Cut it back to half a can AM and PM and see how her weight is. You can always substitute her grain for something like beet pulp (does nothing but add on weight/not an energy giver) OR there is a feed by Purina that is specifically for weight gain but im not too sure what its called. Use those two options if weight begin to be an issue.
As long as she has 24/7 access to hay she should be fine.
A Dog looks up to a man,
A cat looks down on a man,
But a patient horse looks a man in the eye and sees him as an equal