Essential nutrients required in very small amounts as part of the diet. Requirements have been established in all classes of horses for Iron, Zinc, Copper, Manganese, Cobalt, Selenium, and Iodine with others such as Chromium estimated.
Normally supplied naturally by the plant based diet, some soils and hence the plants growing on them are deficient. Deficient soils are not uncommon in Australia, particularly for Zinc, Copper and Selenium so horses grazing on pasture may require supplements including trace elements to avoid deficiency status. Adequacy of trace element supply is especially important in broodmares and young growing horses, so stud rations are commonly fortified with this in mind.