Here are background information and tips on protecting grazing cattle against grass tetany, an often fatal metabolic disorder characterized by low levels of magnesium in cattle's blood serum.
Spring brings risk for metabolic/nervous problems in cattle brought on by acute magnesium deficiency, a condition she called tetany, grass staggers, milk tetany, lactation tetany, winter tetany, wheat-pasture poisoning, crested wheatgrass poisoning, or barley poisoning. Tetany mainly affects mature cattle grazing lush forage, due to a deficiency of magnesium in blood and cerebrospinal fluid.
Calves are rarely affected. Mean-while, mature animals are less able to mobilize magnesium from their bones to meet the body's needs. Plus, they have less stored magnesium that can be drawn on quickly, and less ability to absorb this mineral.
Low blood levels can occur in all ages and in both sexes under certain conditions, but it's most common in lactating cows — beef or dairy — in the first 60 days of lactation. Heavier-milking cows are most at risk, especially when grazing immature cool-season grasses with lush early growth.
Grass tetany can also occur in late pregnancy if forages are low in magnesium, or when potassium and protein/nitrogen interfere with absorption and utilization of magnesium. The relatively high level of potassium and protein in lush immature grasses may tie up availability of calcium and magnesium.
Milk fever (caused by sudden calcium deficiency) and grass tetany (magnesium deficiency) have similar symptoms, but cows with milk fever are more lethargic; cows with tetany are generally more violent. Cattle may have calcium and magnesium deficiency at the same time. Supplements containing both minerals — and treatment to restore proper levels of both minerals — are often used.
Times of most risk
The most common scenario for the occurrence of tetany is cool wet weather in spring with little sunshine, if cattle are grazing young plants that grow best in these conditions and 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, decreasing the animal's ability to absorb magnesium.
Grass tetany may occur in fall/winter in mild climates with lush new growth. It's also common when late-gestation or lactating cows graze immature crested wheatgrass or cereal grains that are growing rapidly and short on magnesium. A cow on lush green pasture at calving is at risk; her requirement for magnesium triples after she calves.
Cows fed cereal greenfeed or silage are at risk, especially if potassium level is high. High rumen levels of potassium may interfere with absorption of calcium and magnesium. Stormy weather or any stress that causes cattle to be off feed 24 hours or more will further reduce magnesium intake. Animals that aren't consuming enough calcium, phosphorus or salt are also at risk.
Magnesium is present in most body tissues, crucial for proper body function, nerve impulses and muscle contractions. About 70% of the body's magnesium is stored in bones and teeth and not readily available if blood levels drop. Daily requirements must be supplied by diet. When feed levels are low, magnesium needed for milk production quickly depletes, lowering the levels in blood and cerebrospinal fluids. This results in loss of normal muscle function and affects the nervous system.
Signs of grass tetany include muscle spasms and convulsions. The first signs may be restlessness, nervousness or flightiness. The cow may leave the herd or stop eating. She may become excited or aggressive. 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.
In addition, rapid eye movements, rapid retraction of the third eyelid, drooling and excessive chewing are common. The animal is very alert, easily excited and may charge at anything that approaches. This belligerent change in attitude is sometimes mistaken for rabies or other conditions that affect the brain.
The animal may run for no reason (bellowing), running into fences or obstacles. She may be uncoordinated and staggering, and collapse when excited or if you try to move her. Eventually she goes down and can't get up. She may lie flat on her side with front legs paddling.
She may throw her head back, drooling and breathing hard, then lapse into coma. Death is usually from respiratory failure during a seizure after she's down. Symptoms often appear suddenly; animals often die within 4-8 hours. You may not see them acting strangely; you just find them dead. Ground around the dead animal is usually disturbed from thrashing.
Animals in early stages must be handled slowly and carefully to avoid stress. Immediately treat the individual where she is, if possible, and quietly move the herd to more mature pasture or a pen to be fed hay. Legume hay is best, with higher levels of calcium and magnesium. If you can't move cattle, get supplemental magnesium into them as soon as possible via drinking water or a concentrate feed they are familiar with and will readily eat. Salt should be provided in ample amounts.
If you find an affected cow while she can still be moved (or down but not yet comatose), the problem can be reversed within minutes by intravenous (IV) administration of 200-500 ml of calcium borogluconate solution containing 5% magnesium hypophosphate. In a lactating cow, it can be put into the big milk vein in front of the udder.
The IV solution should be given slowly and heart rate closely monitored. If magnesium salts are absorbed too quickly they can be toxic, resulting in respiratory failure. Some vets prefer to inject 200-300 ml of magnesium sulfate solution (Epsom salts) under the skin, rather than give an IV.
Generally the cow will get up after treatment. Improvement is usually seen within 3-5 hours, though some cows die if they suffer convulsions before the magnesium is fully absorbed. The cow should be kept as quiet as possible. Some vets administer a tranquilizer.
Another effective treatment is to dissolve 60 grams of magnesium chloride or magnesium sulfate (Epsom salts) in 200 ml of water, given as an enema. It's easier to deal with the cow's rear end than her head (trying to give an IV in the neck) if she's belligerent or thrashing. Use a plastic tube inserted into the rectum, letting fluid flow down into the rectum. She can absorb magnesium through the rectal lining. Blood levels should rise within 20 minutes.
The cow may recover quickly after treatment, but relapses are common. Your vet may recommend follow-up oral treatment after she gets up. Bring her slowly to a place you can restrain her and give an oral mix containing 3 oz. each of magnesium oxide and dicalcium phosphate, plus 1 oz. of salt, mixed in 2 gal. of water, given by stomach tube. Leave her where you can treat her again if necessary.
Cows that develop tetany may do it again. They should be culled, or put in a different feeding program or pastures.
Don't turn cattle out until grass is 4-6 in. tall. If you must turn out when grass is immature, feed supplement containing 1-2 oz. of magnesium oxide or magnesium sulfate and calcium. Magnesium oxide is unpalatable and must be mixed with grain or a flavoring agent like molasses if you want cattle to consume it free choice. A mineral mix should be 6% magnesium, and cattle must eat 2-3 oz./day to prevent tetany on risky pastures.
Tetany may be impossible to completely prevent with mineral supplements because consumption isn't consistent, especially in large pastures. Put mineral feeders near water sources where cattle must go every day. If they drink from a tank, magnesium sulfate (Epsom salts) can be added to the water to make sure every animal is dosed. Don't use magnesium oxide; it's insoluble. After grasses become more mature and/or there are more sunny days, mineral supplement or water treatments are no longer needed.
Another alternative is to put less-susceptible animals (yearlings, dry cows, or cows with calves more than four months old, past their peak of lactation) on riskiest pastures. Adding legumes like alfalfa or clover to diet can help prevent magnesium deficiency, since legumes have higher levels of magnesium and calcium than rapidly growing grasses.
Heather Smith Thomas is a Salmon, ID, rancher and freelance author of books and articles on livestock management. Look for her titles at amazon.com.