Watch for Grass Tetany

Published on: 09:59AM Apr 20, 2009

By Dan Goehl


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Grass tetany, or hypomagnesemia, is a metabolic disorder that can affect adult beef cattle. This syndrome is a result of low magnesium levels in the circulatory and nervous systems, and it is most common when lactating cows graze lush green pastures including annual ryegrass, small grains (i.e., oats, rye, wheat), and cool-season perennial grasses (i.e., tall fescue). 

The occurrence of the syndrome is seasonal and related to the flush of new forage growth associated with the onset of spring. The syndrome is more frequent when pastures have been fertilized with potassium and nitrogen and when solids are naturally high in potassium and low in sodium. Onset of disease is related to low magnesium intake, high potassium and low sodium intakes, and low blood calcium levels (due to peak lactation). 

Early identification, appropriate treatments, and prevention are keys to minimizing negative herd impacts from this disease.


Cows in early stages of grass tetany may be restless, isolated from herd, and stop eating. Individuals may also be easily excitable, and may run away or charge when disturbed.  These animals may walk with a high-stepping action of the forelimbs when excited. 

Later stages involve uncoordinated walking and eventually staggering and falling to ground. Animals will be unable to get up and typically suffer from excessive muscle contractions, seizures and eventually death.  The progression of clinical signs is relatively quick, and the first signs noted may be a down cow or a dead animal.  Diagnosis of the syndrome is typically based on the type of animal affected, time of year, and a physical examination. The diagnosis can be confirmed by your veterinarian with blood work illustrating a low magnesium level.


Due to the rapid progression of the disease, treatment is most successful early in the disease progress. Prognosis for recovery is related to the time interval between onset of clinical signs and institution of treatment. Treatment of affected animals will involve increasing the magnesium blood levels. 

The most rapid and effective method is through intravenous therapy, and you should consult your local veterinarian for prompt treatment of down animals. Success of intravenous treatment will depend on appropriate product selection, rate of administration, and monitoring animal’s response to determine proper dosages.  Follow up treatments for individuals may include oral magnesium gels.


Ideal management of grass tetany or hypomagnesemia is prevention of the syndrome. Prevention techniques involve Mg supplementation and forage management. 

The most reliable method of prevention is supplemental feeding of magnesium on a daily basis immediately prior to and during the grass tetany season. The appropriate levels can be incorporated into the ration as a part of a free-choice mineral or a pre-mix added to a ration. A reasonable goal for magnesium levels is 14% magnesium mineral with a projected intake of four ounces per head per day. 

Supplementation should start four to six weeks prior to expected onset of spring lush grass growth. Remember that magnesium is not very palatable to cows, so mineral consumption should be monitored and formulation adjusted to be sure adequate intake is achieved.

Adding legumes to pastures alters forage composition and increases pasture quality and improves levels of magnesium. Appropriate fertilization techniques should be employed by regularly testing pasture soils and applying appropriate levels of lime, nitrogen, phosphorus, and potassium. 

In summary, low magnesium or grass tetany, can cause a severe disease syndrome in beef cows.  The disease typically affects lactating cows grazing lush grasses.  Treatment success is dependant on early identification of disease and prompt treatment by your veterinarian. 

Prevention is key to management and centers on magnesium supplements for the cow herd during the susceptible period.  If you have more questions about grass tetany in your area, please contact your local veterinarian.


Dan Goehl, DVM, and his wife own and operate Canton Veterinary Clinic in Canton, MO, where Dan works primarily with stocker and cow/calf beef operations. Dan is also partner in Professional Beef Services, LLC, which offers herd consultation and helps in data management and marketing of beef cattle.