Subclinical Hypocalcemia Across Lactation Stages Reflects Potential Metabolic Vulnerability in Korean Holstein Cows.

Subclinical hypocalcemia (SCH) is traditionally associated with the transition period. However, emerging evidence suggests it may persist or recur throughout lactation. This study investigated the prevalence, metabolic profile, and risk factors for SCH in lactating dairy cows in South Korea. A cross-sectional analysis was conducted on 859 cows from 49 commercial dairy farms. SCH was defined as serum calcium ≤ 8.2 mg/dL. The individual-level prevalence was 9.4%, while 69.4% of herds had at least one affected cow, indicating widespread herd-level occurrence. SCH was significantly associated with parity, with cows in their fourth lactation or higher showing the highest prevalence. Although SCH prevalence did not differ significantly across lactation stages, early lactation cows had significantly lower mean serum calcium concentrations than those in later stages. Milk yield and composition did not differ significantly between groups. However, SCH cows showed notable metabolic differences, including higher serum phosphorus and lower albumin, total protein, and cholesterol levels, indicating potential energy and liver function challenges. Hematological values remained within physiological limits. These findings demonstrate that SCH can persist beyond the transition period, especially in older, high-producing cows, and may remain undetected by routine health checks. Broader monitoring and nutritional support are recommended to enhance metabolic resilience in at-risk animals.