Soy is the protein source for 99.9% of horse feeds, ration balancers, etc.
It is high in amino acids which is good to help maintain muscles.
However, soy can also exacerbate inflammation, and have negative personality affects on some horses. I have a horse with soy intolerance; he turns into a real nut case if he eats anything with soy in it.
Alfalfa is the next best choice for feeding high protein and amino acids. That can be over done as well.
Many horses cannot tolerate eating a lot of alfalfa as it amps them up too much.
Also, you know you're feeding too much alfalfa (or soy) if the urine starts to have a stronger than normal ammonia smell. That is how the horse dumps the extra protein his body doesn't need.
At 15, your TB may be old enough to tolerate some extra alfalfa and then again, he might not
When my now 25 yr old TWH was 15, the one pound daily of alfalfa cubes he now gets would have sent him right thru the barn roof.
He also has metabolic issues but is one of the metabolic horses that can tolerate alfalfa (soy is completely out for him).
Equine Metabolic Syndrome (EMS) strikes at muscle mass and the alfalfa has helped him to some degree. I don't ride him but he has 15 acres of hills to wander on, so gets plenty of daily exercise.
These pics were taken for his 25th birthday last Fall. In spite of his health issues, he's doing very well and I think his topline looks pretty good, all things considered. Happy Bday to my 25 yr old TWH
Is the one pound daily of alfalfa I feed him, the magic bullet? No but I feel like it has helped