Grass Tetany in Cattle

Grass tetany remains one of the leading causes of death for adult beef cows in South-eastern Australia. It mostly occurs from late autumn through winter in lactating cows. Grass tetany occurs when there is not enough magnesium in the blood. Adequate magnesium levels are required for proper brain and nerve function.

The magnesium status of cows depends mainly on the balance between the amount of magnesium absorbed from the rumen and that lost in the milk. Any excess magnesium absorbed is excreted in the urine. Grass tetany results when the output of magnesium exceeds the input. Magnesium intake depends on the level of magnesium in the feed and the amount of feed consumed.

Generally, young grass and lush cereals have lower magnesium levels than older grass and cereal crops. Grasses and cereal crops have lower magnesium levels than legumes eg clovers and lucerne.  Further information can be found in Primefact 420.

A sudden deterioration in weather conditions can lead to severe outbreaks of grass tetany by dramatically increasing the number of cows deficient in magnesium. Producers need to be aware of the risks and take steps to prevent this disease in lactating cows.

Clinical signs

Animals with grass tetany become nervous and excitable. They may show muscular twitching, walk with a high goose-step or a staggering gait, bellow, go down, paddle and die. Because of the short duration of clinical signs, affected animals are frequently found dead.

Cows may become aggressive and so measures to avoid personal injury should be taken if the condition is suspected. Magnesium deficiency alters calcium metabolism and thereby increases the risk of "milk fever" in recently calved cows at pasture.  The diagnosis of hypomagnesaemia can be confirmed by post mortem in most cases.

Prevention- Magnesium supplementation

All animals with grass tetany require emergency treatment and therefore your veterinary surgeon should be called immediately. A veterinarian may give magnesium (usually in a solution containing magnesium, calcium, phosphorus and glucose) into the vein or under the skin after taking a blood samples to confirm the condition and rule out other causes.

A producer who strongly suspects this condition can administer this solution under the skin as time is of the essence. Usually 2 bags are given. This treatment does not last long and oral supplements must follow to maintain adequate levels of magnesium. Bags should be warmed to body temperature and the needles should be sterile. Massaging the areas after injection helps with the absorption.

A clinical case of grass tetany usually represents "the tip of the iceberg" as many other cows in the herd are likely to be severely magnesium deficient. Therefore, providing increased magnesium supplementation for the remainder of the herd is also important.

The daily magnesium oxide (Causmag) requirement for cattle is 60 g/head/ day, but up to 100 g/head/day may be necessary in some circumstances. It takes 2–3 days after the before stock are protected. Protection ceases as soon as this supplementation is stopped. Too much Causmag is expensive and can cause diarrhoea.

There are many ways of providing magnesium and these are well covered in Primefact 421. Hay or grain treated with magnesium oxide (Causmag) is the most effective treatment.

Causmag may be provided as loose licks (mixed with limestone and salt and molasses may be added), magnesium may be added to water (variable intake, depends on water source), magnesium bullets (may not provide adequate protection) and magnesium blocks (high cost and not all animals use the blocks).


Soil testing to check for at risk paddocks should be considered. Grass tetany is more commonly associated with acid soils and high potassium (K). See Primefact 785 for more information.

Grass dominant pastures and young growing cereals are more risky especially high risk is the consumption of green shoots after a period of drought as these shoots are high in potassium. Consider introducing more legumes to the pasture although this can be difficult to achieve in some circumstances.

It is also important to consider the age structure of the herd and the time of calving as older cows with young calves are most at risk during the autumn and winter.

Inadequate salt intake increases the risk of grass tetany.

Cows should not be stressed in the last 6 weeks of calving avoid mustering or transporting or any events which may stress them or cause a reduced feed intake. Wind, rain and exposure with low temperatures put cows at risk of grass tetany.

It is important that a veterinarian is called upon in an attempt to reach a diagnosis in down cows and cases of sudden death as other conditions may be hard to distinguish from grass tetany and require different remedies and prevention.

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