Our 25-26 yr old gelding gets a mash made from 3 lbs Purina Equine Senior, with an "Ultimate Joint" supplement added, and a tbsp of Spirulina, when I have it. He also gets about 1/2 flake (5 lbs) of Alfalfa hay daily and free access to Bermuda hay. I like the Purina Equine Senior because it is designed for making a mash, as well as being complete nutrition for a horse who can no longer chew grass/hay. Chief is not that far, but he doesn't chew up the hay well enough to get the full benefit of it. Because that is what it is designed for, the grain becomes a mash within just a few minutes of having hot water added, and I don't have to worry about other supplements.
He is fed according to our vet's direction. (Except the free-fed bermuda part. She had given us a quantity to give him every day, I prefer to leave him with free access to it. Esp since there is no longer really any green grass in the pasture for him.). Vet did not specifically recommend the joint supplement but when I asked about it, she said it would probably be a good idea. Right now, I am just using the one available at our feed store.