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Grass Tetany Can Happen Without Grass
By T.L. Meyer

Many cattle producers consider grass tetany, also known as hypomagnesemia, a problem that affects cattle eating lush, spring grass. However, grass tetany can occur in drylot situations, too, according to a University of Nebraska veterinarian.

"Producers need to be aware that grass tetany can occur in a drylot situation," says Steve Ensley, veterinarian at the university’s North Platte Veterinary Diagnostic Laboratory. "Cattle do not have to have access to green, growing grass to induce grass tetany. Any forage high in potassium has the potential to cause grass tetany."

Interactions between key nutrients in a cow’s body can bring about grass tetany if the diet is unbalanced, he explains. "Low levels of magnesium cause grass tetany. Magnesium enables a cow’s nervous system to function properly. Calcium, phosphorous and potassium are able to interact with — or tie up — magnesium, which can lower the availability of magnesium, even if it is in the diet."

Don Adams, range beef nutritionist at the university’s West Central Research and Extension Center, says producers may have a hard time recognizing the symptoms of grass tetany in a drylot situation.

"Typically, grass tetany is associated with lush, growing grass in the cool season because those plants have low levels of magnesium and high levels of potassium, but it is important [that] producers understand any forage high in potassium has the potential to bring about grass tetany," Adams says.

Because of their diets, grass tetany usually occurs in cows just before calving, he adds. "When the cow is getting ready to lactate and she makes a bag, the magnesium is pulled out of the body, and her serum magnesium level goes down. The free-choice minerals or hay in the cow’s diet are usually high in calcium and potassium. This combination will induce grass tetany."

To prevent grass tetany, Ensley recommends cattle producers replace their current mineral supplement if it is high in potassium (greater than 1%-2% potassium) with a mineral supplement high in magnesium. He also suggests a mineral supplement should contain 7%-14% magnesium at all times. This translates to 10-25 grams of magnesium per head per day.

"Many producers put mineral out in their normal winter mineral supplementation that has high levels of potassium. This time of year, mineral consumption and dry matter intake [are] going up, so more potassium is being consumed, which can promote grass tetany," Ensley says.

Cows with grass tetany can exhibit hyperexcitability, reduced feed intake, and muscular twitching around the face and ears. They also may appear uncoordinated and walk with a stiff gait.

"Many times people see the symptoms but don’t think of grass tetany because the cows are in drylot situations or don’t have access to green, growing grass," Ensley adds.

An intravenous (IV) magnesium-salt solution is used in treating animals with grass tetany. If the condition goes untreated, cows can go down on their sides with convulsions, which leads to death.

For additional information, consult Cooperative Extension NebGuide G73-32A, Grass Tetany, available online at www.ianr.unl.edu/pubs/animaldisease/g32.htm.

Editor’s note: T.L. Meyer is a communications associate at the West Central Research and Extension Center, North Platte, Neb. The University of Nebraska supplied this article.

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