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Magnesium supplementation for dairy cows

Magnesium (Mg) is an important macromineral 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 more 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. Ruminants have no readily available pool of magnesium in their body, so a fall in magnesium content of the diet and/or increased need of magnesium for milk production can cause blood magnesium levels to fall quickly, leading to the typical signs of hypomagnesaemia. If cows do succumb to metabolic issues such as hypomagnesaemia, it can jeopardise their milk production for rest of the season or in worst cases cause cow deaths. Sub-clinical magnesium issues may also put a handbrake on milk production. Calculating how much supplementary magnesium should be fed can be difficult because the amount that is in the pasture depends on soil type, types of forages, weather and speed of growth. Availability is also reduced by high potassium and nitrogen levels.

Supplementation options

Being a reactive mineral, magnesium is normally found in association with other elements as compounds, so it is important to know the percentage of the compound that is elemental (pure) magnesium. Magnesium oxide (MgO) is the most common magnesium supplementation product. The level of elemental magnesium is high at approximately 54 percent. Attention to sourcing high quality, reliable MgO is important. MgO is suitable for pasture dusting, drenching and in feeds such as compound feed and blends but is not soluble, so cannot be used for water treatment. Magnesium chloride and Magnesium sulphate are both readily soluble in water, which lends them towards use in animal troughs. However, the level of elemental magnesium is significantly lower than magnesium oxide and they should be carefully used due to their bitter nature – ensuring the water they are dissolved in does not become so unpalatable that animals abstain from drinking it is extremely important. A multiple route to magnesium supplementation seems to be the most effective strategy, especially during the risky spring period. Use of MgO in feed and/or via pasture dusting, along with suitable Mg sources added to drinking water, will ensure that requirements are successfully met.

For more information, contact your local Nutrition Specialist.

Article supplied by Stacey Cosnett, Nutritionist.

Magnesium requirements and supplementation recommendations

As you can see in the table, the magnesium level in pasture generally does not meet the requirements of lactating cows, so supplementation is important.

% Mg pasture 0.1-0.20% (not able to meet requirements) Supplementary Mg recommended
(g/cow/day of elemental Mg)
Mg Required (% of diet) Jersey Crossbred Friesian
Dry 35% 12 16 20
Lactating 0.28% can differ depending of level of production of animal 15 17 20

Table adapted from Dairy NZ.


A table showing the most common magnesium oxide supplementation options

Mg Source % Elemental Mg (approx. can different between suppliers) Features Limitations Dosage
Mg Oxide 54 High in Mg content compared to other sources.

Very easy to supplement with low wastage if included in dairy feeds.

Can be dusted on pasture.

Not soluble, so cannot be used in water troughs.

50% field loss if used for paddock dusting.

Dusting can be labour intensive.

50-80g if dusting/cow/day.

Higher levels may be required in adverse conditions to offset in-field losses.

Very good
utilisation if included in dairy compound feed and blends feed in-shed.

Mg Chloride 11.7 Soluble, so can be added to water. Palatability issues if included in the water at too high rate. 60-100g/cow/day (levels over this limit decreases palatability of water).
Mg Sulphate 9.8 Soluble, so can be added to water. Can have a laxative effect when used in high concentrations.

Palatability issues if included in the water at too high rate.

75-100g/cow/day (over this limit increases risk of laxative effect and can decrease palatability of water).