Magnesium should be consumed on a daily basis. Mature cows nursing calves and grazing new spring grass in western South Dakota where grass tetany frequently occurs, were protected from grass tetany by feeding 6 grams of supplemental magnesium daily. In Virginia, the magnesium requirement for lactating beef cows was found to be 18 to 22 grams daily to maintain serum magnesium at 2.0 milligrams per 100 milliliters of serum. The amount of supplemental magnesium for cows nursing calves in areas of Nebraska where grass tetany frequently occurs appears to be 9 to 15 grams per head daily, beginning at least 30 days before the normal grass tetany season. Up to 20 grams per day may be needed for heavy milking beef cows on pasture, or for cows grazing small grain or other lush spring pasture.
Magnesium oxide is one of the better and cheaper sources of magnesium.
Since magnesium oxide contains 54 to 60 percent magnesium, the rate of feeding magnesium is about .04 to .08 pound per head daily. As magnesium is not well liked, it should be included in a readily acceptable protein or mineral supplement. Adding magnesium oxide to a supplement at the rate of 75 to 150 pounds per ton when 1 pound per head is fed daily, or 37.5 to 75 pounds per ton when fed at the rate of 2 pounds per head daily, is the best way to provide supplemental magnesium for many cattlemen. It can be added to a high protein mix before cows go to grass. After the cows are turned to grass, a grain cube will probably be more economical and more palatable.