Grass tetany is caused by deficiency of . . . . . . . . in forage crops

Grass tetany is caused by the deficiency of Magnesium in forage crops.
Grass tetany is a metabolic disease involving magnesium deficiency, it occurs in such ruminant livestock as beef cattle, dairy cattle, and sheep. It occurs usually after grazing on pastures of rapidly growing grass, especially in early spring.
Low magnesium intake by grazing ruminants may occur especially with some grass species early in the growing season, due to seasonally low magnesium concentrations in forage dry matter.