lb bag of steamed crimped oats. I would also add beetpulp, and a supplement I believe is called Omega, or a sprinkle supplement called FoalGrow. Yes, it'll be a big chunk of money all at once, but it will seem to last forever.
This mixture is what I use for each and every one of my rescues. Now that I've been through about 20 of them, I'm pretty confident in the mix.
And being on grass, 9/10 times she won't touch the hay until she runs out of grass. The dry stuff is no where near as tasty as the green, juicy stuff!