Effects of Diets Magnesium on Acid-Base Status and Calcium Metabolism of Dry Cows Fed Acidogenic Salts

¹ Dairy Science Department, Institute of Food and Agricultural Sciences, University of Florida Gainesville 32611-0701

The objective was to study effects of dietary Mg on acid-base status and Ca metabolism of Holstein cows fed acidogenic diets with relatively high Ca concentrations. Eight nonlactating, nonpregnant Holstein cows were used in a switchback experiment with three 28-d periods. The normal Mg (.2%, dry basis) diet consisted of corn silage plus a concentrate mix supplemented with NH₄Cl (126 g/d per cow) and (NH₄)₂SO₄ (126 g/d per cow). The high Mg (.37%, dry basis) diet had MgSO₄ substituted for an equivalent amount of s supplied by (NH₄)₂SO₄ in the normal Mg diet. Cation-anion differences of the two diets were –302 (normal Mg) and –289 (high Mg) meq/kg of dietary DM. Compared with cows fed the normal Mg diet, those fed high Mg tended to have higher blood pH and plasma concentrations of total Ca but lower plasma concentrations of P and lower urinary excretion of ammonium and net acid. Cows fed the high Mg diet also tended to increase in urinary excretion with a decrease in urinary excretion of Ca. Metabolic responses to intravenous infusion of Na₂-EDTA were similar among cows fed either diet. Results indicate that increasing salts was of no advantage with regard to Ca metabolism.

Key Words: acid-base status • calcium • magnesium • cation-anion difference

Submitted on May 6, 1991
Accepted on October 9, 1991

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