I also feed Purina Equine Senior for all my older horses. I chose this one as it could also be fed as a complete feed. My horses still have enough teeth to eat hay. In addition to any of the hay substitutes, you can also add a quarter to a half cup of vegtable oil to the Purina Senior).
I have 2 big horses (Dutch Warmblood Mare and a Quarter Horse Gelding).
They are the same height and weight. The mare gets a half gallon twice a day, while the gelding half a quart twice a day.