If a horse has access to pasture and a trace mineral salt block, they do require additional nutrients. Trace Mineral salt blocks are typically 97% salt and 3% trace minerals. Trace minerals often included in these blocks are zinc, iron, manganese, copper, cobalt, and iodine. Pasture and trace mineral salt will NOT satisfy the nutrient requirements of horses. This is especially true if horses and keep in groups and one horse is at the bottom of the pecking order. Selenium is rarely added to a trace mineral salt block and is lacking in soils and pastures in most parts of the world.