ASince there are so little hard data on any of these products, especially in horses, I can’t address these very specific questions. The activated yeast products do confer slight benefits in digestion in some studies. As far as the bacteria/probiotics go, no one really knows other than testimonials, as far as I know.

Single-species inoculants (products with only one bacterial or yeast species), however, do not make good sense in that the population of the equine hindgut is very complex. It has multiple bacterial and protozoal species that are all assumed to serve specific functions in the digestive process, but which have been very poorly characterized. There is some cattle and human literature that indicates benefits from probiotics, but both species are quite different in their digestive processes than the horse!

I would use such products with caution and carefully follow the manufacturer’s directions. If benefits aren’t seen in a reasonable period of time (a week or two), or any sort of adverse effect appears (such as diarrhea, colic, or poor appetite), I’d discontinue their use immediately