Can We Differentiate Supplemental Magnesium Sources Nutritionally?

A laboratory test to differentiate the apparent availability of Mg among supplemental sources would be useful. Evaluation directly with cattle to test the reactivity, solublization (dissolution), and apparent absorption of Mg from various Mg sources in the rumen is tedious, laborious, and expensive. The degree of reactivity in the rumen and release of soluble Mg in the primary absorption site (the rumen) is key to differentiating among various sources. In addition to their solubility, other primary factors that affect solubility include origin (source), specific chemical compound, proper calcining process of magnesium carbonate to yield magnesium oxide (MgO), particle size, and other chemical compounds (e.g., potassium, calcium) in the diet and rumen. When supplemental sources of Mg were tested in the laboratory for solubility, sometimes, but not always, improved lactational performance was detected. Unfortunately, determination and ranking of Mg sources by solubility as an indicator of apparent availability does not appear to be a very reliable test. The “vinegar test” proposed by Goff (2014) is a simple way to characterize the reactivity and alkalizing property of various MgO sources. Test sources that raise the pH in a solution of vinegar (5% acetic acid) are more alkalizing. This also might provide indirect evidence that some MgO sources are more reactive and yield more soluble Mg for absorption from the rumen than others. However, MgO sources ranking differently in the vinegar test have not been evaluated with dairy cows by measuring differences in apparent digestibility or lactational performance to verify that the test reliably differentiated sources for apparent Mg availability.