If it slips under the radar, grass tetany/hypomagnesaemia can suddenly prove very costly to farmers. The transition period post-calving is when animals are most susceptible to metabolic disorders such as this.

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In severe cases – just like any metabolic disorder – it can have adverse affects on an animal’s future reproductive performance. Any disorder that might compromise an animal’s performance should not be overlooked.

This year, due to the exceptional growth rates over the winter, there is an abundance of heavy covers ready to be grazed across the country.

Grass tetany often occurs when cows are let out to graze lush, green, low-fibre pastures; so it is something to be conscious of over the coming months.

Additionally, application of potash before early spring grazing increases the chances of a magnesium (Mg) deficiency so this should be avoided if possible.


The symptoms of grass tetany can occur rapidly and mainly impact the animal’s nervous system.

Symptoms include:

  • Severe muscle contractions;
  • Hyperness;
  • Frothing at the mouth;
  • Staggering while walking or standing;
  • Body tremors;
  • Visual distress;
  • Irregular and loud heartbeat.

If you are suspicious of a cow having grass tetany, you must act promptly. She should be treated as quickly as possible to avoid deterioration or possible death.

Treatment involves administering an intravenous injection of a combined Mg and calcium (Ca) solution. In some serious cases the treatment may need to be repeated.


The best, and most effective way to prevent grass tetany, is to supplement the animals’ diet with Mg.

Mg can be added to the diet through:

  • Pasture dusting with Cal/Mag at a rate of 15-17kg/ha;
  • Feeding a high Mg concentrate;
  • Adding a soluble Mg solution to water troughs;
  • Administering a bolus (although this is very labour intensive);
  • High Mg licks.

It is important to note that if you are supplementing your cows’ diet with Mg via a nut, more than 1kg must be fed to receive the 50-60g of CalMag needed per day.