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Three experiments were conducted with eight lactating Angus cows to study the effects of dietary magnesium levels at early, mid- and late lactation. The individual experiments consisted of two consecutive 15-day trials. Each trial consisted of a 5-day adjustment period and a 10-day experimental period. During the experimental periods the total magnesium content of the four rations averaged 9.7, 20.6, 31.0 and 42.4 gm. per day. Blood and urine samples were taken at the end of the adjustment periods and on alternate days during the experimental periods. Generally, increasing the level of dietary magnesium significantly increased blood serum magnesium during the three stages of lactation but did not influence serum calcium or inorganic phosphorus values. In general, urinary magnesium per unit of creatinine was significantly increased as the level of dietary magnesium increased. As calculated from the regression equations obtained at each stage of lactation for the effect of dietary magnesium on serum magnesium, 20.9, 22.1 and 18.0 gm. per day of dietary magnesium would be required to maintain serum magnesium levels of 2.0 mg. per 100 ml. during early, mid- and late lactation, respectively.

Three 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 5-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.