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Magnesium Metabolism of Sheep Fed Different Levels of Potassium and Citric Acid¹

U. S. Plant, Soil and Nutrition Laboratory, Soil and Water Conservation Research Division, Agricultural Research Service, U. S. Department of Agriculture, Ithaca, New York 14850

Experiments using nutritional balance methods combined with measurements of ²⁸Mg kinetics were conducted to determine the effects of dietary potassium and citric acid on magnesium metabolism of wethers. Treatments in each of four trials consisted of feeding 1 kg/day of a control diet (treatment A, 0.17% Mg and 0.76% K) and diet A plus supplements of either 60 g KCl (treatment B), 30 g citric acid (treatment C), or both (treatment D). Plasma Mg concentration, Mg retention, and exchangeable Mg pool size were unaffected by the treatments, but pool size appeared to decline with age. A model consisting of two compartments (M₁ and M₂) was used to describe the distribution of exchangeable Mg within the sheep. Compartments M₁ and M₂ generally contained about 40 and 60% of the exchangeable Mg, respectively, although in treatment B the pool was distributed about equally between the compartments. Intercompartmental Mg flux decreased in response to K supplementation. Compared to animals fed the basal level of K, increased intake of KCl significantly (P < 0.05) depressed Mg absorption, reduced urinary Mg excretion, and lowered endogenous fecal Mg excretion, but elevated total fecal Mg output. Dietary citric acid, at the level provided (treatment C), had little effect on Mg metabolism. Inhibition of alimentary absorption of Mg and increased retention of Mg by the body cells are possible mechanisms for the development of hypomagnesemia and the altered partition of Mg between urine and feces of ruminants fed high levels of K.

Manuscript received 16 April 1971.