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Calves are rarely affected. Mature animals are more susceptible to grass tetany because they are less able to mobilize magnesium from their bones to meet the needs of the body. Mature cattle have less stored magnesium that they can draw on quickly, and also have a reduced ability to absorb this mineral. The older the animal the more susceptible it is to this problem.

Low blood levels of magnesium can occur, however, in all age groups and in both sexes under certain conditions, but is most common in lactating cows—beef or dairy—in the first 60 days of lactation. Older cows producing a lot of milk are most at risk, especially when grazing immature cool-season grasses, such as lush early growth. Grass tetany can also occur in late pregnancy if forages are low in magnesium, or when certain other nutrients such as potassium and protein/nitrogen interfere with adequate absorption and utilization of magnesium in the body. When cattle are first turned out on lush pasture in the spring, for instance, the relatively high level of potassium and protein in those immature grasses may tie up the availability of calcium and magnesium.

Milk fever (caused by sudden calcium deficiency) and grass tetany (magnesium deficiency) are similar in many ways and have similar symptoms, except that cows with milk fever are more lethargic and cows with grass tetany are generally more violent. Cattle may have calcium and magnesium deficiency at the same time. For this reason, supplements containing both minerals—and treatment with products to restore the proper levels of both minerals—are often used.

Conditions In Which Grass Tetany May Occur
The most common scenario is cool wet weather in spring, with very little sunshine, especially if cattle are grazing young plants that grow best in these conditions and the plants are high in potassium and soluble nitrogen. Cereal grasses such as wheat pastures are most risky, but any high quality lush grass tends to absorb excess potassium while growing rapidly. Eating these grasses decreases the animal’s ability to absorb magnesium.

Grass tetany may also occur in fall or winter in mild climates where rain and lush new growth can occur that time of year. It is also common when lactating cows or cows in late gestation graze crested wheatgrass or immature cereal grains that are growing rapidly and short on magnesium. A lactating cow may not have enough magnesium stores in her bones to overcome the deficiency. A cow on lush green pasture at calving time is at high risk, because her requirement for magnesium triples after she calves.

Grass tetany may occur in winter if cows are fed grass hay that’s low in magnesium. It may also occur if the mainstay of diet is cereal greenfeed or silage, especially if the potassium level in greenfeed or silage is high. High rumen levels of potassium may interfere with absorption of both calcium and magnesium into the bloodstream. Grass tetany may also appear after stormy weather or some type of stress if it causes cattle to be off feed for 24 hours or more, further reducing their magnesium intake. Animals that are not consuming enough calcium, phosphorus or salt in their diet are also more at risk for grass tetany.

Magnesium is normally present in most body tissues and is crucial for proper body function, nerve impulses and muscle contractions. About 70 percent of the magnesium in the body is stored in bones and teeth and not readily available if blood levels drop. Thus the body’s daily requirement for magnesium must be supplied by diet. When magnesium levels in feeds are low, such as in lush spring pasture, magnesium needed for milk production quickly depletes, lowering the levels in blood and cerebrospinal fluids. This results in loss of normal muscle function and also affects the nervous system.

Symptoms
Signs of grass tetany include muscle spasms and convulsions, but the very first signs you’ll notice may be restlessness, nervousness or flightiness. The affected cow may leave the herd or stop eating, and she may become more excitable or more aggressive than normal. Very upright ears, face and ears twitching, muscle twitches in the flanks, and wide-eyed staring are early signs, along with head and neck tremors, frequent urination, getting up and down repeatedly, and high stepping with the front legs.

Muscle spasms, rapid eye movements, rapid and snapping retraction of the third eyelid membrane, drooling and excessive chewing are also common signs. The affected animal is very alert, easily excited and may charge at anyone or anything that approaches her. This belligerent change in attitude is sometimes mistaken for rabies. Symptoms may also be confused with listeriosis or other conditions that affect the brain or cause sudden death.
The animal may run for no reason (with bellowing and frenzied galloping), often running into fences or other obstacles. She may be uncoordinated and staggering, and collapse when she gets excited or if you try to move her. Stress may bring on more symptoms. Eventually the animal 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, and then lapse into a coma. Death is usually the result of respiratory failure during a seizure after she is down on the ground. Often the symptoms come on so suddenly—and these animals die so quickly, within 4 to 8 hours from onset of symptoms—that you don’t see them acting strangely. You just find them dead. The ground around the dead animal is usually torn up, due to the thrashing of the animal as it dies.

One way to determine the cause of death (and thus find out if other cattle in the pasture are at risk) is to have your vet 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 body tissue since the 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 the samples may stress the animal more, resulting in life-threatening convulsions.

Treatment
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 where she is, if possible, and quietly move the rest of the herd to more mature pasture or a pen where they can be fed hay. Legume hay is best, since it contains higher levels of calcium and magnesium. If you can’t move them, get supplemental magnesium into them as soon as possible, putting it into the drinking water or into a concentrate feed the animals are familiar with and will readily eat, otherwise they won’t consume enough to prevent additional cases. Salt 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 her 200 to 500 ml of calcium borogluconate solution that contains 5 percent magnesium hypophosphate. This 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 the big vein in front of the udder, since these are easy to find.

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

Generally the cow will be able to get up after this treatment. Improvement is usually seen within 3 to 5 hours, though a few cows die in spite of treatment—especially if they suffer convulsions before the magnesium is fully absorbed. A relapse may occur 3 to 6 hours after the treatment. The animal should be kept as quiet as possible. Some vets give a tranquilizer, just to keep the animal from becoming excited.

Another effective treatment is to dissolve 60 grams of magnesium chloride or magnesium sulfate (Epsom salts, which can be purchased at a grocery store or drug store) in 200 ml of water, to give as an enema. It is easier to deal with the sick cow’s rear end than her head (trying to give an IV in the neck, for instance) if she’s belligerent or having convulsions and thrashing. 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 the magnesium through the lining of the rectum. Blood levels of magnesium should rise within 20 minutes after giving the enema.

After treatment the cow may recover quickly, but relapses are common. If you have to treat a cow out in the pasture, giving an IV or an enema, your vet may recommend a follow-up oral treatment, after the cow is able to get up and walk. Bring her slowly to a place you can restrain her and give her an oral mix containing 3 ounces each of magnesium oxide and dicalcium phosphate, plus one ounce of salt, mixed into 1 or 2 gallons of water. This can be given by stomach tube. To play it safe, leave her in a corral for a few days where you can treat her again if necessary.

Cows that develop tetany are more likely to do it again in the future. They should be culled, or put into a different type of feeding program or different pastures, where the risk situation is avoided.

Prevention
Grass tetany can be prevented by using pastures that contain mature plants. Don’t turn cattle out on pastures until the grass is at least 4 to 6 inches tall. If you have to turn out earlier than that when grass is very immature, 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 it, 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 be mixed with grain or a flavoring agent if you want cattle to consume it free choice. A mineral mix should be about 6 percent magnesium, and cattle need to eat 2 to 3 ounces of it per day in order to prevent grass tetany on risky pastures. You can encourage salt and mineral consumption by using a salt-mineral mix containing molasses.

But in many instances it’s impossible to completely prevent tetany by use of mineral supplements because consumption is not consistent enough, especially in large pastures. Always put the mineral feeders where every animal will have access, such as near the water source where cattle must go every day. Make sure there is space in the mineral feeders for even the timid ones to have opportunity to eat.

It’s very important that each animal consume an adequate amount daily. 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 in the water. Even though it is a common source of supplemental magnesium, it is insoluble in water. After pasture grasses become more mature and/or there are more sunny days, the mineral supplement or water treatments are no longer needed.

Another alternative is to delay pasture turnout until grass is more mature. Or, you can put the less susceptible animals on the riskiest pastures first. You can often get by on those pastures with yearlings, dry cows, or cows with calves that are more than 4 months old, since the cows will then be past their peak of lactation.

Other ways to prevent magnesium deficiencies include applying fertilizer to problem pastures and soils, to increase the uptake of magnesium by the plants. Testing pasture grasses for magnesium, calcium and potassium content can be revealing, if cool season grasses are a major part of the cattle diet. If the amount of potassium divided by the sum of the forage totals for calcium and magnesium is greater than 2.2 the plants are likely to put grazing animals at risk for magnesium deficiency. Forages with less than 0.2 percent magnesium content can also be risky.

Adding legumes like alfalfa or clover to the diet can help prevent magnesium deficiency, since legumes have higher levels of magnesium and calcium than rapidly growing grasses. If you have problems with grass tetany, consult your vet or county extension agent; they can usually help you come up with a plan for your situation, to reduce the risks.