Preventing Grass Tetany in Cattle

What is grass tetany?

Grass tetany (or grass staggers in sheep) is a condition caused by critically low magnesium levels in the blood (hypomagnesemia). It can cause significant production losses, even in cases where there are no signs of illness.

Cattle store magnesium in their bones and muscles, however, cannot access these stores. As such, they need magnesium in their diet to meet daily requirements (3mg/kg for maintenance). Animals lose magnesium everyday in urine, faeces and milk. Grass tetany occurs when there’s an imbalance between magnesium absorbed (from feed) and lost. It is most prevalent in lactating cattle, due to the added loss of magnesium through milk. A cow in peak lactation (6-8 weeks after calving) needs an extra reliable source of magnesium to replace the large amount lost (120mg/kg milk). Grass tetany is rare in nonlactating cattle, but has occurred when undernourished cattle were introduced to green cereal crops.

Causes of grass tetany

Factors that contribute to low magnesium levels in cattle include:

Magnesium levels are lower in winter grasses and cereals, than in legumes or weeds.
Magnesium is low in grasses grown on sandy, leached, acid soils.
When potash and nitrogen fertilisers are used, and grass growth is vigorous.
High moisture content in grass resulting in rapid gut transit and low uptake.
Reduced magnesium absorption resulting from high rumen potassium and nitrogen, or low rumen sodium.
Low energy intake, fasting or sudden changes in feed.
Bad weather, especially winter storms.
Stress, such as transport or yarding.
Low roughage intake (young grasses often have low roughage).
Decreased intake of phosphorus and salt.
Very thin cows (condition score <2) and fat cows (condition score >3.5).

Clinical signs

Clinical signs of grass tetany include:

Restlessness (bellowing, galloping in a blind frenzy).
‘Staggers’ (walking stiffly).
Over-alert appearance.
Being excitable, or even aggressive.
Frequent urination.
Tachycardia (approaching 150bpm).
Loud heart sounds (audible through stethoscope).
Elevated rectal temperatures (up to 40.5˚C).

Acute/severe cases may cause the animal to exhibit:

Falls.
Severe ‘paddling’ seizures, with chomping of the jaws.
Frothy salivation.
Fluttering eyelids.
Eyes making repetitive, uncontrolled movements.
Death.

The disease can progress to the acute stage in a period of up to 2-3 days, or shorter if the animal is transported or herded to new pasture/yards. If the animal is found dead, there are often marks on the ground from previous seizure activity that can indicate grass tetany as a potential cause of death.

Treatment

Treating grass tetany involves restoring blood magnesium concentration. Any treatment must be prompt in order to be effective.

Veterinary administration of intravenous calcium and magnesium solution produces the best results. However, in acute cases where time is especially critical, producers can inject a combined calcium/magnesium solution (e.g., 4 in 1) under the skin in the area behind the shoulder and over the ribs. Massage the area well after injecting the solution, to spread the fluid and aid its absorption into the blood stream.

Treated animals should be identified and given shelter, so that a response to treatment can be measured. Repeat treatment(s) may also be required. The success rate depends on getting to the animals soon enough, and reducing stress from the weather, etc.

Preventing grass tetany

Prevention is much better than treatment when it comes to grass tetany, as it often occurs without warning and treatment success is variable. Activities that prevent grass tetany include good stock management, pasture management and additional supplementation strategies.

Stock management

Lower the herd age structure, as older cows are at higher risk
Keep the herd around a condition score of 3 (on the beef cattle scale)
Avoid stressors and keep mustering during high-risk times to a minimum
Change calving dates to reduce the duration/severity of the risk period. This won’t eliminate the risk altogether and should be considered with other management factors.
Move lactating cows (especially older animals) to high legume and high dry matter pastures
Avoid sudden changes of feed or feed quality

2. Pasture management

Correct soil acidity with lime or dolomite
Plant clovers
Apply phosphate fertiliser
Limit potash and nitrogen fertiliser applications until soil acidity is corrected, and clovers/legumes are established
Plant tree lines for shelter
Spray pastures fortnightly with magnesium sulphate (2% magnesium sulphate in 1000L/hectare, increase if required)

3. Supplementation

Increase energy and roughage intake, e.g., good quality hay or silage fed daily, or at least every second day
Introduce pellets or grain carefully, if cattle are already accustomed to these feeds
Providing loose licks containing limestone, salt, magnesium oxide and molasses
Using intra-ruminal boluses containing magnesium
Feed hay that has been treated with magnesium oxide (e.g., Causmag) during high-risk periods
Epsom salts or magnesium chloride can be added to water supply (500g/100L for Epsom salts or 420g/100L for magnesium chloride)

If magnesium is fed to cattle for a long period of time, it is important to supply additional phosphorus in their diet as a precautionary measure, as magnesium can decrease phosphorus absorption.

Differential diagnoses

Accurate diagnosis of grass tetany by a veterinarian is important, as there are a number of other diseases that present with similar signs. These diseases include:

Staggers caused by Phalaris/ryegrass/paspalum.
Nitrate or nitrite poisoning (also seen on young, rapidly growing heavily fertilised grasses and cereals).
Lead poisoning.
Exotic diseases such as BSE and Aujeszky’s disease.
Locally occurring bacteria or viruses.
Vitamin B1 Deficiency (Polioencephalomalacia)

Once grass tetany strikes, it is difficult to treat. However, as outlined above, there are many ways to prevent it. Depending on the severity of grass tetany incidences, saving one cow may pay for the cost of prevention strategies.

If you are having trouble with grass tetany, call your local veterinarian. If you have more questions about prevention, give us a call on 1800 243 683.