Horses get all of the minerals they need from good quality forage and/or a balanced commercial ration. Over-supplementing them can cause a lack of absorption of necessary nutrients in the small intestine. It should be avoided.
Source: The Horse
Over-supplementation can lead to problems with proper absorption and nutrient balance. Good quality forage contains many essential minerals to meet basic needs. Properly formulated, fortified grain concentrates also provide essential vitamins and minerals.