Thanks for asking this. I asked a similar question but didn't get any responses and then the thread got lost.
The response for this helps me, too.
Also, A week ago I started my 2 Sr. Horses on Fat Cat (a high protein, amino acid supplement). Wondering how it will work. One of them has some physical limitation to exercise but the other one (who I just got recently) is as sound as can be and worked several times a week so it will be interesting to see how it works individually for each of them.