I don't feed anything but hay unless my horses are working. I get all of my hay in the late spring/early summer - coastal/rye mix, fresh out of the field and buy enough to last me the year (round bales), store it out of the sun and I keep it in front of them free choice. Of course, they have a mineral block out there, too.
Now when conditioning for distance:
I feed Strategy DX
Probiotics (as needed per horse/individual situation)
Then I add a little Red Cell from time to time as a blood builder since hard work can cause a slight anemia - but then I really get into all of the metabolics (I love the clinical pathology side of it) and look at my horses' blood all the time under the microscope and develop individual plans to meet their needs.
Yes, I'm over the top sometimes...LOL!