Colostrum conductivity, pH and Brix index as predictors of passive immunity transfer in foals

Background: Foal immunity relies heavily on the absorption of colostrum immunoglobulins; thus, colostrum evaluation is used to predict the transfer of passive immunity (TPI), and its conductivity is associated with TPI in cattle. Leading up to and at parturition, a reduction in colostrum pH and conductivity is thought to be necessary for TPI; however, this remains to be determined. Objectives: To assess conductivity, pH, and Brix refractometer index of colostrum to predict the TPI in foals. Study Design: Field study. Methods: The colostrum of 241 mares was assessed for conductivity, pH, and Brix index using handheld devices immediately after parturition. Twenty-hour postpartum foals had complete blood cell count and plasma IgG concentrations assessed. Foals were split in complete versus incomplete TPI (i.e., IgG ≥8 g/L vs. <8 g/L). Mare (breed, duration of pregnancy, parity, and age), foal (sex), and colostrum (conductivity, pH, and Brix index) data were computed to assess factors affecting TPI. Multivariate regression and receiver operating characteristic (ROC) curves were employed for analysis. Results: Complete TPI was achieved in 89.4% of foals. The ROC TPI values were conductivity ≤5 mS/cm (AUC = 0.68), Brix ≥23.4% (AUC = 0.70), and pH ≤6.4 units (AUC = 0.73). The odds ratio for TPI in 24 h post-partum was 6.1 (2.1–17.1, 95% CI) for conductivity, 3.2 (1.2–9.3, 95% CI) for pH, and 4.5 (1.5–12.7, 95% CI) for Brix index. Sensitivity and specificity of 93% (88–96 95% CI), 56% (44–7, 95% CI), 92% (87–96, 95% CI), 28% (14–47, 95% CI), 94% (88–97, 95% CI) and 18% (14–47, 95% CI) for conductivity, Brix and pH, respectively. Main limitations: Low incidence of incomplete TPI. Conclusions: Conductivity, pH and Brix are strong predictors of foals that succeed in the transfer of passive immunity; however, all three presented low to moderate specificity. The established cutoff values obtained herein can be used on-field by other foaling programs. All three handheld devices used in the study are inexpensive and ready to use on-farm.