Grass Tetany in Beef Cattle: Prevention and Treatment

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 side during winter and spring are most at risk. Very thin and overly fat animals are also more susceptible. Calves, yearlings and replacement heifers rarely develop grass tetany because they can more easily access body stores of magnesium.

Grass tetany – causes:

Cattle hold magnesium in the bones and muscles but cannot readily access and utilize these stores when needed. The animal constantly loses magnesium in urine, feces and milk, so it needs magnesium in its diet 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 small grain forages than in legumes or weeds.

·         Levels are low in grasses grown on leached acid sandy soils.

·         Levels are low when potash and nitrogen fertilizers 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.

·         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 in loose mineral supplements.

·         Molasses blocks or tubs with elevate levels of Magnesium.

·         Slow-release boluses.

·         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.  Please note that trying to increase magnesium intake through mineral or feed supplementation can be difficult and can actually be detrimental in some situations.  If magnesium levels are elevated too high in the supplement it will actually limit or stop consumption of these products.

Prevention and control:

·         Provide a magnesium supplement to cattle that provide elevated levels of magnesium at least 2 weeks prior to grazing lush or suspect pastures and forages and certainly prior to parturition and the start of lactation.

·         Eliminate factors which reduce magnesium absorption through pasture and grazing management.

·         Provide supplemental roughage in advance of grazing lush pasture.

Immediate actions:

·         Increase energy and roughage intake. Good quality hay is 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 dry matter pastures if available.

·         Reduce stress factors (if corralled do it in a very low stress manner, limit transportation if at all possible).

·         Provide magnesium supplements (see below).

Long term actions:

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

·         Plant clovers if adaptable species will grow in your environment.

·         Apply phosphate fertilizer.

·         Limit potash and nitrogen applications until soil acidity is corrected.

·         Keep good records to inform future management.

For problem pastures, consider forage analysis (primarily the leaves) for magnesium and potassium. Consult your veterinarian for further advice.

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 perennial rye and annual ryegrass toxicity.

·         Nitrate/nitrite poisoning (also seen on young, rapidly growing heavily fertilized grasses and cereals).

·         Lead poisoning – usually from discarded batteries.

·         Locally occurring viruses and bacteria.

How a veterinarian can help:

If cases of grass tetany have been confirmed in the area, work with your veterinarian and be prepared to provide immediate intervention if cattle are found with symptoms of grass tetany.  Work with the veterinarian to develop a management plan to prevent the onset of grass tetany within the herd.  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.