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.
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.
- 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).
- 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.
Diseases similar to grass tetany
Accurate diagnosis of grass tetany by a veterinarian is important because a number of significant diseases have similar signs. These include:
- staggers caused by phalaris, perennial rye, paspalum and annual ryegrass toxicity
- nitrate/nitrite poisoning (also seen on young, rapidly growing heavily fertilised grasses and cereals)
- lead poisoning – usually from discarded batteries
- exotic diseases such as bovine spongiform encephalopathy (BSE) and Aujeszky’s disease
- locally occurring viruses and bacteria.
Early detection of exotic disease
Producers play a vital role in the early detection of exotic diseases in Australia.
If you see:
- unusual disease signs
- abnormal behaviour
- unexpected deaths.
in your stock, ring your private veterinarian, the local Department of Primary Industries and Regional Development, Western Australia (DPIRD) field veterinary officer or the Emergency Animal Disease hotline on 1800 675 888. Early recognition and reporting of an exotic disease will help to minimise the spread and impact on your markets and your community.
How a veterinarian can help
A veterinarian will be able to 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 cattle between 30 months and nine years old showing neurological signs (such as grass tetany 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 under the NTSESP. More information is available at the National Transmissible Spongiform Encephalopathies Surveillance Program webpage.
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.
DPIRD Field Veterinary Officers can provide more information about grass tetany in cattle. To find the contact details of your closest DPIRD Field Veterinary Officer, go to the Livestock Biosecurity program contacts page.