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Grass Tetany in Ewes

Grass Tetany is a common disease of lactating ewes during the spring time. It is an acute and frequently fatal condition that primarily affects lactating sheep at grass. This condition is caused by a deficiency in blood magnesium levels. Michael Gottstein, Teagasc, Head of Sheep has more information

Factors influencing onset of Grass Tetany

The main factors influencing the onset of this condition are;

  • Bad weather conditions especially very wet spells which affect the utilization and intake of grass and the ability of the ewe to absorb nutrients from the grass.
  • Rapid grass growth coupled with high soil potassium levels. High levels of potassium (applications in excess of 70kg/ha (55units per acre) in spring time) affect the grasses ability to take up the magnesium.
  • Stress. Ewes that are under stress because of poor weather, disease, malnutrition, weaning etc.


Very often the first sign of a problem will be a dead ewe. However where ewes are found alive, identifying and rapidly treating the affected animal with 100cc of 25% Magnesium Sulphate solution given at body temperature under the skin at 5 – 6 different places can be successful where the condition has not progressed too far. Some of the more common symptoms include nervousness, staggering, twitching and in the later stages ewes may be found lying on the ground and kicking due to muscle tremors.

Magnesium supplementation

Magnesium is not stored in the body to any great extent, consequently lactating ewes need to be supplemented with magnesium daily. Each lactating ewe requires about 1-2 grams of magnesium per day. During periods of stress and poor weather the absorption of magnesium is reduced which is why we recommend daily supplementation rates of 3 -5 grams per head per day.

There are many ways to supplement ewes with magnesium. When supplementing ewes protection will occur 1-2 days after supplementation has started and will last for 1 to 3 days after supplementation has ceased.

Controlling Grass Tetany

The most popular options for controlling grass tetany are as follows;

1) Meal feeding and incorporating Cal-Mag into the meal at the rate of 10grams per head per day (follow manufacturer’s feeding instructions).

2) High mag buckets/blocks – are generally successful but some ewes will not eat sufficient to ensure protection. During very dry weather the blocks may dry out and will need to be ‘wetted’ in order to get satisfactory intakes. Do not use blocks designed for dairy/suckler cows as they will contain levels of copper which are toxic to sheep.

3) Magnesium bullets – these are very convenient but generally do not allow for the release of the optimal 4-5 grams of intake per day.

4) Pasture dusting with Cal – Mag (powder form only) at a rate of 17kg / ha every week for paddock system and double that amount every 2 weeks for set stocking system. For this to work the paddock being dusted needs to have at least 10cm (4inches) of grass for the Magnesium dust to stick to.

5) Treating the drinking water with magnesium is not a reliable control method for sheep as ewe will drink little or no water during wet weather.  

NB: Take care not to feed concentrates containing Calmag or other high Mg supplements to male sheep due to the risk of them developing urinary calculi.

 More information on Flock health can can be found here