Approximately 70 percent of the body stores of magnesium are in the bone. It is an activator of numerous enzyme systems and affects the nervous system. A magnesium deficiency rarely occurs in feedlot cattle. Grass or winter tetany, a condition that causes many death losses in Missouri beef cow herds, is associated with low magnesium levels in blood serum. Cattle need about 0.04 to 0.1 percent magnesium in the dry matter of their ration.
Potassium is required for many body functions.
Thus, a deficiency results in non-specific symptoms such as poor appetite, followed by thinness, reduced performance and stiffness, especially in the joints of the front legs. There is no evidence that potassium is needed in feedlot rations that contain sizable amounts of silage or other roughage. Rations containing molasses and alfalfa meal are not likely to be deficient in potassium. University of Missouri-Columbia studies have shown potassium will leach from the forage in fescue pasture during winter to levels as low as 0.3 percent of the dry matter. Potassium additions could be beneficial for cattle grazing these pastures from January through March.