Garlic

Research has not proven whether low doses of garlic are beneficial or harmful to horses. No well-designed studies have been done on the relationships between garlic and horses, although some studies have been done on the relationship between onions (garlic's well-known cousin) and horses, when given in high doses.

Both onions and garlic contain a toxin that damages red blood cells, which carry oxygen in your horse's blood. When red blood cells become damaged, the spleen removes them from the system. When onions are fed on a regular basis, the toxin they contain continues to damage red blood cells, forcing the spleen to constantly remove them from the horse's system. This activity will eventually cause the horse to become anemic, meaning that its body cannot release enough red blood cells to make up for the damaged ones. Outward symptoms of anemia can include a yellowish discoloration of the whites of the eyes and gums, and reddish-colored urine. In horses fed low doses of onion, anemia symptoms may show up as decreased stamina and energy level and lower disease resistance.