Magnesium deficiency and sudden death in livestock

Nigel Brown, Northern Tablelands Local Land Services District Veterinarian, Glen Innes

Hypomagnesaemia (grass tetany) is likely to be a real problem on the Northern Tablelands this year. We are already seeing birthing problems related to ‘lazy’ calvers and lambers. Grass staggers and sudden death will likely be seen in grazing animals when the grass starts to grow even more as it warms up.

Clostridial diseases and bloat are also likely to cause sudden death this year.

What is magnesium?

Magnesium is an essential mineral for several energy-generating reactions and transmission of nerve impulses in animals. Livestock need a continuous dietary supply of magnesium to maintain adequate blood levels but magnesium is poorly absorbed by the rumen (only 30 - 50%).

Several factors reduce absorption further including a high dietary intake of potassium and degradable protein in lush, rapidly growing spring pasture, especially after heavy fertiliser. The added stress of cold weather or, worse, the frequent changes between cold, wet, warm and chilling days, further increases the risk of reduced feed intake with a large proportion of the magnesium being absorbed into fat tissue.

How do I recognise magnesium deficiency?

The highest-risk age group of animals for severe magnesium deficiency is usually 4-6 years old and the most common sign of problems is sudden death.

In sheep, the acute form most commonly occurs within 4-6 weeks of lambing with extreme cases collapsing and showing repeated tetanic spasms with all four limbs rigidly extended; some are unable to move, others move stiffly, and some are sensitive to touch and facial muscles tremble.

In the same way, cattle become irritable, show aggression and signs of muscular tremors (often with the ears drawn back). Less severe signs are often associated with milk fever and calving/lambing problems from weak uterine muscles.


Hypomagnesaemia and clostridial diseases are common causes of death among grazing livestock that can be almost completely prevented by adequate preparedness.

Magnesium supplementation should be provided to stock prior to the spring grass flush and other high-risk periods (cattle = 60-100g/head/day magnesium oxide; sheep = 10g/head/day). I recommend that all producers should give grazing livestock ad lib access to common salt (NaCl) all year round. Prior to spring, calving or lambing, magnesium oxide (e.g. causmag) and lime (calcium carbonate) can be added to this salt in equal parts (1:1:1 ratio) with several containers in each paddock.

Clostridial vaccines should be given at least two weeks before changes to lush feed and repeated at least four times a year.

The NSW DPI Primefact 421 (Grass tetany in cattle – treatment and prevention) and several on clostridial diseases and vaccination, provide good overviews and are well worth consulting.

For more information about livestock health, contact your Local Land Services District Vet or Livestock Officer on 1300 795 299.

Media contact: Annabelle Monie on 0429 626 326.

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