Before they can reach the intestines, commercially available supplements partially break down in the animals' stomachs, which can lead to inflammation of the stomach lining. So if you really feel the need to give a horse prebiotics, preparing food supplements so that they don't take effect until they reach the large intestine.
In horses, prebiotics are are indigestible fibers that hope to create certain beneficial bacteria in the large intestine. "Horses have a relatively small, non-diverse core microbiome and are therefore very susceptible to digestive disorders," explains Professor Annette Zeyner, head of the animal nutrition group at Martin-Luther-Universität Halle-Wittenberg.
There isn't evidence that it actually helps.
For the study, the team investigated the effect of using Jerusalem artichoke meal (JAM), a typical prebiotic for horses. In addition to their normal feed, six animals received JAM containing high amounts of certain carbohydrates, so-called fructo-oligosaccharides (FOS) and also inulin. Another group of six horses received a placebo with their normal feed. The researchers then analysed the balance of bacteria in the digestive tract of the animals of both groups. It was discovered that the prebiotics were already being fermented in the stomach by the microorganisms naturally living there - i.e. they were taking effect much too early.
However, the bacterial diversity of the entire digestive tract did increase, which probably also produces the desired protective effect. "Still, the prebiotics are probably more harmful than beneficial when used in their present form," according to Zeyner surmises.