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Watching out for grass tetany

In this article, FRS looks at grass tetany. It explains what it is, the causes, and signs/symptoms.

At this time of year, farmers are busy getting cows out to grass and tending to the new additions to their herd.

For both dairy and suckler farmers, there is one illness they should keep an eye out for at this time of year: grass tetany.

Grass tetany is an acute magnesium deficiency in the blood in lactating cows, which is potentially fatal.

Magnesium is essential for the functioning of nerves and muscles of the animal.

A lack of this macronutrient can impact the temperament and overall performance of the cow. In severe cases, it can also cause sudden death.


The main causes of grass tetany are poor feed intake and weather conditions.

Grass tetany is known to strike when there is cold or wet weather.

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Farmers should pay particular attention to when weather changes.

Also, stress can play a factor at weaning time. All these factors can occur when farmers let lactating cows out to graze lush pastures.

A swift change in diet post-turn out can bring on the metabolic disorder.


Grass tetany occurs quite quickly, as does its symptoms.

Farmers should watch out for animals becoming excited or unsteady on their feet. They may appear to stagger when walking or standing.

In some cases, severe muscle contractions and body tremors are visible. Farmers can also watch for animals in visual distress or with frothing mouths.

Should a farmer or farmworker suspect an animal has grass tetany, they should act immediately.

Their condition can deteriorate rapidly, so it is important for the local vet to treat them as fast as possible.

With all farm illnesses, prevention is better than cure.

Teagasc maintains the cost of one animal lost to grass tetany would cover the cost of years of prevention/protection for that herd.

Animals cannot store magnesium; they need it in a constant supply.

Get magnesium into cows’ diets

There are several ways farmers can ensure their cows get enough magnesium in their diet and reduce the risk of grass tetany.

Firstly, they could use a magnesium treated nut or ration as part of the cows’ daily feed or during critical periods.

It is important to talk to the feed provider to ensure that the feed volume is sufficient for the amount of magnesium required.

Some farmers opt to dust their pastures with Cal-Mag. Teagasc recommends a rate of 15-17kg/ha.

Other options include putting a magnesium block/lick in the field but be watchful as not all animals may lick it.

Alternatively, you could administer a bolus, but this may not suit all farming enterprises as it can be considered labour intensive and will depend on the lactation of cows.

Lastly, another method is to introduce magnesium compounds to the drinking supply.

Work with FRS Farm Relief

Are you interested in helping with herd management? FRS Farm Relief is currently recruiting for farmworkers.

These positions are suitable for those looking for full time and flexible work.

To find out more, contact FRS Farm Relief, call 0818 890 890 or visit this link.

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