Inverse TVS logo

Magnesium in beef cows

Magnesium is required for optimal rumen fermentation. Ruminants deficient in magnesium have lowered cellulose digestion in the rumen resulting in reduced appetite and total nutrient intake.  Magnesium is also essential for key body enzyme systems regulating energy, protein and fat metabolism.  In addition, magnesium is also involved in the uptake and metabolism of calcium, as well as nerve function.

A magnesium deficiency before calving is associated with a reduced voluntary intake, resulting in the inability of cows to gain weight despite above maintenance feeding levels, and an increased incidence of both milk fever (low calcium) and grass staggers (low magnesium).

After calving, magnesium deficiency has been associated with excessive body-weight loss, often with reduced milk production.

The impact of this disorder on your property will depend on a number of nutritional, environmental and management factors. Underfeeding, early calving on lush spring pasture, abrupt changes in diet, age (older cattle are more predisposed) and a large parasite burden are all factors which will increase the likelihood of animals lapsing into a low magnesium state. 

As a general rule, magnesium supplementation should begin in the cows at least three weeks before calving and should continue for at least a month after calving where practical.

Options for magnesium supplementation include:

  • Magnesium capsules – release elemental magnesium in the rumen for up to 90 days. In extensive situations this is often the only way of ensuring that cows are receiving magnesium.
  • Treatment of hay/silage – apply as a slurry (1 part water to 1 part magnesium oxide) @ 50g/cow/day.
  • Pasture dusting – fine particle magnesium oxide (Causmag) flows more easily through spinners, but blows in the wind more easily than coarse particle magnesium oxide @ 70g/cow/day.  Apply in the morning when grass is wet with dew.  Be aware of its side effects.  It is a depressant in humans, so avoid inhaling the dust. Dust into the wind, and/or wear a mask.
  • PETA dispenser delivery in trough – magnesium chloride or magnesium sulphate @ 60g/cow/day.


If you have any questions, please do not hesitate to contact us. It might be worth blood sampling some of your animals to assess whether you need to begin magnesium supplementation or whether it is safe to stop supplementing.