Blood total calcium in the adult cow is closely regulated. Subclinical hypocalcemia (SCHCa) were in this study defined as blood Ca concentration below 2.2 mmol/L (8.8 mg/dl). The prevalence of subclinical hypocalcemia in dairy cows at dry off is unknown. The objective of this study was to determine the prevalence of subclinical hypocalcemia at drying off in dairy herds in France and Denmark. Additionally, to identify risks factors related to the drying-off management procedures leading to the emergence of hypocalcemia at this stage. This is a multicenter prospective cohort study including 381 dairy cows from 37 herds in France and 345 cows from 21 herds in Denmark. Biochemical parameters were assessed at 2 blood samplings; one in mean (SD) 0.9 (0.7) h (BS1) after the last milking and another in mean (SD) 9.2 (1.3) h later (BS2). Data on feeding and management practices at cow level were collected. Specifically, the interaction between dry-off method and incidence of SCHCa at BS2 was assessed using descriptive statistics and ANOVA. Abrupt dry off implied no prior change in either milking or feeding; 180 and 154 cows were dried off abruptly in France and Denmark respectively. Gradual implied any change in these practices; 201 and 175 cows were dried off gradually in France and Denmark respectively. Calcium concentrations at BS1 and BS2 and relative variation between BS1 and BS2 are shown in Table 1. Cows dried off gradually in France had an incidence of SCHCa at BS2 at 17%, in Denmark 21.8%. Cows dried off abruptly had a lower incidence of SCHCa at BS2 in France 5.5% and in Denmark 2.7%. Relative risk (RR) for SCHCa at BS2 is 3.85 (CI95% 2.18:6.78) (P < 0.0001) for cows dried off gradually compared with abrupt. Reduction of blood Ca concentrations after dry-off occurs with varying incidence across countries. A gradual dry off increases the likelihood that cows will have subsequent subclinical hypocalcemia. Table 1. Mean [minimum; maximum] calcium concentration (mmol/L)

Country	BS1	BS2	Relative variationBS1 − BS2
France	2.4[1.6; 2.9]	2.3[1.6; 2.7]	−1.7%[−33.3; 31.3]
Denmark	2.4[1.9; 3.2]	2.3[1.5; 3.1]	−4.0%[−37.5; 13.6]