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Magnesium and Calcium

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Magnesium and Milk Fever Magnesium plays an important role in milk fever prevention (calcium deficiency). Magnesium is required for the production of hormones that are important for the absorption of calcium (Ca) from the gut and the mobilisation of Ca from bones. Supplementing with magnesium for two to three weeks pre-calving will reduce the risk of milk fever, however it does not build up a store of magnesium.

Magnesium supplementation is an important tool to help prevent metabolic issues.

The magnesium requirement of the modern dairy cow has increased, partly due to increased use of nitrogen (N) and potassium (K) fertilisers and partly due to an increase in cow genetic merit. All cows are to some extent deficient in Magnesium in late pregnancy and early lactation. High producing cows are more deficient.

Following are some key points to aid in prevention of metabolic problems:

  • Supplementing with magnesium will reduce the risk of milk fever and grass staggers (grass tetany)
  • Supplement with magnesium daily for two to three weeks pre-calving until after spring pasture growth rates have slowed (around December)
  • It can be difficult to supply cows with enough magnesium just through the water supply
    Combine magnesium through the water supply with magnesium dusted daily onto pasture or silage
  • Dietary requirements of magnesium will vary due to cow liveweight (dependent on breed) as shown in Table 2 (Refer to DairyNZ FarmFact Magnesium supplementation 3-1 for dose rates of magnesium from different sources to meet daily requirements)
  • After calving, provide 150g/cow/day ground limestone (lime flour) daily during the colostrum period- double this rate if dusting; can also consider calcium-enriched molasses
  • Ensure the metabolisable energy (ME) requirement of grazing transition cows is met during the last month pre-calving (Equivalent to approx. 20% of a cow's empty liveweight (LW without the calf))
  • Consider using starter drenches on high risk cows at calving - 7+ year olds, or very fat, or with a history of metabolic problems.


Table 1. Dietary magnesium requirements and supplementation recommendations


Mg requirement

(% of diet)

Supplementary Mg (g/cow/day)