Risk factors associated with postpartum subclinical hypocalcemia in dairy cows.

Modern well-balanced rations in terms of macro-minerals have significantly minimized dietary imbalances which may cause clinical mineral deficiencies. However, subclinical disorders still influence dairy cow health and performance, as the hormonal and metabolic changes from the dry period to the onset of lactation can influence macro-mineral balances. Subclinical hypocalcaemia (SCH) in particular is of concern and therefore it remains a priority to better understand risk factors. Prevention of hypocalcaemia is mainly by means of the diet, with restriction in cations (mainly sodium and potassium) with or without addition of anions in the pre-partum diet. The role of magnesium here is less clear, although hypomagnesaemia is a well-known risk factor in clinical hypocalcaemia. The study of Dr R. C. Neves and his colleagues sought further answers. Their paper was published in the Journal of Dairy Science, Volume 100 of 2017, pages 3796 to 3804, the title being: Risk factors associated with postpartum subclinical hypocalcemia in dairy cows.

The study objectives were to evaluate the relationship of pre-partum plasma magnesium concentrations with SCH at calving and to evaluate the association of other risk factors with SCH at calving or at two days in milk. A total of 301 cows from two dairy herds were enrolled in the study. Blood samples were collected at approximately one week before the expected calving date, within four hours of calving and at two days in milk. Pre-partum blood samples were analyzed for concentrations of calcium, potassium, magnesium, phosphorus, albumin and β-hydroxybutyrate. Post-partum SCH was defined as calcium concentrations of equal or less than 2.1 mmol/L.                                                                  

The prevalence of SCH at calving was 2, 40 and 66% for first, second and third or greater lactations, respectively. Only 4% of cows could be classified with pre-partum subclinical hypomagnesaemia, which unfortunately did not provide enough statistical power to effectively determine the association of plasma magnesium with post-partum calcium concentrations and its effect on SCH. Cows with more than one lactations which had calcium concentrations equal or less than 2.1 mmol/L in the pre-partum period had a higher risk of being categorized as SCH at calving. The risk of SCH at two days in milk was correlated with the interaction of calcium status at calving and lameness score. Non-lame cows with calcium concentrations equal or less than 2.1 mmol/L and lame cows with normal calcium levels at calving were more likely to be SCH at two days in milk compared with non-lame cows with normal calcium levels. 

In conclusion, the researchers quantified a pre-partum calcium threshold (about 2 mmol calcium/L) to identify cows that are more likely to have SCH at calving. However, different risk factors were associated with SCH depending on the timing of diagnosis relative to parturition.