Magnesium plays a key role in the nutrition and physiology of ruminants. Therefore it is important to make sure that a magnesium source is used in ruminant diets that is highly soluble and absorbable in the rumen as the majority of Mg is absorbed in the rumen.

1. Magnesium plays an essential role in the metabolism, because it influences the activity of more than 300 cellular enzymes that are involved in energy metabolism and protein synthesis (Aikawa,1981; Ryan,1991).
   

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2. In cows, clinical hypomagnesemia (plasma Mg < 0.4 mmol/L) causes grass tetany (Sjollema,1930), and subclinical hypomagnesemia (plasma Mg values ranging from 0.4 to 0.8 mmol/L) seems to be associated with an increased incidence of milk fever (Van Leengoed,1979; Barber et al.,1983).
   

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3. High potassium levels in the rumen can reduce the difference in charges (apical membrane potential) between the rumen fluid and rumen epithelial cells. The smaller the difference due to high levels of potassium in the rumen epithelial cells, the more difficult it becomes for the animal to absorb Mg from the rumen fluid into the rumen epithelial cells (Schonewille, 2013).
   

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4. The rumen and reticulum are most probably the only site of Mg absorption (Martens & Gabel,1986; Martens & Rayssiguier,1980; Pfeffer et al.,1970).
   

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5. According to NRC 2001, a 30-litre cow need 5 grams of absorbed Mg to function (Faecal losses = 3mg/kg body weight; Urine losses = major route for excretion, small endogenous loss; Growth = 0.45g/kg gain; Foetus = 0.33g/day; milk = 0.15g/kg).
   

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6. Meta-analysis from 137 published experiments involving 2,545 calvings, found that an increase of the Mg content of the pre-calving ration significantly reduced the incidence of milk fever (Lean et al., 2006).
   

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7. There is a great variation in Mg absorption from MgO sources, ranging from 9.9 % – 73.7%. Average absorption of MgO is about 26% (Schonewille, 2008).
   

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8. Particle size of Mg-salts is highly correlated with the solubility. The finer the particle, the higher the solubility (Schonewille,1992).
   

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9. The time period and temperature at which Magnesium-carbonate is heated to convert it to MgO influences the solubility of MgO in the rumen. The lower the temperature and shorter the time period, the lower the solubility of MgO (Bhatti et al.,1984).
   

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10. Marine MgO is manufactured from sea water and contains 52 – 55% Mg. More than 90% of the Marine MgO particles is smaller than 11 microns, resulting in an increase in solubility in the rumen.
    

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11. Solubility of Marine MgO in the rumen range from 75 – 90% (Alimetrics, Finland). Little variation in solubility between different Marine MgO batches.
    

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12. Replacing supplemented 36g MgO with either 18g or 9g of Marine MgO gave similar Mg levels in blood plasma of dairy cows (Cruywagen, 2010).
    

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