Magnesium is closely related to calcium and phosphorus in its dispersion and functions in the body. Most of the body’s magnesium is found in bones and muscles. Grass tetany, which is characterized by low magnesium in plasma and cerebrospinal fluid, typically occurs in lactating animals either grazing lush, spring pasture high in potassium and low in calcium and magnesium or those fed harvested forages low in magnesium. It appears the incidence of grass tetany increases in cattle grazing lush, spring pastures with applied nitrogen and potassium.
aAlthough forages contain about twice as much magnesium as cereal grains, its content depends on plant species, soil magnesium, and stage of growth, season and environment. Several studies have shown that apparent absorption of magnesium from forges ranged from 20-45%. Supplementation of ruminants with various sources of readily fermentable carbohydrates has improved magnesium utilization. It is generally accepted that absorption of magnesium from intestinal tract and tubular resorption of magnesium in the kidney are lower in older animals than in the younger animals. Magnesium in the sulphate > acetate and > chloride forms was better bioavailable in ruminants. Magnesium oxide also has been a good source of supplementation due to higher Mg content and moderately better bioavailability.