Plasma Ionized Calcium and Magnesium Concentrations and Prevalence of Subclinical Hypocalcemia and Hypomagnesemia in Postpartum Grazing Holstein Cows from Southern Chile

The objective was to determine the plasma concentrations of ionized Ca (iCa) and Mg (iMg) and to establish the prevalence of subclinical hypocalcemia (SCHC) and hypomagnesemia (SCHM) in dairy cows at calving (within 6 hours of parturition) and at 7 days postpartum (pp) in Chilean grazing herds with spring parturitions. Plasma iCa and iMg were assessed using a clinical analyzer. A total of 113 and 175 cows in 18 herds selected at random were sampled at calving and at 7 days pp, respectively. From these 18 herds, 11 herds provided reliable records of lactation number and 8 cows per herd were scored for body condition and sampled at calving and then at 7 days pp. Ionized Ca concentrations for the 18 herds were 0.99 ± 0.16 mmol/L (calving) and 1.01 ± 0.13 mmol/L [7 d pp (P > 0.05)]. Ionized Mg concentrations were 0.58 ± 0.12 mmol/L and 0.51 ± 0.09 mmol/L (P ≤ 0.05). For the 11 herds, iCa concentrations at calving were 1.06 mmol/L (lactation 1), 1.02 mmol/L (lactation 2) and 0.89 mmol/L (lactation ≥ 3), while iMg concentrations were 0.63 mmol/L, 0.60 mmol/L, and 0.61 mmol/L, respectively. Herd prevalence for SCHC (iCa < 1.0 mmol/L) at calving was 64.8%. Prevalence by parity was 40%, 54.5% and 86.7% for lactations 1, 2 and ≥ 3, respectively. Herd prevalence of SCHC on day 7 pp was 30.1%. For SCHM (iMg < 0.52 mmol/L) prevalence was 21.6% and 48.9% at calving and at 7 days pp, respectively.