Grass tetany

Grass tetany (hypomagnesemia) is a disorder in cattle where magnesium levels in the body become insufficient. 

Magnesium is an element required for muscle function and many metabolic processes. There are no body stores of magnesium, and decreased magnesium levels in feed or an increase in demand for magnesium (i.e. through excretion in milk to calves) can decrease magnesium in the body below critical levels, resulting in clinical signs or death. 

Late pregnant cows and cows with calves at foot, especially those on a lush green feed diet, are most at risk.

Risk Factors

Time of year

Grass tetany occurs more in:

• autumn/winter calving cows than spring calving cows.
• cows grazing on pastures made up of young grasses and cereals which have lower magnesium levels than older grass and cereal crops.

Stress levels

A number of changes to the cow's surroundings can increase the chance of grass tetany including:

• weather like wind, rain or a sudden lowering of temperature
• sudden change in feed and feed quality
• mustering
• transport.

Do not transport cows in the last six weeks of pregnancy.

Age

Older cows are more susceptible to grass tetany because:

• there is more excretion of magnesium through milk (milk production tends to rise until four years of age and then stabilises)
• the absorption rate of magnesium decreases with age
• fat cows are more prone to grass tetany, and older cows tend to lay down more fat.

Breed

Some breeds are more susceptible, including; 

• Angus
• Angus crosses
• Murray Greys.

Symptoms

In most cases, the first sign of grass tetany is finding dead cows. 

Dead cows may exhibit:

• froth around the mouth and nose
• disturbed ground around their legs due to leg ‘paddling’ or thrashing before death.

Mildly affected animals can appear normal and may only exhibit symptoms when stressed (for example, when yarding, mustering, trucking etc.) and may then show signs such as twitching of the face and ears, a wary appearance and a stiff gait. These early signs are subtle and often missed.

Intermediately affected cows may show more obvious signs, such as ‘goosestepping’ front legs, holding their tail high, and appearing blind.

Severely affected animals may show:

• excitement
• galloping
• bellowing
• staggering.

They will then quickly go down on their side with legs outstretched and ‘paddling’ back and forth.  

Animals are likely to die within minutes of being seen staggering and going down.

An individual cow suffering from grass tetany is a sign that other cows could be at risk or suffering from hypomagnesaemia. If grass tetany is diagnosed on a property, the risk of recurrence in the following years is high.

Prevention

Prevention of grass tetany is aimed at reducing the factors that lead to the condition and/or increasing magnesium in the diet. Prevention techniques include:

• adding magnesium oxide (i.e. Causmag®) to hay
• adding magnesium licks, blocks or slurries (it can be harder to ensure that each cow will consume what they require when using lick blocks)
• providing roughage to help slow gut transit time and improve magnesium absorption
• reducing the average age of cows in the herd as older animals are more susceptible
• reducing stress factors, providing shelter, and avoiding transport or mustering during the risk period.
• changing calving season as the worst months for grass tetany are from June until August.

Treatment 

Treatment is by administering magnesium solutions (i.e. flow-pack 4 in 1 products) to affected animals, but relapse is common, and you may not find the animal while treatment is still possible.