Grass tetany in beef cattle: prevention and treatment | Agriculture and Food

Grass tetany can be treated with an intravenous dextrose-based commercial preparation of magnesium and calcium purchased from a local veterinarian.

Grass tetany is a highly fatal disease associated with low levels of magnesium in the blood.

Grass tetany can affect all classes of cattle, but older cows with calves at foot during winter and spring are most at risk. Very thin and overly fat animals are also more susceptible, as are Angus cattle and their crosses.

Grass tetany - causes

Cattle hold magnesium in the bones and muscles but cannot readily access and utilise these stores when needed. The animal constantly loses magnesium in urine, faeces and milk, so it needs magnesium in its feed to meet daily requirements.

A cow in peak lactation (6–8 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.

Low magnesium in the blood of an animal can be caused by low magnesium levels in feed and/or reduced magnesium absorption.

Contributing causes are:

magnesium levels are lower in cool season grasses and cereals than in legumes or weeds

levels are low in grasses grown on leached acid sandy soils

levels are low when potash and nitrogen fertilisers are used and growth is vigorous

high moisture content in grass causing rapid gut transit and low uptake

reduced absorption of magnesium resulting from high rumen potassium and nitrogen and 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 have low roughage and often poor palatability)

low intake of phosphorus and salt.

Signs

Animals suffering from grass tetany are often found dead. There may be marks on the ground beside the animal indicating they were leg paddling before death (lying on their side with stiff outstretched legs that thrash backwards and forwards).

Early signs include some excitability with muscle twitching, an exaggerated awareness and a stiff gait. Animals may appear aggressive and may progress through galloping, bellowing and then staggering.

In less severe cases, the only signs may be a change in the character of the animal and difficulty in handling.

Treatment of affected cattle

Blood magnesium levels must be restored. Veterinary administration of an intravenous calcium and magnesium solution produces best results. However, in acute cases where time is critical, producers can inject a calcium and magnesium solution under the skin.

Producers should also provide oral sources of magnesium to affected herds to prevent relapses. These include:

magnesium oxide powder for dusting onto feed or pasture

magnesium lick blocks

slow-release capsules

magnesium sulphate or soluble magnesium chloride added to hay or silage

adding magnesium to concentrates or pellets.

These products are available from your veterinarian, feed supplier or rural supplies company.

Prevention and control

Management should aim to:

eliminate factors which reduce magnesium absorption and

provide a magnesium supplement.

Immediate actions:

Increase energy and roughage intake. Good quality hay and silage are suitable

Pellets or grain can be added if introduced carefully and cattle are accustomed to these

Provide salt if a natural source is not available

Move lactating cows (especially older animals) to high legume and high dry matter pastures

Provide shelter

Reduce stress factors (yarding, transport)

Provide magnesium supplements (see below).

Long-term actions:

Correct soil acidity with lime or dolomite (dolomite contains some magnesium)

Plant clovers

Apply phosphate fertiliser

Limit potash and nitrogen applications until soil acidity is corrected and clovers are established

Plant tree lines for shelter

Keep good records to inform future management.

For problem paddocks, consider pasture leaf analysis for magnesium and potassium. Consult your veterinarian for further advice.

Other options for magnesium supplementation

 Treat hay rolls with magnesium sulphate:

Lactating cows need at least two grams of magnesium per kilogram of dry matter.

Add 3g magnesium sulphate per kilogram of hay to help make up any shortfall (a 350kg hay roll needs 1kg of magnesium sulphate added).

Add molasses to improve palatability. Dissolve 1kg of molasses with 1kg of magnesium sulphate in 20 litres of hot water. Soak this into the hay roll before feeding, or spray onto hay after it is rolled out.

Apply dry magnesium oxide to pasture at up to 30kg/ha every three weeks during risk periods (of all supplements, magnesium oxide has the highest magnesium content).

Spray pasture with magnesium sulphate fortnightly (2% magnesium sulphate in 1000L per hectare = 20kg/h). Use higher rates of magnesium if needed.

Observe label directions where provided.