Phone: (02) 4832 1977 

Grass Tetany

Grass tetany, or hypomagnesaemia (low blood magnesium), is primarily seen in autumn and winter. A potentially fatal metabolic disease of cattle, grass tetany can decimate herds in cold rainy seasons. Bountiful autumn and winter grasses might look lush, but they are often high in water content and low in magnesium and calcium. 

Mostly seen in older, high milk producing cows pre-calving or with calves at foot, the disease is also initiated when high potassium levels hinder magnesium absorption from the rumen – classically on heavily fertilised pastures. Contributing factors include:

  • Stress of calving and lactating, especially in older cows with lower reserves and poorer absorption of magnesium. Low calcium occurs at the same time, particularly in lactating cows.
  • Grazing on tetany-prone pastures. These include grass dominant pastures, cereal crops, acid soils, high use of potassium fertilisers.
  • Cold weather, particularly sudden drops in temperature.
  • Other factors include the stress of feed changes, transportation and periods of starvation.

Symptoms of hypomagnesaemia vary but affected cows are always excitable, and sometimes dangerous. Early cases show slight muscle twitching, a stiff gait and appear wary. This progresses to staggering, charging, falling to the ground and paddling with head stretched back. The animal may appear blind.


Treatment involves intravenous administration of magnesium and calcium and it needs to be delivered quickly. The prognosis following treatment is unpredictable, and severe cases may still die.


Prevention through magnesium supplementation is by far the best option. This can be achieved by the feeding of magnesium oxide products such as Causmag at 60gm to 100gm per head per day. Long-acting magnesium capsules delivered into the rumen provide 80 to 90 days of protection.

Other products include loose licks – granulated or powdered preparations administered from feeders. Magnesium blocks are not effective – they’re too costly and it’s difficult to manage a precise and adequate delivery of the required magnesium per cow.

Providing hay for cows and calves is also good practice. Legume hay especially supplies nutrients that help reduce the incidence of grass tetany.