Calf-, calving-, and colostrum-related factors associated with transfer of passive immunity in dairy calves: an observational cohort study

Newborn calves depend on colostrum intake for transfer of passive immunity (TPI), but the factors influencing this process under field conditions remain incompletely understood. This observational cohort study investigated factors associated with serum IgG concentration in 68 dairy calves from three farms in southern Portugal. Calf, calving, colostrum, microbial, and feeding-management variables were recorded. TPI was classified as Poor (<10 g/L), Fair (10.0–17.9 g/L), Good (18.0–24.9 g/L), or Excellent (≥25 g/L), with Poor/Fair and Good/Excellent grouped as suboptimal and adequate TPI, respectively. Farm-adjusted logistic regression (n = 66) and continuous-outcome sensitivity analysis were performed. Eight calves (11.8%) had Poor, 20 (29.4%) Fair, 21 (30.9%) Good, and 19 (27.9%) Excellent TPI. A one-standard-deviation increase in colostrum pH (0.208 units) was associated with lower odds of adequate TPI (OR = 0.26; 95% CI: 0.07–0.61) and a 3.86-g/L lower serum IgG concentration (HC3 95% CI: −5.63 to −2.10). Colostrum pH was therefore associated with both serum IgG concentration and TPI classification. However, the modest sample size, unequal farm contributions, and confounding between farm and management practices limit causal inference and generalisability. These findings warrant further investigation in larger, balanced, prospective studies.