Milk fever in cattle is caused by

Milk fever in cattle is caused by Decrease in the level of calcium in the tissue fluid.
Milk fever, postparturient hypocalcemia, or parturient paresis is a disease, primarily in dairy cattle but also seen in beef cattle and non-bovine domesticated animals, characterized by reduced blood calcium levels (hypocalcemia).
It occurs following parturition, at onset of lactation when demand for calcium for colostrum and milk production exceeds the body's ability to mobilize calcium.