What is the condition in a cow that has given birth that is treated with magnesium?

There is no known illness that has cows having a deficiency of magnesium immediately after calving! You might be thinking of calcium, not magnesium, that dips suddenly in the cow right after calving. This is called Milk Fever. Milk fever occurs when the cow is unable to mobilize reserves of calcium from her bones and thus has low blood calcium levels. As a result she decreases in eating, becomes constipated, and shows general depression. Nervousness and collapse soon follow along with unconsciousness. The head is noted to be usually turned back. Treatment with calcium is important to save the cow via IV injection.

Magnesium deficiency only occurs in beef cows in early lactation when they graze spring pasture containing less than 0.2% magnesium. Generally occurs during the first 2 weeks of pasture season (not necessarily 2 weeks after calving!) Early lactation are not considered as "right after birth." This magnesium deficiency is called Grass Tetany. The symptoms include nervousness, twitching of muscles (usually of head and neck), head held high, accelerated respiration, high temperature, gnashing of teeth, and abundant salivation. Slight stimulus may precipate a crash to the ground, and finally death. It is treated with magnesium or calcium salts via IV injection, but must be done as soon as possible to save the animal from this highly fatal illness.

***NOTE the DIFFERENCES between Milk Fever and Grass Tetany!!!!!!!