I would feed Purina over Nutrena, and in horse feed, Purina is actually pretty good, but Pennfield, and Triple Crown are the best, and so far every where I've been, they sell Triple Crown. Pennfield I don't think is nearly as common. I would have an actual equine dentist come out and look at her teeth, as they can give you a much better opinion than a vet, and the one in my area actually charges a bit less for a general teeth "floating" session than the vet. I would also suggest adding something like Farnum Weight Builder, or weight building supplement as well. I have a hard time believing that her teeth are fine if she's dropping a lot of feed, and not keeping weight on. I haven't ever heard of Buckeye or Progressive, so I'll have to ask my equine nutritionist friend of mine for my own curiosity, but I would definitely go with a senior feed over a general sweet feed type of feed. For my dads 30 some odd year old horse, we fed equine senior, rice bran, 4-flex, and cob, and we soaked it so it was nice and mushy for him. Knowing what I do now, I would have changed a few things, but that kept him in good weight up until the day we put him down, and he was still running barrels and jumping small fences.