Grass Tetany – Hypomagnesaemia

Quick Facts


Grass tetany (Cattle), Grass staggers (Sheep) and Hypomagnesaemia are names given to a metabolic disorder that can affect grazing ruminants in late autumn, winter and spring.

Grass tetany occurs when blood magnesium levels fall below a critical level, hence the term Hypo (low) magnesium (Mg), due to their diet been low in available magnesium or an interference / reduction of magnesium absorption, while the livestock requirements are high. Such examples include but not limited to, cows or ewes in late pregnancy and or lactating, while grazing grass dominant lush pastures or cereal crops.

Symptoms may include; restlessness, staggers, over-alter appearance, muscle termers, excitable, “downers cows”, go into convulsions or just die without noticeable warnings. Often the illness decreases appetite and the ewe or cow eat very little or nil feed. This may then lead to pregnancy toxaemia condition.

Common factors that may induce hypomagnesaemia include but not limited to;

  • Cows of 5 years or greater.
  • Grazing dominant grass or lush cereal crops (often without fibre / hay supplementation). Grass dominant pastures in late autumn, winter and early spring commonly have lower Mg and Calcium (Ca) concentrations and higher potassium (K) levels, especially when grazing short pasture. Grazing cereal crops are generally high risk because they are inherently low in Mg.
  • Livestock that have recently moved to a different paddock that may have been heavily fertilized with Nitrogen (N) or Potash (K) on pastures.
  • Cold & Wet weather with little or no shelter for the livestock.
  • Short periods of fastening during this weather event.
  • Lack of adlib / feeding of hay (contains Calcium).
  • Livestock may be very fat, losing condition or very thin.

A subcutaneous (under the skin) combination 4in1 mineral solution injection consisting of; Calcium, magnesium and glucose (Flopak Plus).

Shelter for effected the livestock.

After the cow has recovered from the initial episode, consider an oral drench treatment of Epsom salts (Magnesium sulphate) to prevent a relapse.

While magnesium is stored in the body (bones), it isn’t rapidly available, at the critical time, prevention is the preferred option to treatment. A significant amount of magnesium is lost via milk, urine and faeces, hence a daily intake is required.

Common methods of supplementing magnesium during high risk periods include but not limited to;

  • Lick Blocks- Grass Tetany Blocks – A disadvantage is invariable intake of sufficient magnesium.
  • Causmag® – Magnesium oxide may be sprayed on adlib hay and or Total Mixed Ration (TMR) supplement ration or grain ration supplement.
  • Epsom salts (Magnesium sulphate) added to water supply.
  • Magnesium intra Ruminal bolus – slow release.
  • Supplementation of magnesium oxide may be added to feed pellets.
    Caution: Should note the levels of magnesium oxide added to feed pellets, at excessive levels may predispose the cow to Salmonella.
    Prolong periods of magnesium supplementation may reduce phosphate absorption. Hence phosphate supplementation needs to be considered.
    • Avoiding pastures where cows have a history of grass tetany.
    • Avoiding sudden changes of feed or feed quality.
    • Lowering the herd age structure as older cows are at higher risk.

Reference Publications and Resources:
Agriculture Victoria- Grass Tetany Ag Note 0579, 2003.
Kellaway R (1994) Cattle Health and Production: Dairy Production University Sydney.
MLA More Beef from Pastures Program-Grass Tetany 2015.
NSW Department of Primary Industries Primefact Sheet 420 & 785.2010.
Rabiee A. & Lean I., Hypomagnesemia (grass tetany) in dairy cows.2005.
Westwood C.T.,et, al. Animal Health Production Module ~ Strategic Bovine Services 2005.

Compiled by:
Andrew Schmetzer – Farm Consultant
B.App.Sci.Ag. Post.Grad.Ruminant Nutrition.