Magnesium (Mg) deficiency in dairy cows was recognised in New Zealand in the 1970s and supplementing with magnesium in late pregnancy and early lactation has become routine on most farms.

Magnesium supplementation will reduce animal health problems and potentially increase milk production. However, the magnesium requirement of the modern dairy cow has increased, partly due to increased use of nitrogen (N) and potassium (K) fertilisers and partly due to an increase in cow genetic merit.