Grass tetany is a disorder caused by an abnormally low amount of magnesium in the animal's blood. Beef cattle producers in Kentucky have generally been successful in reducing the incidence of tetany. However, the potential still exists for this disorder to be a problem.
Grass tetany occurs most often in cows grazing lush spring forages, especially small grains and cool-season perennials, such as fescue. It is most common in spring calving cows, especially if they are high producers in their third to fifth lactation.
Several factors contribute to the increased incidence of tetany at this time. The magnesium requirement of cows doubles from late gestation to early lactation (from 9 grams to 21 to 22 grams).
When this rapid change in magnesium needed by the cow is coupled with lowered magnesium in the plant along with certain components that lower the availability of magnesium (such as high applications of nitrogen and potassium fertilizers), tetany can develop.