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Apparent Magnesium Absorption in Dry Cows Fed at 3 Levels of Potassium and 2 Levels of Magnesium Intake^*

Department of Nutrition, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands

Corresponding author: J. T. Schonewille; e-mail: j.schonewille{at}vet.uu.nl.

In vitro experiments with isolated rumen epithelium have shown that the relationship between the ruminal K concentration and either the apical membrane potential difference or the mucosal-to-serosal Mg flux reach plateau values at high ruminal K concentrations. Hence, it may be hypothesized that the inhibitory effect of supplemental K on Mg absorption becomes smaller at high initial K intakes. To test our hypothesis, 6 ruminally fistulated, nonpregnant dry cows were fed 6 experimental diets in a 6 x 6 Latin square design with a 2 x 3 factorial arrangement of treatments. Four cows were of a Friesian-Holstein x Holstein-Friesian cross, and the 2 remaining cows were of a Meuse-Rhine-IJssel x Holstein-Friesian cross. The diets provided either 40.6 or 69.1 g of Mg per day and contained 20.7, 48.0, or 75.5 g of K per kilogram of dry matter. The dietary variables were obtained by mixing KHCO₃ and MgO into the basal concentrate. Absorption of Mg and the urinary Mg excretion was significantly decreased by supplemental K and significantly increased after the intake of supplemental Mg. In contrast to apparent Mg absorption, the urinary excretion of Mg was not affected by the dietary K x Mg interaction. Postfeeding ruminal K and Mg concentrations were increased with increasing K and Mg intakes. Postfeeding ruminal K concentrations and the urinary excretion of Mg showed a linear negative correlation; the slope was not significantly affected by Mg intake. Therefore, our hypothesis was rejected. Furthermore, these data indicate that supplemental Mg can effectively counteract the suppressant effect of K on Mg absorption in cows.

Key Words: magnesium • potassium • absorption • cow

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