Availability of Magnesium from Magnesium Oxide Particles of Differing Sizes and Surfaces

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Abstract
Magnesium oxides were unground prills sieved to 30-100 screen mesh size (30-100), or 12 and 40 mesh (12-40), or forms ground from the 12-40 to pass a 200 mesh (-200) or a 20 mesh (-20) screen. In the 1st experiment, Mg recoveries, the increase in urinary excretion of Mg above baseline for 2 days following oral administration of 100 g of magnesium oxide to cattle, were 0, 0, 10.2% and 6.29% for the respective screen sizes. Recovery from 12-40 increased to 4.47% following a series of 4 daily 100 g oral loads. These amounts represent the relative availability of Mg and magnesium oxide forms examined, not the absolute availability of Mg from any 1 form. In a 2nd experiment, magnesium oxide supplementation, as either 12-40, 30-100 or -20, to a control restricted roughage ad lib concentrate diet fed to lactating Holstein cows increased milk fat concentrations (2.84 vs. 2.19%) total fat production (580 vs. 470 g/day) and Mg concentrations in blood serum (2.96 vs. 2.68 mg/dl). The -20 generally produced a greater increase above control in these traits than did the unground 12-40 or 30-100 forms. The rumen proportion of propionate was decreased by the -20 and 12-40 sizes (32.0 vs. 35.4), but only the -20 increased the molar proportion of acetate (55.3 vs. 51.0). Although degree of availability differed, Mg from all magnesium oxide forms examined is available for absorption. Differences may result from the lower rate of solubilization in rumen fluid of the unground vs. ground magnesium oxides that was noted during 24-h in vitro incubation.