Ruminal Magnesium Transport Revisited: Apparent Absorption and Estimate of Requirement.

The pathogenesis of hypomagnesemia is caused in most cases by impaired magnesium (Mg) absorption and not by an insufficient Mg intake. Recent improved knowledge about the site and mechanism of ruminal Mg absorption allows studies of modulating factors regarding Mg absorption in vitro, in vivo, with the washed-rumen technique, and with classical balance studies. The obtained data show: a depressive effect of K excess and Na deficiency, transient inhibition by high ammonia concentration and stimulation by CO2 and short-chain fatty acids, the impact of pH and solubility of Mg in the rumen fluid and the influence of the rumen volume and the outflow on Mg absorption. Current knowledge allows a reliable prediction of apparent Mg absorption with increasing K intake and the recommendation of an adequate Mg intake for dairy cows. Furthermore, the analysis of data from studies with dairy cows shows that the content of nitrogen, Na (≥ requirement) or fat in the diet has no effect on Mg absorption. However, hypomagnesemia is still a problem in grazing ruminants without sufficient supplementation.