Answer

By Your Horse

Nutritionist Katie Williams advises:

As travelling causes stress to the digestive system, a supplement designed to promote gut health should be beneficial.
Prebiotics are materials that are not live, but stimulate the microbial population in the digestive tract. Some types of prebiotic such as fructo oligosccharides (FOS) provide food for beneficial bacteria, whereas others like mannan oligosaccharides (MOS) stick to harmful bacteria, preventing them from becoming established. Yeast can also be used to promote gut health and improve fibre digestion, and is referred to as a probiotic.
Inactive yeast or yeast metabolites can be used, which are thought to provide the raw materials that bacteria in the gut need to function effectively. However, probiotics that contain live bacteria rather than live yeast are not approved for horses under EU legislation.
Supplements that would be suitable for your mare include Dengie’s Digestive Health Plus, Dodson & Horrell’s Sure Bio, or Baileys’ Digest Plus.