Grass tetany

What is grass tetany?

Grass tetany, also called grass staggers, is an acute shortage of magnesium in the blood. The technical term is hypomagnesaemia. Magnesium is needed to pass nerve signals on to the muscles. When it runs short, nerves and muscles become overexcitable, which leads to trembling and cramps.

Cattle have no significant magnesium store they can draw on quickly. They depend on a daily intake from their feed. Just a few days with too little magnesium can be enough for the blood level to drop. The normal blood value is around 0.8 to 1.2 mmol per litre.

Why at grass?

Most cases occur in spring, in the first weeks after turnout, and less often in autumn. Young, fast-growing grass has several properties that work together:

• little magnesium and little sodium
• a lot of potassium, especially after slurry or potash application. Potassium inhibits magnesium uptake through the rumen wall.
• a lot of rapidly degradable protein, which forms a lot of ammonia in the rumen and also slows uptake
• little fibre structure, so the feed passes through quickly and can cause diarrhoea, which loses more magnesium

On top of this comes stress from cold, wet weather, transport or bulling, because the cow then eats less. Older, high-yielding cows in early lactation and suckler cows with calves are most at risk.

Signs

Grass tetany can progress very quickly. Typical signs are:

• restlessness, nervousness and heightened alertness
• muscle trembling, especially of the head and shoulder
• stiff gait and staggering
• cramps, teeth grinding, salivating
• lying flat on the side with paddling leg movements

Some animals are found dead in the field without anything having been noticed beforehand. Besides the acute form, there is a subacute, creeping magnesium deficiency with lower feed intake and yield.

What to do in an emergency?

Acute grass tetany is an emergency: call the vet straight away, who will correct the deficiency with an infusion. Do not drive the animal, as excitement can trigger a seizure. After recovering from tetany, the cow needs a reliable magnesium supply over the following days, otherwise there is a risk of relapse. How grass tetany relates to other reasons a cow cannot get up is covered in Calved; Why doesn’t my cow get up?

Prevention

• Make the change gradual: build up grazing time over one to two weeks and feed hay, straw or silage alongside at the start.
• Supplement magnesium daily, starting two to three weeks before turnout, for example with a magnesium-rich mineral feed. Individual animals take to lick buckets and lick blocks very differently.
• Provide sodium, as sodium deficiency strengthens the effect of potassium.
• Be sparing with potash on grazing land in spring.
• Offer shelter and forage in cold and rainy weather.

More on mineral supply during grazing in Grazing: keep a close eye on minerals.

Magnesium in the shed

Magnesium can also run short indoors, for example in rations with a lot of potassium-rich grass silage or in dry cows. Around calving the cow needs magnesium to mobilise calcium. A shortage can therefore also make milk fever more likely.

Frequently asked questions

All terms in the glossary