Blood total calcium concentrations in the fourth week postpartum of dairy cows continue to play a key role in uterine involution.

In the present study, we examined the relationship between uterine involution, serum metabolites and calcium concentrations in clinically healthy postpartum dairy cows with or without subclinical endometritis (SCE). A cohort of 43 Holstein cows, sampled between 21 and 29 days postpartum, was selected based on the absence of clinical endometritis or other clinical postparturient diseases. Reproductive tract measurements included the diameters of the cervix and uterine horns, and blood sample analyses included serum total calcium, total cholesterol, triglycerides, non-esterified fatty acids, and β-hydroxybutyrate. Using the cytobrush, endometrial cytology samples were collected to assess the proportion of endometrial polymorphonuclear leukocytes (PMNs) at 21-29 days postpartum. Results indicated no association between SCE, defined as >5 % PMNs in endometrial smears, and either uterine dimensions or serum calcium levels. However, a general linear model, accounting for random effects of parity and days postpartum, demonstrated that lower serum calcium concentrations were associated with greater cervical and uterine horn diameters (P < 0.001), suggesting a linear dependence of uterine structural dimensions on calcium concentration. For all the other metabolites, no associations were found with cervical nor uterine horns diameters. This indicates blood calcium's role in supporting uterine involution during the late postpartum period. Interestingly, the PMN% in endometrial samples (characteristic of SCE) did not correlate with changes in uterine size, suggesting that other factors related to the periparturient period rather than serum calcium levels influence the development of SCE in the fourth week postpartum.