To prevent milk fever, dry cows should receive a diet that will ensure their metabolism is in a calcium (Ca) deficient state.
Effective regulation of Ca also relies on an adequate magnesium (Mg) status as it stimulates the mobilisation of Ca from the bone.
A general rule of thumb is to supplement magnesium at 10-15g Mg/cow/day from three to four weeks before calving to 12-14 weeks after calving.
Dry cows require feeds lower in calcium such as maize silage, which contains less than 2.5g Ca/kgDM. In comparison, grass silage contains around 4.5g Ca/kgDM.
However, supplements like citrus pulp should be fed to dry cows. Citrus pulp contains around 18g Ca/kgDM, which is very high and increases the risk of metabolic issues.
A cow that gets too fat and calves down with a body condition score above 3.5 will have an increased likelihood of milk fever and ketosis. After calving, increase the intake of Ca to restore skeletal Ca reserves.