Grass Tetany in Cattle

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Grass Tetany in Cattle

Also known as

Grass staggers, milk tetany, lactation tetany, magnesium tetany, winter tetany, wheat pasture poisoning, crested wheatgrass poisoning, barley poisoning


Grass tetany primarily affects mature cattle grazing lush forage and is due to deficiency of magnesium in the animal’s bloodstream and cerebrospinal fluid (around the brain and spinal cord).

This condition is seen most commonly in cattle grazing early spring pastures or pastures with sudden growth after rainfall following drought or weather changes such as freezing. It was first described in Britain in 1930, associated with magnesium deficiency and calcium deficiency (“milk fever”) and excess potassium in the blood of affected animals.

Magnesium (“grass”) tetany is usually associated with very fast-growing, lush grass that contains a lot of nitrogen. However, it doesn’t have to be lush and green; sometimes winter tetany occurs when cattle are eating grass hay that’s high in potassium with marginal magnesium levels.

Grass is the culprit, as opposed to legumes, because grass picks up extra potassium in what is called “luxury consumption”. Grasses love potassium. The potassium is required for growth but if the soil contains excess potassium, grasses take in more than they need. 

Legumes are not guilty of this greedy consumption; potassium levels in alfalfa and clovers are not high enough to interfere with magnesium absorption in the animal.

When animals die from grass tetany, it’s usually because of weather and grass growth. If there is not enough magnesium, or it’s not being absorbed in the gut at high enough levels, there will be problems. It a certain percentage of animals in the herd are not eating enough of the trace mineral mix during that period, some will develop tetany.

Milk fever and grass tetany are similar. Milk fever is a calcium deficiency and tetany is due to magnesium deficiency, but both have similar signs--except that cows with milk fever are more lethargic and cows with grass tetany are generally more violent.

Some cows have calcium and magnesium deficiency at the same time, so supplements containing both minerals are often used.

Signs of tetany include muscle spasms and convulsions, but the first signs may be restlessness, nervousness or flightiness, leaving the herd, poor appetite, excitability or aggressiveness.

Very upright ears, ears and face twitching, muscle twitches in the flanks and wide-eyed staring are early signs, along with frequent urination, getting up and down repeatedly, head and neck tremors and high-stepping with the front legs. Muscle spasms, rapid eye movements, rapid and snapping retraction of the third eyelid, drooling and excessive chewing are common.

The affected animal is very alert, easily excited, and may charge at anyone or anything that approaches. The belligerent change in attitude might be mistaken for rabies. Some symptoms may be confused with listeriosis or other conditions that affect the brain or cause sudden death.

The animal may run for no reason (often running into fences or other obstacles), with bellowing and frenzied galloping, or may collapse when moved or excited. Stress can bring on clinical signs.

The cow may be uncoordinated and staggering. Then she goes down and can’t get up. At this stage she may lie flat on her side with front legs paddling.  She may thrash or throw her head back, drooling and breathing hard, then lapse into a coma.

Death is usually the result of respiratory failure during a seizure after the cow is down. Often the signs come on so suddenly and these animals die so quickly (within 4 to 8 hours from onset of signs), that you don’t see them acting strangely; you just find them dead.

The ground around the dead animal is usually disturbed, due to thrashing of the animal as it dies.

Since these cattle are often found dead, one way to determine cause of death (and know if other cattle in the pasture are at risk) is to have your veterinarian collect a sample of fluid from the eye or from brain fluid.

This can be analyzed for magnesium content, and is more accurate than a sample of blood serum or tissue since magnesium levels in these may return to normal at death. Blood samples of live animals are accurate for diagnosis, but putting them into a chute to collect samples can create stress--resulting in life-threatening convulsions.


  • Restless, nervous, flighty, leaving the herd
  • Poor appetite
  • Excitability
  • Aggressiveness
  • Upright ears
  • Twitching ears, face
  • Muscle twitches in the flanks,
  • Wide-eyed staring
  • Frequent urination
  • Getting up and down repeatedly
  • Head and neck tremors
  • High-stepping with front legs
  • Muscle spasms
  • Rapid eye movements
  • Rapid retraction of the third eyelid
  • Drooling and excessive chewing
  • Incoordination
  • Convulsions


Low blood levels of magnesium are the cause, and can occur in all age groups and both sexes, but is most common in lactating cows (beef or dairy) in the first 60 days of lactation, especially older cows producing a lot of milk and grazing immature cool-season grasses. It may also occur in late pregnancy.  

The problem develops when forages are low in magnesium, or when other nutrients such as potassium and protein/nitrogen interfere with absorption and utilization of magnesium in the body. In lush fast-growing grasses, the relatively high level of potassium and protein in these grasses tie up availability of calcium and magnesium.

Calves are rarely affected. Mature animals are more susceptible to grass tetany because of their inability to mobilize magnesium from their bones to meet the needs of the body.  They have lower magnesium stores and reduced ability to absorb this mineral. The older the animal, the more susceptible it is.

Tetany cases often coincide with cool, rainy weather or regrowth of plants after frost or drought damage when sodium levels in certain forage plants plummet, while nitrogen and potassium levels spike. The dead cattle have high levels of potassium in their eye fluid.

The low blood levels of magnesium in affected cattle (which affect nerve impulses) cause dramatic signs like incoordination and convulsions. As a general rule, magnesium levels in forages above 0.18% are generally safe, while levels down to 0.12% are risky. Levels below 0.12% are high risk. These percentages can be influenced by high levels of potassium.

Magnesium tetany is a complex syndrome. Potassium and calcium levels can alter the proper balance. Forage analysis of what the cattle are eating can give the percentages of potassium, calcium and magnesium, and if the ratio is skewed and magnesium levels are marginal, the cattle are at high risk for tetany.  

The levels can change in grass plants, depending on several factors. Cereal grasses are most risky, but any high-quality lush grass tends to absorb excess potassium when growing rapidly, and thus decrease absorption of magnesium.

Problems are common when cows in late gestation or lactating cows graze crested wheat grass or immature cereal grains that are growing rapidly and short on magnesium. A heavily-milking cow may not have enough magnesium stores in her bones to overcome the deficiency.

A cow on lush green grass at calving time is at high risk, since her requirement for magnesium triples after she calves.

Grass tetany may also occur if cows are fed grass hay low in magnesium, or whenever the mainstay of diet is cereal green-feed or silage--especially if potassium in those feeds is high. High rumen levels of potassium may interfere with absorption of both calcium and magnesium.

Sometimes the problem occurs after stormy weather, stress or any other reason that causes cattle to be off feed 24 hours or more, further reducing magnesium intake. Cattle that are not getting enough calcium, phosphorus or salt in their diet are also more at risk for grass tetany.

Magnesium is present in most body tissues and is crucial for normal body function, nerve impulses and muscle contraction. About 70% of the magnesium in the body is stored in bones and teeth and not readily available if circulating blood levels drop.

The body’s daily requirement for magnesium must be supplied by diet. When magnesium levels in feeds are low, magnesium needed for milk production quickly depletes the levels in the blood and cerebrospinal fluids. This results in loss of normal muscle function and also affects the nervous system.


Recommended prevention for this condition has been supplemental dietary magnesium, to try to flood the body systems with more magnesium atoms, making sure cattle have plenty of magnesium during risky situations.

Grass tetany can be prevented by using pastures that contain some mature plants. Delay turnout on pastures until grass is at least 4 to 6 inches tall, or feed a mineral supplement containing relatively high levels of magnesium (1 to 2 ounces of magnesium oxide or magnesium sulfate) and calcium, if cattle will eat enough of the supplement, or use a mineral mix containing magnesium in a palatable base.

Magnesium oxide is very unpalatable and cattle won’t readily eat it. It must usually be mixed with grain or a flavoring agent if you want cattle to consume it free choice. A mineral mix should be about 6% magnesium, and cattle need to eat 2 to 3 ounces of it per day, to prevent grass tetany.

They also need plenty of salt. You can encourage salt consumption by using salt-mineral mixes containing molasses.

In many instances it’s impossible to completely prevent tetany by using supplemental minerals since consumption is not consistent enough, especially in large pastures. Place mineral feeders where every animal has access (near the water source where animals go every day) and make sure there is space in the feeders for timid ones.

If cattle are watered in a tank, magnesium sulfate (Epsom salts) can be added to the water to make sure every animal is dosed. Magnesium acetate or magnesium chloride will also work. Don’t use magnesium oxide (a common source of supplemental magnesium); it is insoluble in water.

After pasture grasses become more mature and/or there are more sunny days, mineral supplement or water treatment are longer needed. Another alternative is to delay pasture turnout until grass is more mature, or put less susceptible animals on those pastures first (yearlings, dry cows cows with calves more than 4 months old, past peak lactation).

Cows that develop tetany are more likely to do so again.  They should be culled or put into a different type of feed program where risk situations are avoided

Other ways to prevent magnesium deficiencies include applying fertilizer to problem pastures and soil types, to increase uptake of magnesium by plants. Test pasture grasses for magnesium, calcium and potassium content, if cool season grasses are a major part of the cattle diet.

If the amount of potassium divided by the sum of the forage values for calcium and magnesium is greater than 2.2 the plants are likely to induce magnesium deficiency in grazing animals. Forages with less than 0.2% magnesium content can also be a problem.

Adding legumes like alfalfa or clover to the diet may help prevent magnesium deficiency, since legumes have higher levels of magnesium and calcium than rapidly growing grass, but tetany usually occurs during early spring growth when soil temperatures are still low--because grass gets started ahead of the legumes that haven’t started to grow yet.

This sets the stage for problems, especially if the animals aren’t getting enough salt. Providing adequate salt is an important aspect of prevention.

Salt (sodium chloride) is always important in the diet of cattle, but becomes even more important in helping prevent the complex chain of events that result in grass tetany.

High levels of potassium can reduce magnesium uptake by plants, and can also reduce magnesium absorption by the animal (so fewer magnesium atoms cross into the blood from the digestive tract), yet the effect of high potassium levels may be less important than a low level of sodium.

Sodium is part of the physiological system for absorbing magnesium from the gut.

Herds with good access to salt have fewer cases of grass tetany than herds where cows do not eat much salt. Sometimes those herds consume standard trace mineral mixtures, but if those mixtures contain only low percentages of salt, the cattle may still get grass tetany.

Scientists in Europe conducted laboratory experiments on the effects of sodium chloride. They looked at magnesium absorption sodium levels in the gut and found a physiological, biochemical mechanism by which tetany occurs when salt levels are too low.

During seasons when there is risk for tetany, producers make sure trace mineral mixtures contain enough salt, and that the cattle are actually consuming it. This may depend on what else is in the mineral mix. If the driving force of intake is salt, the animals will eat enough of it, desiring the salt.

If there are other ingredients that are tasty, like soybean hulls or distillers grains, they will eat the mix because of that—and if the salt level in the mineral mix is low they may not be getting enough salt.

Some mixes, called pre-mixes, are designed to be added to salt—as stated on the label. If you follow the label directions the cattle will have plenty of salt.

Pay attention to whether the animals are eating the mineral mix. Trace mineral are usually provided in a salt block or mineral feeder in the pasture, hoping the cattle will eat it. Some do and some don’t.  If 10% of the animals don’t eat it—for whatever reason, which might include being timid and chased away by bossier individuals—that 10% might be at risk for tetany.


Standard treatment has been to give affected cows oral and/or intravenous magnesium. Animals in early stages of tetany must be handled slowly and carefully to avoid stress. 

If you find an animal with tetany, immediately treat the affected individual(s), quietly move the rest of the herd to more mature pastures or a location where they can be fed hay (preferably legume hay, which contains higher levels of calcium and magnesium) or get supplemental magnesium into all of them as soon as possible (such as via the drinking water or a concentrate feed the animals are familiar with and will eat--otherwise they won’t get enough to prevent additional cases).

Cattle on dry feed tend to eat more salt, which should be provided in ample amounts.

If you find an affected cow soon enough, while she can still be moved, or even if she is down and can’t get up but is not yet comatose, the problem can be reversed within minutes by giving 200 to 500 ml of calcium magnesium gluconate intravenously.

There are also commercial preparations of calcium borogluconate solution that contain 5% magnesium hypophosphate.

The calcium solution can be put into the jugular vein. In a lactating cow (especially a dairy cow with big milk veins) it can be put into a vein in front of the udder, since those are easy to find.

The solution should be given very slowly, and the animal’s heart rate closely monitored during IV administration, since magnesium salts can be toxic if absorbed too quickly, resulting in respiratory failure. To avoid this risk, some veterinarians prefer to inject 200 to 300 ml. of a magnesium sulfate solution (Epsom salts) under the skin rather than give an IV injection.

Generally the cow will get up after treatment. Improvement is usually seen within 3 to 5 hours, though a few cows die in spite of treatment--if they suffer another convulsion before the magnesium is fully absorbed.

Relapses may occur 3 to 6 hours later. The animal should be kept as quiet as possible; some vets give a tranquilizer just to keep the animal from getting excited.

Another effective treatment is to dissolve 60 grams of magnesium chloride or magnesium sulfate (Epsom salts) in 6.5 ounces of water, to give as an enema. It is easier to deal with the cow’s rear end than her head (for an IV in the neck) if she’s belligerent or having convulsions.

Use a collapsible plastic bottle attached to a plastic tube inserted into the rectum, letting the fluid flow down the tube and into the rectum. The cow can absorb magnesium through the membrane lining of the rectum. Blood levels of magnesium will rise within 20 minutes after you give the enema.

After treatment the cow may recover quickly, but relapses are common. If you had to treat a cow in a pasture (giving an IV or an enema), your vet may recommend follow-up treatment orally—after she is able to get up and walk.

Confine the cow in a chute and give 3 ounces each of magnesium oxide and dicalcium phosphate, plus one ounce of salt, mixed into 1 or 2 gallons of water, via stomach tube. To play it safe, leave her in a corral for a few days where you can treat her again if necessary.

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EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.