Maintaining an adequate concentration of calcium in blood during the transition period is very important. The start of a new lactation challenges the cow’s ability to maintain normal blood calcium. Colostrum and milk are very rich in calcium, and cows must quickly adjust to these sudden losses. Second and greater lactation cows are generally more affected by this drastic calcium decline.
There is a subclinical form of hypocalcemia that can progress into the clinical form. In the subclinical form the signs are more subtle and difficult to detect, however the earlier its detection the greater the chances for a successful treatment. When a cow becomes recumbent, we are already likely in presence of clinical hypocalcemia.
The subclinical form predisposes to other metabolic diseases. Slight drops in blood calcium concentrations lead to reductions in feed intake, decreased smooth muscle (internal organs) tone, and increased incidence of retained fetal membranes, displaced abomasum, and mastitis. A critical step to maintain adequate blood calcium levels is to achieve an equilibrium between positive and negative ions. The balance between cations (positive charge) and anions (negative charge) should tend to neutrality.
What is the key to maintain blood calcium levels in dairy cows?
When considering close-up cow diets however it is recommended that the dietary ionic balance be negative (more anions than cations) because a negative balance leads to more calcium mobilization and absorption. Bone metabolic markers in blood have been used to monitor the adequacy of Ca mobilization in dairy cows during the transition period. One example is osteoprotegerin, an anti-resorptive protein that downregulates osteoclast differentiation in the bone.
Tartrate-resistant acid phosphatase (TRAP5b) secreted by the osteoclasts provides a good assessment of the number of these active cells present. Since osteoprotegerin downregulates the activity of osteoclasts, it is logical to expect that its concentration will be reduced at calving. The differentiation of the osteoclasts is triggered by calving and regulated by the inducer of its ligand called RANKL (nuclear activator NF-κB ligand) that acts as its inhibitor. From this perspective then the osteoprotegerin/RANKL ratio reflects the amount of bone resorption occurring and the amount of calcium in circulation.