Grass tetany, or hypomagnesia, is caused by low blood levels of magnesium.
It is most prevalent when cows and ewes that are in late gestation or heavy lactation graze lush spring growth, usually in April and May, but cases can be seen in the fall. Rapidly growing forages have low levels of magnesium, and the availability of magnesium is further reduced by high levels of protein and potassium in the forage.
Grass tetany also is known as grass staggers and wheat pasture poisoning. Grass tetany is most prevalent in spring pastures that had very short grazing heights the previous fall or contain little to no carryover residue.
Species most likely to cause grass tetany include cool-season grasses (crested wheatgrass, bromegrass, bluegrass and timothy), annual cereal grasses (wheat, rye, oats) and native range that lacks standing litter from the previous year.
Leaving sufficient forage in the fall will not only trap snow, retain subsoil moisture and reduce runoff potential, but it also will providesufficient magnesium in the old growth to help balance the mineral profile and reduce the potential for grass tetany in the spring. In addition, pastures containing legumes or legume/grass mixes will reduce the chances of livestock developing tetany.
Pastures fertilized with nitrogen (N) and potassium (K) fertilizers also are more likely to induce grass tetany. The plants’ uptake of nitrogen and potassium from the fertilizer further decreases the amount of magnesium available to the animal. Test the soils to ensure that nutrient rates in the fertilizer are not in excess of plant requirements for growth.
Susceptibility to Tetany
Cows and ewes in heavy lactation and late gestation are most susceptible to the disease. If conditions are favorable for grass tetany, dry cows and heifers, stocker cattle and cows with calves older than 4 months can be grazed on those pastures.
Cows grazing grass-dominant pastures account for most cases of tetany. Cloudy, windy, rainy weather with the daytime temperaturebetween 40 and 60 F seems to be the environmental conditions most likely to induce grass tetany symptoms.
Tetany seldom occurs when legumes or legume-grass mixtures are a major portion of the animal’s diet. Legumes usually contain at leasttwice the magnesium as grasses grown in the same soil.
Animals with grass tetany may experience excitable and erratic behavior. They can appear to be blind. Clinical signs can includehypersensitivity to touch, muscle tremors, frequent urination, staggered walking pattern, separation from the herd, convulsions, coma and,ultimately, death.
The onset of the condition can be very rapid, and the first clinical sign producers may see is a dead animal. The ground surrounding a dead
cow should be studied for marks indicating whether a cow was lying down and thrashing her feet while convulsing prior to death.
A veterinarian should investigate all unexplained sudden deaths immediately. Anthrax also can cause unexplained sudden deaths (see NDSU Extension publication “Anthrax,” V561).