Best Vit/Min Supplement?

I apologize if I am repeating something that must have been repeatedly posted before... For a horse on 24/7 pasture/hay with a couple of handfuls of senior feed a day, what do you all suggest for a supplement? The hay now is grass hay, don't know the details of its nutritional content--he gets a flake or two of alfalfa when he comes in to eat once a day. Even though he is an 11 yr. Old (?) Thoroughbred he is an easy keeper (way unlike my last TB, who was skinny and would never finish his grain), working lightly 3 - 4 times a week. He seems to be a perfect weight for the winter just now, very slightly leaning to the plump side. But I want to be sure he is getting all the nutrition he needs. Any suggestions?

Instead of the senior look for a ration balancer it is a vitamin/mineral supplement on steriods :) Provides more nutrition then any other supplement and elimanates most horses need for grains IF they are not super hard keepers

Here this might help you abit
What is a ration balancer and where can you find one?

I highly suggest finding a locally made ration balancer. I use Hoffman's Horse Ration for all of my horses, and because it is formulated for my area it is all I need to use.
Also, scrap the senior feed that stuff usually does more harm than good.

Do you have reason to believe that they hay you are feeding isn't good enough quality?
I personally believe that we (as horse owners) over-supplement our horses. If you're worried, I would suggest getting an analysis done on your hay, and go from there. Many horses don't need anything more than a good quality hay.
A good nutritional supplement that I can think of off the top of my head is Equi-Liq.