Hypocalcemia in Dairy Cows: A Systematic Review of Metabolic Implications and Management Strategies

Background/Objectives: Hypocalcemia is a major transition-cow disorder in dairy cattle, with clinical and subclinical forms differing in detectability, severity, timing, and herd-level consequences. This systematic review integrates evidence on calcium (Ca) homeostasis, classification of clinical hypocalcemia (CHC) and subclinical hypocalcemia (SCH), diagnostic interpretation, risk factors, systemic effects, and preventive and therapeutic strategies in dairy cows. Methods: Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 statement, PubMed, ScienceDirect, SpringerLink, and CAB Abstracts were searched in February 2026 for 1994-2025 publications, and 23 reports were included in a qualitative synthesis; meta-analysis was not performed because of methodological and outcome heterogeneity. Results: The evidence indicates that hypocalcemia should be interpreted as a failure of homeorhetic adaptation to abrupt mammary Ca export rather than as a simple mineral deficiency. The parathyroid hormone-vitamin D axis, skeletal Ca mobilization, renal Ca conservation, intestinal Ca absorption, magnesium (Mg) status, dietary cation-anion difference (DCAD), dry matter intake, parity, and acid-base balance jointly determine whether blood Ca is maintained during early lactation. Total calcium (tCa) thresholds are useful decision aids for herd-level monitoring, but their interpretation depends on sampling time, parity, persistence pattern, clinical signs, and the relationship between tCa and ionized calcium (iCa). Subclinical hypocalcemia is most relevant when it is delayed, persistent, or occurs in high-risk cows because reduced Ca availability can impair smooth muscle function, feed intake, immune competence, uterine health, and metabolic resilience. Management should therefore combine prepartum ration control, Mg adequacy, DCAD and urine pH monitoring, selective Ca testing in high-risk cows, targeted oral Ca supplementation for standing cows, and intravenous Ca treatment for recumbent CHC cases. Conclusions: The evidence supports a risk-based, context-aware strategy rather than universal threshold-driven treatment.