I would make sure you switch her slowly to your new grain. The hay she might not like that much. She'll eat it when she gets hungry enough
. Make sure there are 50% more piles of hay than horses. So if you have 4 horses, you should have at least 6 piles of hay. I like to do double the hay piles. That way the horses have to move around from pile to pile to eat, and no one gets left out.