I would just like to make a comment here:
First of all, if a horse shows a definitive need for probiotics, there is a very natural way for any horse to get the RIGHT BACTERIA that actually help the digestive process and the immune system with equine specific bacteria and enzymes.
It is called manure.
When a newborn foal starts to eat solid food, one of the first things it eats it its mothers manure. They all do it. If you keep their stalls and pens so clean that there is no manure in them, they do not grow or do well. They have to have it.
When I was young, manure was the only probiotic available and it was VERY effective. You keep a fresh pile in front of horses that have been sick and are not doing well and they will eat it like candy. I knew old Vets back in the 60s and 70s that prescribed 'manure tea'. They prescribed soaking manure from a fat healthy horse, straining it through a piece of cloth and using it to make bran mashes.
If I have a recently sick or puny horse, I will keep fresh manure in front of it that comes from a parasite free 'fat', 'easy keeper' that is healthy and on on a similar diet.
It is cheap, effective, has the 'right' bacteria in it and it is 'natural'.