As ewes and lambs are turned out to grass, it’s important farmers keep in mind the dangers that are associated with grass tetany (hypomagnesia).

It is a disease that is seen during spring in lactating ewes at grass and is often fatal. It is caused by a deficiency in blood magnesium levels.

Many factors contribute to this disease, such as poor weather (high rainfall) which we have had lately; rapid grass growth, which is something that we will be hopefully seeing over the coming weeks if the weather improves; both coupled with high soil potassium (K) levels.

It can also be triggered when a ewe is under stress.


Unfortunately, the first sign of this problem will be when the ewe is dead. Although, 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.

Teagasc recommends that when affected animals are detected early, treating them with magnesium sulphate solution is often successful.

When ewes aren’t detected early, the outcome is often not good and can lead to permanent brain damage and death.

Ewes will need to be supplemented as magnesium (Mg) is not stored in the body to any great degree.

Again, it is advised that each lactating ewe get about 1-2g of Mg/day. During periods of stress and poor weather it is advised to increase daily supplementation rates to 3-5g/head/day.

When supplementing ewes protection will occur one to two days after supplementation has started, and will last for one to three days after supplementation has ceased.

Options for controlling grass tetany

There a number of preventative measures farmers have at their disposal to help prevent this issue from occuring.

Some measures include: 

  • Meal feeding and incorporating Cal-Mag into the meal;
  • High mag buckets/blocks;
  • Magnesium bullets;
  • Pasture dusting with Cal-Mag (powder form only);
  • Treating the drinking water with magnesium (not a reliable control method).