I think the answer might depend on each horse and their age.
My two mid-20's Fellas are on some form of microorganisms for the rest of their lives.
The 25 yr old TWH with the hind gut ulcers is on "Succeed", which is a pre-biotic.
The 26+ Arab with gastric stomach ulcers is on Daily Start by EquiShine because it's a pre-probiotic. I've started him on this, so it's too soon to tell if it's going to help him.
It's tought to find an affordable product that is both a prebiotic and a probiotic.
Prebiotics are Oligosaccharides,so anytime you see that word (that I copy/pasted
, there are prebiotics in the product.
This is a credible article that describes the difference between pre and probiotics. The Horse | Pre- and Probiotics for Horses | TheHorse.com