Cattle need a continuous supply of magnesium primarily to facilitate numerous energy-generating reactions in their tissues and for the orderly transmission of nerve pulses. The kidneys maintain magnesium homeostasis, and a renal threshold exists below which magnesium excretion is sharply reduced. Cattle rely on absorption of magnesium from the rumen to meet most of their needs. The animal cannot modify magnesium absorption and absorbs only a small proportion (<30%) of the magnesium consumed.
There are two types of hypomagnesaemia, hypomagnesaemic tetany in calves, which appears to be due to a straightforward deficiency of magnesium in the diet, and lactation tetany, in which there may be a partial dietary deficiency but in which nutritional and metabolic factors reduce the availability, or increase the body loss, of magnesium.
Slower grass growth and lack of artificial fertilisation on organically managed pastures may be protective against lactation/grass tetany in cows.