Expert knowledge. Expert opinion.
Expert formulas.

Preparing to supplement cows with magnesium oxide

Magnesium (Mg) is an important mineral that has many functions within the body, including nerve and muscle function,
immune system function and bone health.

As magnesium relaxes nerve impulses after transmission, a minor deficiency can manifest itself as excitability and nervousness in cows, whereas excessive intakes can cause sedation, with cows becoming more lethargic and non-responsive. If cows do succumb to hypomagnesaemia (grass staggers) it can jeopardise their milk production for the rest of the season, or in the worst cases cause cow deaths.

Calculating how much supplementary magnesium that should be fed is difficult because how much is in the pasture depends on soil type, types of forage, weather and speed of growth. Availability is also reduced by high potassium and nitrogen levels.
The magnesium level in pasture (typically 0.1–0.2 percent) generally does not meet the requirements of lactating cows (approximately 0.28 percent, depending on the level of production) so supplementation is normally required in the spring.

Magnesium oxide (MgO) is the most common magnesium supplementation. Its key benefit over other products such as magnesium chloride, magnesium sulphate and magnesium phosphate is its alkalising effect in the rumen i.e. magnesium oxide helps to lift rumen pH when the ration lacks fibre. Magnesium oxide also increases milk fat concentration which is more valuable than it used it to be.

Now that farmers are being presented with different MgO options due to the disruption in global trade, it pays to know how to assess the quality of various products. Especially given that this supplement is part of our food chain – consumers are increasingly scrutinising suppliers’/manufacturers’ accreditations – both for direct sales to farmers and use in manufactured feeds and blends.

Magnesium oxide is not dug out of the ground as-is but is produced by the high-temperature furnace calcination (oxidation) of magnesium carbonate. The elemental magnesium content is important for cost effectiveness but also the availability of the magnesium which is influenced by:

Particle size – grinding magnesium oxide to a finer particle size has been shown to increase solubility and availability for ruminal absorption. Finer grades are more suitable for dusting. If dusting magnesium oxide on pasture, the amount required to be consumed has to be doubled or possibly tripled to allow for field losses.

Temperature at which magnesium oxide is calcined – affects the reactivity and availability to the animal. Higher temperatures result in greater surface area by breaking down the magnesium carbonate particles, thus increasing the potential for solubilisation and release of magnesium into the ruminal fluid.

Origins of the rock used – affects the level of elemental magnesium and the level of impurities. Three non-essential minerals in particular are classed as highly toxic – cadmium, lead and mercury and can be damaging to animal health and the environment.

While not a source of the disease, feeding granular magnesium oxide to dairy cows may increase their risk of contracting salmonellosis so the level fed is normally limited at 22–36g per cow per day.

Adding magnesium oxide to blends and compound feed is a good option. Magnesium absorption is improved with more readily degradable carbohydrates, the palatability of which can also help offset the bitterness of magnesium oxide.


For more information contact your NRM Nutrition Specialist or the friendly team at your local
Farmlands store.

Article supplied by Dr Rob Derrick,
NRM Lead Nutritionist.