Grass tetany – how to stop the drop - Elders Rural Services

Grass tetany – how to stop the drop

Incidents of grass tetany (hypomagnesaemia) and milk fever (hypocalcaemia) are currently on the rise.

“The recent abundant rain, current winter weather and feed conditions are combining to produce something of a perfect storm for these metabolic disorders,” says Elders livestock production manager Rob Inglis, who is very concerned about the number of calls he is receiving about affected animals.

“Farmers should be hyper-vigilant right now.”

What is grass tetany?

Grass tetany is a highly fatal disease associated with low levels of magnesium in the blood. Grass tetany can affect both cattle and sheep, with pregnant or lactating stock on grazing crops or improved pastures most at risk.

The vigorous growth in pastures, while looking lush, is often high in potassium while low in magnesium and calcium. A cow in peak lactation (4 to 8 weeks following calving) needs a constant source of magnesium to replace the large amount lost from the body in milk. Even when feed levels of magnesium are low, the loss of magnesium in the milk remains the same.

“Potassium is the primary antagonist of magnesium in the rumen, in other words high levels of potassium in feed interferes with the absorption of magnesium” explains Mr Inglis.

“Mammals do not have the capacity for long term storage of magnesium, therefore require daily dietary intake.”

“It is generally accepted that the potassium to calcium plus magnesium ratio (K/(Ca + Mg), expressed in milliequivalents (mEq)/kg should be less than 2.2.”

As an example, the feed test results below are from a wheat crop at early to mid-tillering stage (Zadox 23). As the figures show this crop would be regarded as a high risk feed for pregnant or lactating livestock. K/(Ca + Mg) = 1162/(95+156) = 4.6, which is well above the threshold of 2.2.

Potassium (K)4.54 1162 mEq/kg
Calcium (Ca)0.1995 mEq/kg
Magnesium (Mg)0.19156 mEq/kg
Ammonia (crude
protein equivalent)
Sulphur (S) 0.38
Chloride (Cl)0.27

Symptoms to watch for

Sadly, says Mr Inglis, often the first sign animals are suffering grass tetany is when they are found dead.

“A telltale sign that the animal was a victim of the disorder is rub marks on the ground beside them where they convulsed prior to death”.

Early symptoms include staggering, excitability, involuntary muscle contractions, convulsions, frothing at the mouth and bellowing.

What can be done?

Mr Inglis has put together a risk assessment matrix of both animal and environment you can download and help determine the risk to your stock.

Where risk has been established, seek advice from a qualified ruminant nutrition specialist or veterinarian before depasturing stock. Magnesium supplementation, such as loose licks (preferred) or lick blocks in addition to low protein roughage should be recommended, as a supplementary feed source.

For advice regarding all animal health issues contact your local Elders livestock production advisor.

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The information contained in this article is given for the purpose of providing general information only, and while Elders has exercised reasonable care, skill and diligence in its preparation, many factors (including environmental and seasonal) can impact its accuracy and currency. Accordingly, the information should not be relied upon under any circumstances and Elders assumes no liability for any loss consequently suffered. If you would like to speak to someone for tailored advice relating to any of the matters referred to in this article, please contact Elders.


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