New concepts and developments in metabolism and homeostasis of inorganic elements in dairy cattle. A review.

Major routes of homeostatic control animals use to adapt to widely variable intakes of minerals and the necessity of keeping tissue content of functional forms relatively constant are changes in: (1) percentage absorbed, (2) urinary excretion, (3) tissue deposition in harmless or mobilizable reserve forms, (4) secretion into milk, and (5) endogenous excretion via feces. The importance of each varies greatly among elements. Changes in absorption are of major importance with calcium, iron, zinc, and manganese but not with cadmium or iodine. Urine is a major control route for magnesium, fluorine, selenium, iodine, sodium, chlorine, and potassium. Milk iodine varies almost directly with intake. Tissue deposition and withdrawal are key routes for iron, copper, fluorine, and calcium. Endogenous (metabolic fecal) excretion is crucial in manganese homeostasis. Biochemical details of most homeostatic control mechanisms have not been established. Apparently changes in body tissues control percentage absorption of several elements, including zinc, at the intestine which is the control site. Other major factors which may affect metabolism of several elements include chemical form, whether the elements include chemical form, whether the element is absorbed or injected, and genetic differences among animals. Several common pitfalls and special research problems are discussed. New research indicates that silicon, vanadium, nickel, tin, and fluorine are probably essential.