Both clinical and subclinical deficiency of calcium and magnesium may cause problems in dairy cows. Clinical hypocalcaemia most commonly occurs at calving and onset of lactation and is associated with milk fever, while clinical hypomagnesaemia occurs under certain dietary conditions. Factors affecting the calcium and magnesium status in dairy cows were examined in this thesis.
The effect of dietary magnesium (0.19 and 0.43 % of dry matter) and potassium (1.9, 2.8, and 3.7 % of dry matter) on magnesium digestibility and magnesium balance was assessed using a Latin square design with six lactating cows. The effect of supplying dietary calcium to dry cows (0.49, 0.93, and 1.36 % of dry matter) on calcium and magnesium homeostasis was investigated in 29 periparturient cows. A case-control study was used to investigate whether mineral feeding during the last part of the dry period differed between 30 herds with high incidence of milk fever, and 22 herds with no milk fever. Finally, the effect of prepartum milking for 1 to 7 days on calcium homeostasis was investigated in 15 cows around calving.
Magnesium uptake in lactating cows was found to depend on the level of dietary magnesium, but not on the potassium concentration. Dietary calcium had no effect on hypocalcaemia at calving, and was not different between herds with high milk fever incidence and herds without milk fever. However, high levels of calcium in the diet resulted in decreased magnesium absorption. High amounts of potassium in the diet were associated with increased risk of high milk fever incidence, while high amounts of dietary magnesium were associated with decreased risk of high milk fever incidence. Prepartum milking decreased plasma calcium levels and activated the calcium homeostatic mechanisms. However, there were no differences in the degree of hypo¬calcaemia at calving, and plasma concentration of calcium decreased within 1 h after calving, indicating that factors other than milk removal alone were responsible for the decrease in plasma calcium levels at calving. The conclusion was that cows should be fed high amounts of magnesium and low amounts of potassium during the last part of the dry period to avoid milk fever, while the potassium concentration in the diet of lactating cows is of less importance regarding magnesium uptake. Milk removal affects calcium homeostasis, but the effect on the risk of milk fever is unclear
