I still can’t get through to that site. Here is my favorite probiotcs
https://www.horseherbs.com/products/probioplus
I thought they had saccarides yeast but now the label just says nutrtional yeast.
I also have a human probiotic on hand Florastor that is sacchyrides b. yeast. It clears up bouts of diarrhea effectively.
The herbs for horses product has been effective for me in correcting diet induced diarrhea for Instance switching from pasture to hay.
A big question would be: what part of the digestive system are you hoping to improve? The stomach, the small intestine, or the colon? Different things go on in each place. Different problems, different solutions. The further back you go the harder to be sure the remedy is making it through. Things go into the stomach more reliably.
On the other hand problems in small intestine digestion can create problems in the colon if undigested carbs get through. And medication for stomach ulcers can cause colon problems.
So a first question is: what digestive problems do you want the yeast to fix?
Also what do you mean by gluton free in the context of a horse diet? The only possible thing a horse could eat with gluten in it is wheat, and that is only present in small amounts in some commercial feeds. Nothing else in a horses diet has gluten in it, including oats, corn, bran, flax, etc.
On the other hand, undigested carbs can impact the colon which is intended to ferment fibre.
I think probiotics including yeast are relatively harmless and it can’t hurt to try them. But if your mare does not have obvious digestive problems like diarrhea or gas or hind gut colic my guess is they won’t have a lot of effect.
These may also be of interest.
https://www.ncbi.nlm.nih.gov/m/pubmed/23161811/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4895607/