Open Veterinary Journal, (2025), Vol. 15(7): 3006-3011

Research Article

10.5455/OVJ.2025.v15.i7.11

Determination of the impact of subclinical hypocalcemia on milk production in the first third of lactation in intensively farmed cows in Lima, Peru

Sonia Gutarra M.¹, Rocío Sandoval-Monzón², Milena Montenegro¹ and Luis Ruíz-García^2*

¹Faculty of Veterinary and Biological Sciences, Universidad Científica del Sur, Lima, Peru

²Faculty of Veterinary Medicine, Universidad Nacional Mayor de San Marcos, Lima, Peru

*Corresponding Author: Luis Ruíz-García, Faculty of Veterinary Medicine, Universidad Nacional Mayor de San Marcos, Lima, Peru. Email: lruizg [at] unmsm.edu.pe

Submitted: 10/03/2025 Revised: 07/06/2025 Accepted: 27/06/2025 Published: 31/07/2025

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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 with fixed factors, with statistical significance (p ≤ 0.05).

Results: Cows with severe SCH had significantly lower cumulative milk production (2,998.58 ± 139.74 kg) compared to normocalcemic cows (3,228.68 ± 119.03 kg, p=0.030), whereas mild SCH cows showed a slight but non-significant reduction (3,160.73 ± 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.

Keywords: Peak production, Stored milk production, SCH, Time to peak production.

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Introduction

Milk production plays a fundamental role in the livestock industry and global food security, with dairy cattle being major contributors by ensuring a stable and sustainable supply of dairy products worldwide. In the context of dairy cows, the transition period, spanning three weeks before and after calving, is a critical phase with significant implications for subsequent production and health (Beñaldo, 2012; Arévalo, 2017). During this period, dairy cow metabolism experiences a substantial increase in calcium demand. A cow requires approximately 23 g of calcium to synthesize 10 l of colostrum on the first day postpartum (Risco, 2004; Jawor et al., 2012), which is nearly eight times higher than normal plasma calcium concentrations.

Calcium is an essential mineral for vital physiological functions such as muscle contraction, enzymatic activation, and cellular stability (Smith, 2010). Although homeostatic mechanisms regulate calcium levels, the elevated demand during the transition period can trigger subclinical hypocalcemia (SCH), a metabolic condition that may compromise milk production and overall animal health (Muiño et al., 2018). The physiological regulation of calcium involves hormones such as parathyroid hormone, calcitonin, and vitamin D. However, these mechanisms may be insufficient to meet the increased demand, possibly due to elevated dietary calcium levels before calving (Muiño et al., 2018).

SCH is characterized by serum calcium levels below 8.5 mg/dl, in contrast to clinical hypocalcemia, which presents with observable clinical signs and typically occurs at a lower incidence (Rodríguez et al., 2017; Seely et al., 2021). Some studies have classified this condition as transient SCH, persistent hypocalcemia, and late hypocalcemia. Transient SCH is generally associated with an effective homeostatic response, whereas persistent and late forms are linked to an increased risk of postpartum disorders and are more detrimental to milk production (McArt and Neves, 2020).

The incidence of SCH is closely related to parity. In the study by Reinhardt et al. (2011), reported prevalence rates were 25%, 41%, and 50% for cows in their first, second, and third lactations, respectively. Similarly, Melendez et al. (2023) observed incidences of 40%, 54.5%, and 86.7% for cows in their first, second, and third lactation. These findings suggest that the overall incidence of SCH frequently exceeds 40% in dairy herds, particularly among multiparous cows.

The presence of SCH, although subclinical, not only directly affects milk production but also increases the risk of other diseases, leading to both direct and indirect economic losses for producers (Reinhardt et al., 2011). SCH interferes with muscle contraction and reduces the motility of the rumen and abomasum, consequently decreasing feed intake (Goff, 2008). It has also been associated with complications such as dystocia, uterine prolapse, retained placenta, and a suppressed immune response, which facilitates infections such as mastitis (Kimura et al., 2006; Zhang et al., 2016).

Previous studies have reported mixed findings regarding the impact of SCH on milk production (Jawor et al., 2012; McArt and Neves, 2020). However, few investigations have specifically examined intensive dairy systems in South America, particularly under the climatic and management conditions characteristic of Lima, Peru. This study aims to address this gap by evaluating the impact of SCH on milk production within this specific regional context.

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Materials and Methods

Location of execution

The study records from three intensive dairy farms located in the province of Lima, Peru. The farms are situated at an altitude of 17 m above sea level. The average annual temperature is 22.66°C, with February being the hottest month (25.92°C) and August the coldest (14.18°C).

Study population

The three farms are characterized by intensive farming and exclusively raise Holstein cattle. They have a total herd of approximately 1,500 lactating cows, from which 483 were monitored from calving to 100 days of lactation and were individually identified. As selection criteria, only cows that met the following conditions were included: at least one calving, no history of clinical hypocalcemia or other related conditions, and complete data records.

Experimental procedure

Blood samples were collected from the coccygeal vein during the first week postpartum, as this period typically shows the lowest serum calcium concentrations. The samples were centrifuged at 200g for 15 minutes to separate the serum, which was subsequently stored at −20°C (Ruiz-García et al., 2022).

Serum calcium levels were measured using spectrophotometry with a commercial diagnostic kit (Ca Color AA, Wiener®). Three calcium levels were defined: Normocalcemia (≥8 mg/dl), mild SCH (≥6.8 and <8 mg/dl), and severe SCH (<6.8 mg/dl). These calcium levels were referenced from the studies by McArt and Neves (2020) and Tsiamadis et al. (2021), which propose subclassifying SCH based on time and severity.

Milk production data were obtained from monthly milking records. Each cow had an individual production record including: daily milk yield, control date, calving date, number of parities, and history of previous diseases.

Statistical analysis

To evaluate the impact of SCH on milk production during the first third of lactation in intensively managed dairy cows in the province of Lima, the cumulative milk yield over the first 100 days was calculated as the total volume of milk produced during that period. Peak milk yield was defined as the highest daily production recorded within those 100 days. Time to peak production was determined as the number of days from calving to the peak yield, typically occurring around day 60. The presence of SCH was assessed during the first week of lactation.

Cumulative milk production, peak production, and time to peak production were compared among cows classified as normocalcemic, with mild SCH, and with severe SCH. Additionally, the effects of parity (first-parity, second-parity, and multiparous) and calving season (summer and winter) were evaluated. Statistical analyses were conducted using IBM SPSS Statistics 22, employing a GLM with fixed factors, with statistical significance (p ≤ 0.05). Prior to GLM analysis, data normality was assessed using the Shapiro–Wilk test, and homogeneity of variances was verified using Levene’s test. When necessary, non-normally distributed data were log-transformed to meet statistical assumptions. The Bonferroni post hoc test was applied to identify significant differences between groups for variables for which the GLM yielded a p-value < 0.05.

Study model

A linear model was used: Y_ijkl=μ + H_i + E_j + P_k +e_ijkl, where Y_ijkl is the observed value of the dependent variable (cumulative milk production, peak production, or time to peak) for the i-th calcium level, j-th calving season, k-th parity group, and l-th observation. µ is the mean; H_i represents the effect of the i-th calcium level; E_j, the effect of the j-th calving season; P_k, the effect of the k-th parity group; and e_ijkl is the random error associated with the ijkl-th observation. Two calving seasons were established: summer (from October to March) and winter (from April to September). Parity was categorised into three groups: first-parity (one calving), second-parity (two calvings), and multiparous (three or more calvings).

Ethical approval

The study was approved by the Institutional Committee on Ethics in Animal Research and Biodiversity at the Universidad Científica del Sur (certificate No. 087-CIEI-AB-CIENTÍFICA-2022). Consent was obtained from the farm owners, who were informed about the nature and objectives of the study. Their anonymity was ensured.

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Results

A total of 483 cows were monitored from calving until 100 days into lactation and were classified into three groups: 167 with normocalcemia (34.6% of the study population), 206 with mild SCH (42.7%), and 110 with severe SCH (22.8%).

Means and 95% confidence intervals (95% CI) were calculated for cumulative milk production (kg), peak milk yield (kg/day), and time to peak production (days) to assess the impact of SCH on each variable. Parity and calving season were also considered to evaluate their effects on milk production.

Table 1 shows that cows with normocalcemia had the highest cumulative milk production (3,228.68 ± 119.03 kg), followed by those with mild SCH (3,160.73 ± 108.09 kg), while cows with severe SCH had the lowest production (2,998.58 ± 139.74 kg). The difference in cumulative yield between the normocalcemic and severe SCH groups was statistically significant (p=0.030), indicating that SCH negatively affects cumulative milk production, particularly in more severe cases.

Multiparous cows produced significantly more milk (3,454.42 ± 114.84 kg) than second-parity (3,296.42 ± 128.19 kg) and first-parity cows (2,637.15 ± 121.14 kg), with a highly significant difference (p < 0.001). Calving season also influenced milk production, which was higher in winter (3,229.91 ± 72.96 kg) than in summer (3,028.75 ± 144.27 kg), a statistically significant difference (p=0.014).

Table 2 shows that peak milk yield did not differ significantly among the calcium level groups (p=0.556). Cows with normocalcemia, mild SCH, and severe SCH reached peak yields of 37.26 ± 1.23 kg/day, 37.02 ± 1.12 kg/day, and 36.30 ± 1.45 kg/day, respectively. These results suggest that although SCH impacts cumulative production, it does not significantly affect the maximum daily yield achieved.

Parity had a significant effect on peak yield (p < 0.001). Multiparous cows achieved the highest peak production (40.58 ± 1.19 kg/day), followed by second-parity (37.85 ± 1.33 kg/day) and first-parity cows (32.14 ± 1.25 kg/day). This indicates that previous calving and lactation experience enhances a cow’s ability to reach a higher peak milk output. Calving season also influenced peak yield, with cows calving in winter achieving significantly higher values (37.81 ± 0.75 kg/day) than those calving in summer (35.91 ± 1.49 kg/day; p=0.025).

Table 3 shows that time to peak production did not differ significantly among calcium level groups (p=0.914). Cows with normocalcemia, mild SCH, and severe SCH reached peak production, on average, at 60.73 ± 5.37 days, 60.74 ± 4.88 days, and 62.21 ± 6.30 days, respectively. These results suggest that SCH does not significantly affect the timing of peak lactation.

Similarly, no significant differences were observed among parity groups (p=0.393), with first-parity, second-parity, and multiparous cows reaching peak production at 62.73 ± 5.46, 58.35 ± 5.78, and 62.59 ± 5.18 days, respectively. Although not statistically significant (p=0.069), a trend was noted regarding calving season: cows calving in summer reached peak production slightly later (64.59 ± 6.51 days) than those calving in winter (57.86 ± 3.29 days).

Table 1. Mean and 95% confidence intervals (CIs) of cumulative milk production at 100 days (kg) and results of variance analysis by calcium level, parity, and calving season.

Table 2. Mean and 95% CI of milk production on the peak production day (kg/day) and the results of the variance analysis according to calcium level, parity, and calving season.

Table 3. Mean and 95% CI of the time to peak production (days) and results of variance analysis according to calcium level, parity, and calving season.

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Discussion

In numerous studies, the association between SCH and milk production has been investigated. However, the results have not been consistent. Earlier studies assessed this relationship without considering additional variables such as the duration or timing of SCH after calving. As a result, no significant differences in milk production were found between groups of cows with SCH and those without the condition (Jawor et al., 2012; Gild et al., 2015). However, more recent research has shown that differences do emerge when these additional factors are considered (Neves et al., 2018; McArt and Neves, 2020; Seely et al., 2021; Ruiz-García et al., 2022). Therefore, the present study aims to determine the impact of SCH on milk production during the first third of lactation in intensively managed cows in Lima, Peru.

The reported incidence of SCH in Holstein dairy cows varies widely. Reinhardt et al. (2011) found an occurrence of 54% in recently calved cows, while Roche (2003) reported 33%, and Fiorentin et al. (2018) reported 17.1%. This considerable variability across studies can be attributed to differences in production systems, methodological approaches, cut-off values, and timing of calcium evaluation, all of which complicate direct comparisons.

In the present study, an overall SCH incidence of 65% was observed, with 43% of cows classified as having mild SCH and 23% as having severe SCH. This incidence aligns with the findings of Reinhardt et al. (2011) and Caixeta et al. (2017), who reported rates of 54% and 73%, respectively, in studies using intensive production systems and similar cut-off thresholds. In contrast, Fiorentin et al. (2018), who applied a lower cut-off point (7.5 mg/dl) and studied cows in free-stall and semi-confinement systems, reported a much lower prevalence.

The results show that cows with mild SCH (6.8–8.0 mg/dl) had slightly lower cumulative milk production than normocalcemic cows, whereas those with severe SCH (<6.8 mg/dl) exhibited significantly lower cumulative yields than the normocalcemic group (≥8.0 mg/dl; p=0.030). These findings indicate that SCH negatively affects cumulative milk production, particularly in more severe cases.

This is consistent with the study by Chapinal et al. (2012), which reported that hypocalcemia around calving is associated with reduced milk yield. Similarly, McArt and Neves (2020) found that first-parity and multiparous cows with persistent or late-onset hypocalcemia—where calcium levels were assessed after the second day postpartum, as in the present study—had lower milk production and a higher risk of early lactation diseases compared to normocalcemic cows. Overall, the present results suggest that the severity of SCH plays a crucial role in determining lactation performance.

In contrast, studies that assessed calcium levels within the first 24 hours postpartum have not found a negative association between low calcium concentrations (<7.21 mg/dl) and milk production (Jawor et al., 2012). In fact, lower calcium levels during this period were associated with higher milk yields in weeks 2, 3, and 4 compared to control cows. These results are consistent with McArt and Neves (2020), who reported that cows experiencing SCH but quickly adapting to early lactation did not exhibit reduced milk production. It has therefore been proposed that transient SCH has a lesser impact on milk yield than persistent or late-onset cases. This highlights the importance of distinguishing between cows that recover from SCH and those that maintain the condition over time or develop it later during the postpartum period.

Regarding peak daily milk yield (kg/day), no effect of calcium status was observed; however, both parity and calving season had a significant influence. Similarly, peak production (days) was not significantly affected by calcium levels, parity, or calving season. Although this study did not find an association between SCH and either the timing or magnitude of peak lactation, it clearly demonstrated that SCH significantly impacts cumulative milk yield.

These findings underscore the importance of managing this condition, as cows with SCH may experience greater physiological discomfort and reduced water intake (Jawor et al., 2012). This is consistent with other studies indicating that hypocalcemia can contribute to postpartum complications and reduced nutrient utilisation efficiency (Venjakob et al., 2018).

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Conclusion

SCH significantly affects cumulative milk production at 100 days of lactation, with a more pronounced reduction observed in severe cases. Parity and calving season significantly influence cumulative yield and peak milk yield. Peak milk yield and time to peak production were not significantly affected by either mild or severe SCH.

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Acknowledgments

The authors are grateful to the farm owners and staff for providing the information necessary to conduct this study, and to Dr. Karla Arévalo and Dr. Joe Pizarro for their valuable technical assistance.

Funding

This research was supported by the Universidad Nacional Mayor de San Marcos – RR N° 006081-R-23 and project number A23081581.

Author contributions

All authors participated in the study, wrote the article, and participated in the monitoring and processing of the results. All authors read and approved the final manuscript.

Conflict of interest

The authors declare no conflict of interest.

Data availability

The data supporting the findings of this study are not openly available due to sensitivity reasons. However, they may be obtained from the corresponding author upon reasonable request.

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