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Factors Influencing Magnesium Absorption and Metabolism in Ruminants¹

Virginia Polytechnic Institute and State University, Blacksburg 24061 and U.S. Department of Agriculture, Ames, IA 50010

Abstract

Magnesium is a nutrient required for all animals, but it is especially critical for ruminants. A physiological deficiency of Mg results in hypomagnesemic tetany. Typically, only female ruminants are affected, and the disturbance usually occurs during the early stages of lactation. Magnesium functions at three biochemical levels, as a cofactor at the enzymatic level, at the structural level in assembly of ribosomes, and at the whole cell level as a stabilizing force in membranes. Parathyroid hormone (PTH) can affect Mg metabolism by decreasing urinary Mg excretion and stimulating bone resorption, thus releasing Mg into the extracellular fluid. Renal excretion of Mg and Mg absorption from the gastrointestinal tract both are increased by 1,25-dihydroxyvitamin D₃. A number of dietary factors depress Mg absorption in ruminants. Of these, high dietary K has the greatest and most consistent effect. Feeding substantial quantities of readily digested carbohydrates increases Mg absorption, but the mode of action is not clear. High concentrations of Al in forage sometimes are associated with a high incidence of grass tetany, but this effect does not appear to be related to Mg absorption. Interrelationships of Al with Mg, Ca, P and PTH are implicated.

¹ Presented at a symposium titled "Magnesium Metabolism and Grass Tetany" at the ASAS 80th Annu. Mtg., New Brunswick, NJ.

² Virginia Polytechnic Inst. and State Univ.

³ USDA-ARS, Natl. Anim. Dis. Center, Ames.

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