Grass tetany, caused by a dietary deficiency in magnesium, typically occurs in cattle during spring, when lush, rapidly growing forage provides an inadequate level of the mineral. Left untreated, grass tetany can be fatal to cattle. University of Nebraska Animal Scientist Rick Rasby, PhD, says inadequate dietary sodium also plays a role, as does excess potassium. Sodium helps transport magnesium into cells, he says, and excess potassium consumption interferes with magnesium absorption from the gut, exacerbating the condition of low dietary magnesium.

In a paper titled “Grass Tetany in Cattle,” University of Florida specialists Y. C. Newman, M. J. Hersom, C. G. Chambliss and W. E. Kunkle say signs of the condition include nervousness, lack of coordination, muscular spasms, staggering and death. They recommend contacting a veterinarian immediately if you suspect grass tetany, to diagnose the problem and initiate treatment.

Magnesium oxide is the typical supplement for preventing grass tetany, Rasby says. Its bitter flavor can discourage cattle from consuming adequate levels, but feed companies have worked to produce more palatable supplements containing magnesium oxide. Spring supplementation with magnesium is critical in areas where grass tetany has occurred. Rasby also recommends supplementing salt to provide adequate sodium, and taking care not to supplement excessive potassium.

The University of Florida specialists recommend these prevention and treatment steps:

  • Feed mineral supplements that contain magnesium. Commercial mineral mixtures containing 10 to 15 percent magnesium are available for feeding during periods of increased grass tetany probability. Cattle need to consume 6 to 12 ounces per head per day of the mineral.
  • In herds that have had previous grass tetany problems, increase supplementation to one-half to 1 ounce of MgO per cow per day from two weeks prior to grazing until pastures are more mature and risk subsides.
  • In herds with clinical cases of grass tetany, increase the Mg intake to 1 to 2 ounces per head per day through the high-risk period. 
  • Place mineral feeders in convenient locations around the pasture. Move them closer to watering and resting areas if necessary to ensure consumption.
  • A calcium-to-phosphorus ratio of two-to-one and energy intake at least at maintenance levels can help prevent grass tetany. 
  • Remove animals from pasture or limit grazing during periods of rapid growth, and allow access to hay or dry pasture.
  • On soils that need liming, use dolomitic limestone. If lime is not needed, include magnesium in mixed fertilizers. Do not exceed recommended levels of nitrogen or potassium on pastures.

For the full paper from the University of Florida, click here.