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Grass Tetany (Hypomagnesaemia)

 Grass tetany (also known as hypomagnesaemia) is a disorder in cows where the magnesium levels in the blood drop too low resulting in muscles unable to function normally which then leads to death. It is common in dairy herds and has been the most common cause of death in adult beef cows in south-eastern Australia over the last 40 years. Sometimes the low level of magnesium is due to insufficient levels of magnesium in the diet but often it is more complex than that with other factors affecting the absorption of magnesium from the rumen. Autumn, winter and spring are when grass tetany is most likely to occur and cows in late pregnancy, with calves at foot or milking are most at risk.


Grass tetany develops very quickly so unfortunately the first signs that may be seen are dead cows. If seen in the earlier stages you may notice:

  • twitching of face and ears
  • goosestepping and staggering when walking
  • may appear blind
  • bellowing with wild and aggressive when being driven
  • down but struggling wildly to get up when disturbed.

If cows are found dead, a sign that it may be due to grass tetany is the ground and grass around the cow will usually be rubbed and dug up where the cow has been moving violently before death. Also they will often have froth from the mouth and nose.

 Milk fever can have similar symptoms and happen at a similar time of the year but it usually only occurs immediately around calving and those cows tend to be very lethargic and subdued rather than wild.

 Risk Factors

Cows that are lactating lose a large amount of magnesium in the milk. The body has minimal stores of magnesium so cows require a constant intake of magnesium in their diet to maintain stable blood levels. There are multiple possible factors affecting magnesium absorption but some of the main ones include:

  • Insufficient      magnesium in feed –
    • herbage containing       <2g/kg DM magnesium (immature, fast-growing grass dominant pasture).
    • OR pasture that       is less than 1,000kg DM/hectare
  • Reduced      absorption of magnesium –
    • potassium in       the rumen reduces magnesium absorption so feed that contains >20g/kg DM       potassium is a risk factor
    • nitrogen in       the rumen reduces magnesium absorption so feed that is high in nitrogen       is a risk factor
    • phosphorous in       the rumen increases magnesium absorption so feed that     is phosphorous deficient is a risk       factor   
    • older cows       (usually 6 years or older) absorb magnesium less well
    • fatter cows       have less magnesium in body fluids so are more susceptible

 Therefore lush young grass and cereal crops (particularly June to August) or pastures that have been heavily fertilised with potassium and/or nitrogen fertilisers are risky pastures for Grass Tetany and we would strongly advise avoiding putting older cows on these pastures when possible or providing supplemental feeding and preventative measures.


There are several preventative measures available when the cows are at risk of Grass Tetany. These include feeding out hay treated with causmag; magnesium salts in water (though this unpredictable as water intake during the risk periods is variable); magnesium licks (useful if cows are used to licks); and magnesium capsules which are inserted into the rumen.


If cows are found showing symptoms of Grass Tetany, urgent treatment is necessary. A large dose of magnesium can be given under the skin along with a dose of 4-in-1 (calcium, magnesium, phosphorous and glucose). If treated immediately then there is a reasonable chance of saving the animal. Magnesium should never be given directly into the vein as this will likely kill the animal.

 If Grass Tetany is an ongoing concern on your farm it is definitely worth seeking advice on the risk factors for your farm and the best preventative strategies.



Proper management of heifers and ensuring adequate growth rates can lead to quite significant economic benefits in both beef and dairy production systems. If properly managed, heifers should be able to calve at 2 years of age rather than 3 years of age without significant calving problems, get back in-calf at the same time as the remainder of the herd and in dairy herds maintain good lifetime production of milk. This leads to significant economic benefits.

 Heifers should be weighed regularly so that any problems can be managed early. Target growth rates are:

  • 6      months – 30% mature adult weight
  • 15      months – 60-65% mature adult weight
  • 22      months – 90% mature adult weight

Mature adult weight is defined as a 5 year old cow of the same breed in condition score 3/5.

 Heifers will not start to cycle at a given age, rather they start cycling when they reach approximately 52% of mature adult weight. However to ensure that the majority of heifers are cycling well they need to reach critical mating weight, which is 60-65% of mature adult weight. Therefore if you are planning to mate them to calve down at 2 years old then they need to reach this weight by at least 15 months of age.

 To achieve target weights heifers will need to grow at about 0.5-1.0kg per day until 2 years of age (note that for dairy heifers growth rates greater than 0.7kg per day between 6 and 12 months should be avoided as this is a risk factor for fatty udder syndrome which leads to a reduction in lifetime production of milk). To achieve these sorts of growth rates feed will have to have adequate energy (10MJ/kg DM) and protein (>16%). Only good quality pasture will meet these requirements so supplemental feeding may be required to meet these demands, particularly over the drier months.

 For heifers that have reached critical mating weight you can expect 85-90% to be in calf with a 6 week mating period or 95% with a 9 week mating period. In general we would recommend using a 6 week mating period for heifers and culling empty animals as this ensures that only higher fertility animals are reproducing which results in higher fertility progeny. Also it is usually worth mating heifers about 6 weeks before the cows as they often have a slightly longer gestation period and are more likely to have problems when calving so this gives a larger margin for getting back in calf the following season without extending out the calving period.

 Pregnancy testing from 6-12 weeks after the end of joining will help to detect non-pregnant animals early so they can be culled sooner rather than later and not use up valuable resources. Also this will help determine early vs late calving animals accurately which can then be used to help in managing resources at calving time.

 In dairy herds a good indication of how well heifer rearing practices are performing is to measure heifer milk yield in their first season compared with the mature cows. If the level is less than 75% of mature cow yields then heifer management should be reviewed.

 Of course all production systems are different so if you are looking for advice specific to your farm we are always happy to have a chat with you.



 Clostridial diseases are quite common and the vast majority of clinical cases of infection with a clostridial organism result in death. The main clostridial diseases in this area are Blackleg, Pulpy Kidney (Enterotoxaemia), and occasionally Tetanus. Black Disease and Malignant Oedema are also common clostridial diseases in other regions. These diseases affect sheep particularly but also cattle throughout Victoria. Other animals, especially goats, are also susceptible. Individual properties sometimes have substantial losses.

Clostridial organisms of various types are found in the soil, where they can survive for a long time. Most clostridial organisms can also occur quite naturally in the gut of healthy animals without causing problems. They then pass in the manure, and consequently contaminate the soil. When conditions are favourable for the uncontrolled growth of clostridial organisms they produce powerful toxins. The effects of the toxins are usually fatal.

Animals are most at risk when on energy dense, abundant feed (ie spring pasture). Young stock are most likely to be affected, usually less than 6 months, but are at a higher risk until 18 months. Management procedures also contribute to the level of risk with marking, transporting and shearing being common times for problems with clostridial diseases to arise.

Treatment can be attempted but this is expensive and usually unrewarding. The good news is that vaccination against these diseases is relatively easy and provides good protection to prevent the disease in the first place. The basic vaccination is a 5-in1 vaccination (covering the 5 main clostridial diseases) but 6-in-1, 7-in-1 and 10-in-1 vaccinations are also available which cover additional diseases. The ideal vaccination program involves a booster for the ewes approximately 1 month before lambing as this results in high levels of antibodies being passed onto the lambs via the ewe’s milk which provides good immunity for the lambs in their first weeks of life. This should be followed up with a vaccination for the lambs at marking (one of the highest risk times for clostridial diseases) as the protection passed on to them from the ewe wanes. This should be followed with a booster 4-6 weeks later to maintain their immunity for a year. Follow-up booster vaccinations can then be done annually.


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