I would suggest talking to your vet to see what you need. Have you had the vet out to see if there is an underlying problem to her weight, or lack of? This year during the spring Jesse had lost about 100 lbs so we had the vet out to see if there was a health issue or he was just at the bottom of the pecking order (he's always been an easy keeper).
It ended up being that he was just on the bottom of the totem pole. So she gave him a viatamin shot and suggested to start feeding him Senior feed (our hay quality is bad also). Now he's almost completely back to his good weight.
"You know, for as long as I can remember, I've had memories." ~Colin Mochrie