In order to have a hedge that horses will not eat or damage in other ways such as rubbing on it, it would be best to have the hedge outside the pasture or other areas where the horses are kept. Plant the shrubs far enough away that the horses will not reach them over the fence. With many plants, limited information on their safety for animals is available because no incidents have been reported of horses eating the plant.
Avoid planting any hedges from the Japanese yew family. They are known to be toxic to horses.
Yews contain a group of highly toxic alkaloids. All parts of the plant — green or dried, except for the fleshy part of the aril — surrounding the seed are toxic. The highest concentration of alkaloid is generally found in the leaves in winter time. Adult horses have been fatally poisoned with as little as 8 to 16 ounces of yew leaves. Dried leaves are toxic.
The Guide to Poisonous Plants Web site at Colorado State University is an excellent site to refer to as you select a hedge variety. It will indicate whether your choice would be potentially toxic to horses.
To protect your horses and your hedge, keep them separated by a fence.
