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Milk fever is a metabolic disorder caused by insufficient calcium, commonly occurring around calving.

Milk fever, or hypocalcaemia, is when the dairy cow has lowered levels of blood calcium. Milk fever generally occurs within the first 24 hours post-calving, but can still occur two to three days post-calving. It can be either clinical or subclinical.

Clinical milk fever includes both “downer” and “non-downer” cows with less than 1.4mM blood calcium.
Sub-clinical milk fever includes cows with less than 2.0mM but more than 1.4mM blood calcium.
Milk fever increases the risk of other metabolic diseases and infections, such as ketosis and metritis, and approximately 5 percent of downer cows do not recover.

In 2000, a New Zealand study indicated that cows suffering from clinical milk fever produced 14 percent less milk, while cows with sub-clinical milk fever produced 7 percent less milk. The impact of this disorder on the average dairy farm was estimated to be $8,000 per 100 cows.

The dairy cow obtains calcium from her diet or from stores in her bones. Although there are substantial amounts of calcium available from these sources, the absorption from the intestines, or resorption from bone, is under tight hormonal control and is affected by other minerals (e.g. phosphorus and magnesium) and vitamins (e.g. vitamin D).

With the onset of lactation, and production of colostrum, the cow’s requirement for calcium increases substantially (400 percent increase in a day). To meet these calcium requirements, the cow must increase both the absorption and resorption processes. Any factors that interfere with these processes mean the cow cannot meet the increased demand for calcium, and this results in lowered blood calcium concentration and milk fever.