Good-quality forage found in pastures goes a long way in meeting a horse's nutrient requirements. Nutrients abundant in green grass are water, digestible fibers, protein, vitamin A (in carotenes), vitamin E and vitamin D (from sunlight). There can be sufficient amounts of macrominerals such as calcium, phosphorus and magnesium. Microminerals like zinc, copper and manganese are present but may be inadequate. Selenium can be deficient, adequate or excessive depending on the area of the country. Sodium is usually low in the forage, so free-choice salt (sodium chloride) should always be available. If the pasture is poor, either in quality or quantity, then the supply of the above-mentioned nutrients would be less. 

Three general types of supplements may be used to balance nutrients that are deficient or missing in the forage, and these vary in what nutrients they offer: vitamin/mineral fortified salt (formulated for horses), vitamin/mineral supplement (fed in small quantities, only appropriate if more calories or protein are not necessary), or commercial concentrate (contain additional calories and protein; the amount of calories needed will dictate whether it would be a low-intake balancer type or a regular commercial feed).