Grass tetany is caused by a magnesium imbalance in cattle often due to young, rapidly growing forage with an increased potassium content, usually the result of fertilization in the growing season. The increased potassium reduces the animal’s ability to absorb magnesium and hypomagnasemia can result.

Information from Jeremy Powell, DVM, University of Arkansas, says that grass tetany is often seen under-conditioned or over-conditioned mature, lactating beef cows. Clinical signs of grass tetany early in the disease include decreased appetite, decreased milk production, tendency to stay away from the herd, increased alertness and a stiff or unsteady gait. As the disease progresses, cattle may become recumbent and unable to get up. They will exhibit muscle tremors (spasms), protruding third eyelid, increased pulse and respiratory rates and eventually death if untreated.

Animals in an early tetany stage should be handled carefully to avoid creating additional stress. In the Beef Cattle Handbook, Kvasnicka and Krysl recommended that if tetany occurs, take one or more of the following steps as soon as possible:

  • Immediately treat affected animals as previously described.
  • Get supplemental Mg into all of the animals in the herd as soon as possible. At this time, mineral supplementation is not the method of choice. This step does not guarantee that all cows will get Mg immediately. If the animals are familiar with concentrates, feed this fortified with Mg so that each animal consumes 1 to 2 ounces of magnesium oxide daily or the equivalent. If the animals are not familiar with concentrates, they will probably not eat it soon enough to prevent an outbreak. Adding Mg o the water under the conditions described earlier will work.
  • Move the herd quietly to more mature pastures or pastures containing legumes.
  • Move the herd from the tetany-prone pastures to a location where they could be fed hay, preferably legume hay.
  • If tetany is anticipated, but has not yet been seen, watch the herd closely; take blood, urine, or forage samples to predict the tetany status of the animals, and be prepared to act. Previous nutritional history will not modify the response of animals to a tetany situation. To prevent this malady, Mg must be supplied on a daily basis when tetany-prone conditions exist.