They eat their grain in their stalls but the hay outside. Separating them for that really isn't an option (time and herd bound issues - which I know can be retrained but at 30+, I'm not going to start now).

They are certainly getting enough good quality senior feed, and more from the supplement, so I will look into the alfalfa pellets if he doesn't improve. Good to know that the hollowness could be coming from a deficiency rather than just him going downhill fast.

While I consider/search for the pellets I'm going to get some better-quality grass hay in the meantime. The stuff I have now is fine for maintenance horses (my other guy mostly needs to eat to keep busy!) but now that they're getting older they actually do need the nutrients.

Hopefully it'll rain and the grass will finally start to grow, too! Then I can start weaning them back onto pasture. That'll probably also work wonders. They've always slimmed down this time of year, but this year just seems worse than others.