Why does my horse eat oak leaves, branches and bark? Oaks have the reputation of being poisonous but contain many beneficial phytochemicals. The acorns contain rapidly degradable tannins that are quickly taken into the blood stream, whereas the leaves and bark contain tannins which are less able to be absorbed across the gut wall, increasing the benefits to the gut and reducing internal toxicity.
Oaks contain more tannins than any other plant meaning the horse needs to eat less, 1g will provide protection to the mucosal membrane, against the effects of an increase of gastric acid production (during times of stress). If your horse seems to be seeking out oak leaves and eating more than a few leaves, he may be using the increase to reduce gastric acid production by the proton pump, repairing and preventing hypersecretion. Tannins also inhibit the production of pepsin, produced by the equine gut in times of stress.
Horses with access to oaks produce proline (a protein) which protects them against the toxic effect of tannins, in other words the horse becomes conditioned to eating food containing tannins. Horses that have access to some oak leaves will use protein better and will also gain condition. Tannins reduce parasites, horses secrete more eggs, parasites also seem to be less fertile. tannins especially target nematodes or round worms, the most common to affect horses from this group is the small redworm.