Grass tetany is generally caused by magnesium deficiency and is most common when cattle are grazing fast growing, lush, cool season grasses in the early spring. It is most common in lactating beef cows that are in the peak of milk production. It is seldom seen in feeder cattle.
Grass tetany is most frequently seen when grazing lush immature grasses or small grains and is most likely during cool cloudy weather. Symptoms are uncoordination, staggering, salivation, excitability and finally convulsions and death.
Pasture fertility plays a role in this problem. Low phosphorus level soils may contribute to the problem, thus phosphorus fertilization of low P soils can result in increased uptake of Mg content of grass pastures. At the same time research has shown high levels of potassium and nitrogen can cause reduced Mg absorption in grazing cattle. So a balanced pasture fertilization program with conservative levels of N and K but higher levels of P may help prevent grass tetany.
When conditions for tetany are suspected cows should be provided minerals or supplements carrying high levels of Mg. Mineral mixtures containing 12 to 15 percent Mg should help if 3 to 4 ounces per head per day is consumed. Mg is not very palatable and must be mixed with something in order to get proper consumption. Also providing calcium in the mineral or grain mixture is important.
Source: Al Kennett, University of Missouri Extension