How grass tetany develops
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.
Cattle store magnesium in the bones and muscles but they cannot readily utilise these stores. Cattle excrete magnesium in urine, faeces and milk, cattle need magnesium in feed to meet daily requirements.
A cow in peak lactation (6–8 weeks following calving) uses magnesium from the body in milk. During this time a feed source low in magnesium such as rapidly growing grass will not provide sufficient magnesium to compensate for the increased demand in milk production.
Low magnesium in the blood of an animal can be caused by low magnesium levels in feed and/or reduced magnesium absorption.
Causes of low magnesium levels
Low magnesium in the blood of an animal can be caused by low magnesium levels in feed and/or reduced magnesium absorption.
Magnesium availability is lower:
- in cool season grasses and cereals than in legumes or weeds
- in grasses grown on leached, acid, sandy soils
- when potash and nitrogen fertilisers are used, increasing plant growth.
Magnesium absorption is reduced by:
- high moisture content in grass causing rapid gut transit
- high rumen potassium and nitrogen
- low energy intake, fasting or sudden changes in feed
- low roughage intake (young grasses have low roughage and often poor palatability)
- low intake of phosphorus and sodium.
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Signs and treatment of grass tetany
In less severe cases, the only signs may be a change in the character of the animal and difficulty in handling.
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.
- be found lying on their side with stiff outstretched legs that thrash backwards and forwards.
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.
Blood magnesium levels must be restored.
The most effective restoration is veterinary administration of an intravenous calcium and magnesium solution. However, in acute cases where time is critical, producers can inject a calcium and magnesium solution under the skin.
To prevent relapses, producers should also provide oral sources of magnesium to affected herds. 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
- magnesium added to concentrates or pellets.
These products are available from veterinarians, feed suppliers or rural supply companies.
Preventing grass tetany
Prevention measures should aim to:
- reduce factors which limit magnesium absorption
- provide a magnesium supplement to cattle most at risk.
- increase energy and roughage intake (good quality hay and silage are suitable)
- careful introduction of pellets or grain, provided cattle are accustomed to these
- provide a salt lick
- move lactating cows (especially older animals) to higher legume and higher dry matter pastures
- provide shelter
- reduce stress factors (yarding, transport)
- provide magnesium supplements (see below).
- 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.
Treat hay rolls with magnesium sulphate:
- Lactating cows need at least 2 grams of magnesium per kilogram of dry matter.
- Add 3 g magnesium sulphate per kilogram of hay to help make up any shortfall (a 350 kg hay roll needs 1 kg of magnesium sulphate).
- Add molasses to improve palatability. Dissolve 1 kg of molasses with 1 kg 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.
Treating pasture:
- Apply dry magnesium oxide to pasture at up to 30 kg/ha every 3 weeks during risk periods (of all supplements, magnesium oxide has the highest magnesium content).
- Spray pasture with magnesium sulphate fortnightly (2% magnesium sulphate in 1000 L per hectare = 20 kg/h). Use higher rates of magnesium if needed.
- Observe label directions where provided.
How a veterinarian can help
A veterinarian can investigate whether grass tetany or another disease is occurring. Producers who detect signs of grass tetany in their stock, or notice any other unusual signs, should contact their private veterinarian or local DPIRD veterinary officer.
If a veterinarian autopsies on cattle aged between 30 months and 9 years old showing neurological signs, the producer may be entitled to claim a payment from DPIRD as part of the National Transmissible Spongiform Encephalopathies Surveillance Program (NTSESP). Incentive payments are also available for private veterinarians participating in the program.
These payments encourage reporting of neurological signs so that Australia can show it has tested sufficient animals with negative results to prove we are free of diseases such as bovine spongiform encephalopathy (BSE). Australia needs this proof of freedom to maintain export markets.
Diseases which look similar to grass tetany
Accurate diagnosis of grass tetany by a veterinarian is important because many significant diseases have similar signs. These include:
| Disease | Signs of disease | Comments |
| Plant poisoning | Depression, staggering, weakness, sometimes convulsions and sudden death. |
Staggers caused by phalaris, perennial rye, paspalum and annual ryegrass toxicity. Other toxic plants include gastrolobium or bracken fern or heliotrope. Occurs when feed is scarce or animals graze unfamiliar areas. |
| Lead poisoning | Staggering, blindness, convulsions, death. | Report immediately if you suspect lead poisoning. Often linked to old batteries or lead paint ingestion. |
| Nitrate/nitrate poisoning | Fast breathing, blue gums, weakness, grey diarrhoea, sudden death. | Also seen on young, rapidly growing heavily fertilised grasses and cereals. |
| Bovine spongiform encephalopathy (BSE) | Progressive nervous signs, behavioural changes, incoordination and wasting. | Exotic disease. Suspicion must be reported immediately to a veterinarian or the Emergency Animal Disease hotline. Subsidies apply for a disease investigation through your veterinarian. |
| Aujeszky's disease | Intense itching (usually head/neck), self-trauma, salivation, difficulty swallowing, ataxia, paralysis, rapid death. | Exotic disease. Suspicion must be reported immediately to a veterinarian or the Emergency Animal Disease hotline. Subsidies apply for a disease investigation through your veterinarian. Cattle are dead-end hosts infected via contact with pigs. Mimics rabies ('mad itch'). |
Early detection of exotic disease
Producers play a vital role in the early detection of exotic diseases in Australia.
If you see any of the following in your stock:
- unusual disease signs
- abnormal behaviour
- unexpected deaths
Contact your private veterinarian, the local DPIRD vet or the Emergency Animal Disease hotline on 1800 675 888.
Early recognition and reporting of an exotic disease helps minimise the spread and enables economic containment if present. Veterinary investigations that confirm a suspect exotic disease is not present also provides supporting evidence of freedom from market sensitive diseases. This evidence enables Australian cattle and beef to enter important international markets.