A lot of press has been given to feeding live yeast culture to horses. It claims to improve phytase activity of the hindgut of horses, which in turn improves phosphorous and fiber digestion. Claims have been made for it increasing milk quality, thereby increasing foal growth. What is known for certain about live yeast is that it is a good source of B vitamins. Also it is known that live yeast, unless protected in some way, would be killed in the stomach before it would get to the hindgut.
More research needs to be done before supporting other claims.
Enzymes fed to a horse will be digested in the stomach and small intestine before getting to the hindgut where they are to improve fiber digestion. Therefore, unless these enzymes are somehow protected until reaching the hindgut, they are probably of no importance in fiber digestion as claimed.