Magnesium for Gestating Beef Cows

T HREE experiments were conducted with eight gestating Angus cows to study the effects of feeding different magnesium levels at 145,200 and 255 days gestation. Each trial consisted of a S-day adjustment period and a 10-day experimental period. The average daily magnesium intake for the cows fed the basal ration was 3.4 grams. Supplemental magnesium was added at levels of 0, 4.1, 8.2 and 16.4 g per day. Blood and urine samples were taken at the end of the adjustment periods and on alternate days during the experimental periods. At the three stages of gestation, increases in dietary magnesium significantly increased serum magnesium levels. Serum calcium and inorganic phosphorus levels were not consistently altered by the dietary magnesium level. Increases in dietary magnesium significantly increased urinary magnesium values per unit of creatinine. As calculated from regression equations obtained at each stage of gestation for the effect of dietary magnesium on serum magnesium, 8.5, 7.0, and 9.0 g of dietary magnesium would be required per day to maintain serum magnesium levels of 2.0 mg per 100 ml at 155, 200, and 255 days gestation, respectively.