I would also highly recommend Purena Senior Feed. I had a Belgian mare that could not eat hay because she had no teeth. I started feeding her the Senior Feed and within about a week she had put on weight and was looking healthy again. Later on I also tried substituting some of the Senior Feed for beet pellets, but even with very few beet pellets mixed in she didn’t like it at all. So if your horse is a really picky eater she may not like the beet pellets.
Yes, I know Senior Feed is a bit expensive, but it is a more complete feed than just adding extra grain would be. With my Belgian mare I went through one 50lb bag in two days(!), but that was all she could eat.