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Grass Tetany Concerns

By Russ Daly

While the quick green-up on spring pastures is the first risk factor that comes to mind for grass tetany, regrowth during warm fall weather can be a culprit, too. Grass tetany is a metabolic disorder that is associated with lush pastures due to low levels of blood magnesium concentration, which results in nerve impulse failure and death, if not treated very quickly.

Multiple factors play a role in causing grass tetany, including:

  • Low magnesium (Mg) content of rapidly growing grasses and pastures
  • High potassium (K) content of rapidly growing grasses and pastures
  • High crude protein content of grasses and pastures
  • Bad weather, storms, stress, etc., that cause cattle to be “off feed” for 24-48 hours
  • Lactation: losses of Mg and calcium (Ca) in the milk
  • Various combinations of the above factors resulting in low blood Mg or Ca

The key to prevention of grass tetany is being proactive. Provide a high Mg supplemental mineral or mineral mix containing at least 8-12% Mg. Palatability and consumption can be challenges, resulting in some of the animals consuming an inadequate amount of the mineral on a daily basis. If possible, providing dry forages while cattle are on lush pastures also helps decrease the risk. Dry forages can act as carriers to provide additional Mg and Ca to the animals at a critical time. If the drinking water source can be controlled (i.e., water tanks), soluble Mg salt may be added to the water. Some examples of soluble Mg salts are magnesium acetate, magnesium chloride, and magnesium sulfate (Epsom salts). The most common form of Mg, Magnesium oxide, is not soluble in water and therefore cannot be used for this purpose. If available, graze pastures with legumes mixed with grasses, as legumes have higher levels of Mg and Ca than do immature grasses.

Older, lactating cows which are still nursing calves will be more susceptible to tetany than steers, heifers, or dry cows. Mature cows are less able to mobilize Mg from bones to maintain the necessary level of Mg in their system. Also, cows that are still nursing and producing milk require additional Ca and Mg.

Cattle will exhibit symptoms of grass tetany, but death can happen quickly – often within 4 to 8 hours. Progressive signs the animal will exhibit include grazing away from the herd, irritability, muscle twitching in the flank, wide-eyed and staring, muscular incoordination, staggering, collapse, thrashing, head thrown back, coma, and death. Affected animals should be handled in a quiet manner, since sudden death can occur if animals incur more stress.

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