Abstract
Background:
Subclinical hypocalcemia (SCH) is a metabolic disorder in dairy cows occurring during early lactation due to increased calcium demand. It is associated with health complications, reduced feed intake, and economic losses. However, its direct impact on milk production remains controversial.

Aim:
This study aimed to determine the effect of SCH on cumulative milk production, peak milk production, and time to peak production during the first third of lactation in intensively managed Holstein cows in Lima, Peru.

Methods:
A total of 483 Holstein cows from three intensive dairy farms were monitored from calving until 100 days in lactation. Cows were classified into normocalcemic (≥8 mg/dL), mild SCH (≥6.8 and <8 mg/dL), and severe SCH (<6.8 mg/dL) groups based on serum calcium levels measured during the first postpartum week. Milk production data, including cumulative, peak, and time to peak production, were recorded. The effects of calcium levels, parity (first-parity, second-parity, and multiparous), and season (summer and winter) were determined using a general linear model (GLM) with fixed factors, with statistical significance (p ≤ 0.05).

Results:
Cows with severe SCH had significantly lower cumulative milk production (2998.58 ± 139.74 kg) compared to normocalcemic cows (3228.68 ± 119.03 kg, p = 0.030), whereas mild SCH cows showed a slight but non-significant reduction (3160.73 ± 108.09 kg). No significant differences were observed in peak milk production or time to peak production among the calcium level groups (p = 0.556 and p = 0.914, respectively). However, parity significantly influenced cumulative and peak milk production, with multiparous cows producing the highest volumes (p < 0.001). Seasonal variation was also significant, with higher cumulative and peak milk production recorded during winter (p = 0.014 and p = 0.025, respectively).

Conclusion:
SCH, particularly in its severe form, negatively affects cumulative milk production but does not significantly affect peak production or time to peak production. Multiparous cows and those calving in winter exhibit higher milk production. While SCH alone may not drastically impair production, other associated factors could exacerbate economic losses in dairy farms.

Key words: Peak production, Stored milk production, SCH, Time to peak production