Grass Tetany in Grazing Animals

Be alert to the potential risk for Grass Tetany in high potassium soils and when other conditions are favorable for the development of this livestock illness.
Grass Tetany in Grazing Animals - Articles


John D. Byrd, Mississippi State University,

I was called to a farm this week because of six dead cows. It was a sad sight especially since it could have been avoided. The cause was hypomagnesmia (low magnesium) which is more commonly known as Grass Tetany. In this situation, the conditions that increase the risk of grass tetany aligned perfectly (see below).

What Is Grass Tetany?

Grazing animals have low magnesium levels in their blood because the grass they are grazing contains low magnesium. It occurs almost exclusively in grass and not legumes.


  • Low blood Mg (Mg less than 1 mg/100ml of blood)
  • Animals appear nervous and hypersensitive
  • Muscle twitching and staggering
  • Convulsions
  • Death


  • Grazing lush grasses in the spring
  • Low soil temperature depresses Mg uptake by plants
  • High soil moisture depresses Mg uptake by plants
  • High soil K depresses Mg uptake by plants
  • Plant cation ratio [K/Ca + Mg > 2.2] is critical for grass tetany to occur.
  • Animal susceptibility is highest 5 days after mean daily air temperature exceeds 57° F following a period when mean daily air temperature was below 57° F.

Prevention and treatment:

  • Use Mg supplement
  • Don't over fertilize with K
  • Intravenous injection with Mg salts