BACKGROUND: Subclinical hypocalcemia (SCHC) is a major metabolic disorder in postpartum dairy cows that impairs health and productivity. Although its pathophysiology is known, the cool and warm periods and physiological factors affecting its severity remain insufficiently understood.

OBJECTIVES: This study evaluated cool and warm periods, physiological, and biochemical determinants of SCHC and milk fever (MF) in postpartum dairy cows.

METHODS: Six hundred Holstein cows from a commercial herd in Shiraz, Iran, were evaluated in two cool and warm periods. Cows were classified as Normal, SCHC, or MF and grouped by parity (1 to ≥5). Blood samples collected within 48 ± 8 hours postpartum were analyzed for calcium, phosphorus, magnesium, albumin, vitamin D, and NEFA. Data were analyzed using ANOVA and Pearson correlations.

RESULTS: Calcium concentrations were lowest in MF cows, intermediate in SCHC cows, and highest in Normal cows (P<0.001), while cool and warm periods did not affect calcium. Phosphorus was lowest in MF and highest in Normal cows (P<0.001), with higher values in warm period. Magnesium was significantly lower in warm period (P<0.001). Albumin was lowest in MF cows (P<0.001) and decreased in cool period. Vitamin D was higher in SCHC and MF cows in cool period (P<0.05). Significant correlations were observed between calcium and phosphorus, magnesium, and albumin. Higher-parity cows struggled more to maintain calcium balance.

CONCLUSIONS: SCHC affects postpartum mineral homeostasis. Although mean serum calcium concentration did not differ significantly between cool and warm periods, seasonal changes in magnesium and albumin were significantly correlated with calcium levels and may indirectly affect calcium homeostasis. Cool and warm periods feeding adjustments, mineral supplementation, and monitoring of key biomarkers are essential to reducing SCHC risk and supporting herd health.