So, Triple Crown's website says their senior feed can be fed as the sole ration. It's not actually a grain, but contains alfalfa, beet pulp (which is a good ration), etc... So it's possible that she just finds the senior feed more palatable.
However, if you have to rely only on the senior feed, she'll need more than what you're feeding her. And it would have to be fed in several small meals throughout the day.
Anyway, you'll probably still want to talk to a vet and try to get her going on hay again.