This mineral is required by the animal but it is not made in the animal. it plays a key role in immune response (function) and iodine metabolism. Grazing animals may be at more of a risk to deficiency.
Its main role in the body is as an antioxidant enzyme glutathione peroxidase. We can measure this in blood to help look at mineral status (long term) while plasma serum levels are used to measure short term intakes.