The IgG-to-Protein Ratio Influences the Relationship Between Brix Values and Immunoglobulin G Concentration in Bovine Colostrum

This study investigated the relationship between Brix values and the nutritional and immunological composition of bovine colostrum. Colostrum samples (n = 82) were collected from Holstein-Friesian cows on four dairy farms. Dry matter (DM), solids non-fat (SNF), crude protein (CP), fat, IgG concentrations, and Brix values were determined. Correlation, regression, and receiver operating characteristic (ROC) analyses were used to evaluate associations among variables and identify a Brix cut-off for high-quality colostrum (IgG ≥ 50 g/L). Brix was positively associated with both CP (ρ = 0.90) and IgG concentration (ρ = 0.75), although the relationship between Brix and IgG was significantly influenced by the IgG/CP ratio. A robust regression model including SNF and fat provided the best prediction of Brix values (R2 = 0.98). ROC analysis identified a Brix cut-off of approximately 25%, which detected high-quality colostrum with 83% sensitivity and 76% specificity. Colostrum above this threshold had higher DM, SNF, CP, and IgG concentrations, but not fat concentration. These findings suggest that Brix values primarily reflect the soluble solids fraction of colostrum and that variation in the proportion of IgG within total protein may contribute to the variability in Brix–IgG relationships and threshold values reported among studies.