Cal Mag or otherwise known as Calcined Magnesite or Magnesium Oxide (MgO) is used in the prevention of Grass Tetany in both dairy cows and Ewes on Spring and Autumn grass but can occur year-round if magnesium levels in the diet become low enough. Calcined refers to the manufacturing process. Cal Mag contains no calcium. Feed-grade Cal Mag is 50% Magnesium (Mg) and is the standard for feeding recommendations: – Cows: 60g/day to help prevent Grass Tetany on pasture. In challenging situations cows may need 100grams/day. Dairy feeds with Cal Mag are not suitable for other stock. Ewes: 10g/day, to help prevent Grass Tetany on pasture. Cal Mag should not be fed to dry sheep, particularly rams, to avoid kidney stones (Urinary Calculi). Very little usable magnesium is stored by the cow and therefore it must be fed daily.

The manufacturing or calcining process involves heating crushed magnesite (natural magnesium carbonate) ore in a kiln at a high temperature to drive off unwanted materials, yielding a more bio-available magnesium oxide. Control of the process is essential to ensure an end-product with optimum availability characteristics.

Research has shown the ideal calcining temperature to be in the range of 800 – 1000 degrees Celsius. Heating for prolonged periods or at higher temperatures results in what’s called “dead-burned” product with little or poor bio-availability.

Magnesium is generally thought of a single function i.e. its role in the prevention of grass tetany. However, magnesium is a more important mineral than this. Magnesium plays a role in over 300 biochemical functions in the cow. These include energy metabolism, bone formation, nerve and muscle function, and calcium and phosphorus metabolism. For dry cows magnesium is a critical element, playing an important role in the metabolism of calcium around calving. This is important in reducing the risk of milk fever, slow and difficult calving’s, retained placenta, and a range of metabolic and immune issues related to calcium metabolism. To maximise production and fertility of cows, magnesium plays a critical role in the dairy cow metabolism.

Research has also shown that of the magnesium oxides available on the European market, Spanish material is the most bio-available material as a result of its physical and chemical makeup (B.M Tas, 2012).