Grass tetany

Dairy cow with grass tetany

What to look for:

Can range from mild signs to a rapidly fatal illness:

  • Sudden death
  • Strange behaviour or aggression
  • Convulsions and muscle spasms
  • Over reaction to sounds or movement
  • Collapse
  • Milder cases show poor appetite, slight agitation and reduced milk production

Cause - low magnesium intake

Cattle cannot store magnesium so require a daily intake to maintain adequate blood levels. Symptoms occur when the cow’s magnesium levels drop below a threshold level. Animals are most at risk when grazing lush pasture especially if it has been fertilized with nitrogen or potash. Outbreaks of grass tetany may be set off by a sudden change of pasture or by bad weather that reduces grazing time.

Animals likely to be affected

Dairy cattle within the first 6 weeks after calving. Susceptibility increases with age. Grass tetany is generally a herd problem.

Other diseases with similar signs

Other causes of sudden death:

Diseases causing nervous signs:

Confirming the diagnosis

Blood magnesium levels are not a reliable diagnostic test in dead animals, so your vet may take a fluid sample from the back chamber of the eye.


The aim is to increase blood levels of magnesium as quickly as possible without damaging the heart. Intravenous administration of a magnesium solution is difficult in an animal with convulsions and is best done by your vet. Additional treatment involves injecting magnesium solution under the skin and allowing the animal to recover in a quiet environment.

Risk factors

  • Grazing on lush pasture, especially if it has been fertilized with nitrogen or potash
  • Cows in early lactation
  • Bad weather or sudden change in pasture


Magnesium compounds are effective but require daily administration e.g. daily drenching, adding the treatment to feed in the bail, spraying it on to hay, adding it to water or using magnesium blocks.

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