Iodine requirements increase with milk production levels, so feed rations in regions furthest from the sea are rich in brassica supplements. Selenium requirements are high in regions with acid soils.
Iodine is key to growth, lactation, breeding and fattening young cattle. Selenium raises the efficiency of iodine intake but protects the thyroid from latter's oxidant properties. Selenium combines with iodine, which is an oxidant, making a combination that has a marked effect on growth, boosting natural defences and breeding.