Postpartum magnesium status as an important risk factor for delayed and persistent subclinical hypocalcaemia in grazing Holstein cattle

Subclinical hypocalcaemia (SCH) is a significant metabolic disorder affecting dairy cattle. Research has shown that dairy cows experience three states of SCH, known as transient, delayed, and persistent SCH. Our objective was to identify potential risk factors for dairy cows with delayed and/or persistent SCH that are managed under grazing conditions. Grazing Holstein cows (n = 158) were randomly selected from eighteen farms in southern Chile, of which 39 were in their first lactation (24.7%), 30 were in their second lactation (19.0%), and 89 were in third or more lactations (56.3%). Lactation data and body condition score (BCS) at calving were recorded for each cow. Blood samples were collected at calving and 7 days postpartum to determine concentrations of plasma ionised calcium (iCa) and ionised magnesium (iMg). Using plasma iCa concentration thresholds of ≤ 0.92 mmol/L at calving and ≤ 1.05 mmol/L at 7 days postpartum, cows were classified as normocalcaemic (33.4%), transient SCH (13.3%), delayed SCH (29.3%), and persistent SCH (24.0%). Older cows had significantly higher SCH prevalence at calving compared to younger cows. However, by day 7 postpartum, the proportion of delayed/persistent SCH cases was similar across all three parity groups. The prevalence of SCH over this period remained consistent for first- and second-parity cows. A linear regression model was built to analyse the concentration of plasma iCa at 7 days postpartum. Results showed that plasma iMg concentration at 7 days postpartum, plasma iCa concentration at parturition, parity, and BCS at calving were significant predictors in the final model. A logistic regression model was used to identify risk factors for SCH at 7 days postpartum. The only significant variables in the final model were BCS at calving and plasma iMg concentration at 7 days postpartum. Cows with plasma iMg concentrations ≤ 0.5 mmol/L were 6.26 times more likely to develop persistent or delayed SCH at 7 days postpartum compared to those with plasma iMg concentrations > 0.5 mmol/L. Overall, our results demonstrate that plasma iMg at 7 days postpartum is an important risk factor associated with plasma iCa concentrations and the occurrence of delayed/persistent SCH in grazing dairy cows. Other potential risk factors, such as parity, plasma iCa concentration at calving, and BCS at calving, may also play a role in the presentation of these conditions.