Lush, rapidly growing grasses present a considerable risk for grass tetany in the spring.
The combination of high potassium and low magnesium and calcium in lush, rapidly growing forages is thought to be the primary cause of grass tetany. Because of the increased forage production and growth rate, cattle grazing nitrogen-fertilized pastures are generally at higher risk.
Grass tetany tends to occur most often in the early to mid-spring following a cool period (less than 60 degrees F), when grasses are lush and growing rapidly but can also occur in the fall with new growth of both small cereal grains (such as wheat, oats, rye) and cool-season grasses, especially crested wheatgrass.
Pastures and hays comprised of legumes and legume-grass mixtures are good sources of magnesium and calcium and are generally not a concern for grass tetany. Feeding legume-based hay or grazing pastures with a legume-grass mixture early in the grazing season can provide some supplemental magnesium to livestock.
The most effective management strategy to address grass tetany is to prevent it. One method to minimize the risk of grass tetany is to delay turnout until the forage is more mature and is past the rapid growth stage (at least 6 inches tall), allowing mineral concentrations in plants to stabilize or to delay nitrogen fertilizer application until after initial spring grazing.
Grass tetany occurs most frequently in cows that are nursing young calves. Growing cattle, dry cows, or cows with calves older than four months of age are generally at a lower risk for grass tetany. Grazing non-lactating animals on high risk pastures can reduce or eliminate problems with grass tetany.
One of the most effective management strategies to prevent grass tetany involves supplementing magnesium while the cattle are grazing high-risk forages. Cattle should receive at least .5 to 2 ounces of supplemental magnesium per head per day to prevent tetany. Magnesium is not effectively stored in the body, so supplementing more than two weeks before grazing does not improve effectiveness.
Supplementing magnesium through a high magnesium block or mineral mix usually works best when livestock are out on range. Supplemental magnesium may also be added to protein supplements, silage, or as a liquid. Mixing magnesium supplements with a more palatable feed (such as dried molasses, distillers grain, or soybean meal) will help improve consumption.For more information regarding grass tetany, refer to SDSU Extension Extra 2055 at: