PS10-25. Validation of a Brix Refractometer as an On-farm Tool to Assess Colostrum Quality and Passive Immunity Transfer Status in Lambs.

Abstract Adequate transfer of passive immunity is critical for neonatal lamb survival, yet laboratory determination of immunoglobulin G (IgG) concentration is impractical for routine flock management. Brix refractometry has been shown to provide a rapid on-farm method to estimate IgG concentrations in cattle, but validation in different sheep breeds under commercial management conditions is limited. The objective of this cross-sectional diagnostic study was to evaluate the relationship between on-farm Brix refractometer readings and laboratory-measured IgG concentrations in ewe colostrum and lamb serum, and to determine diagnostic thresholds for identifying inadequate colostrum quality and transfer of passive immunity. The study was conducted on a commercial Rideau Arcott sheep flock in Ontario, Canada. Colostrum samples were collected from ewes within 12 h of lambing (n = 277) and lamb were blood sampled once between 24 h and 7 d of age (n = 546). Serum was spun on farm, split, and digital Brix readings were performed. Frozen samples were shipped to an off-site lab to determine IgG concentration using radial immunodiffusion (RID). Relationships between Brix values and IgG concentrations were assessed using Pearson correlation and linear regression. Receiver operating characteristic (ROC) curve analysis was used to evaluate the ability of Brix values to classify inadequate colostrum quality (< 50 and < 60 g/L IgG) and inadequate passive transfer (< 10 and < 15 g/L IgG), using suggested values from previous work. On-farm colostrum Brix values were moderately correlated with laboratory IgG concentrations (r ≈ 0.42, P < 0.001), explaining 18.1% of the variability in IgG concentrations. Diagnostic performance for identifying inadequate colostrum quality was fair (AUC = 0.69 and 0.68 for < 50 and < 60 g/L IgG, respectively). Optimal Brix thresholds identified by ROC analysis were 24% for detecting IgG < 50 g/L and 25.4% for detecting IgG < 60 g/L, yielding sensitivities of 90% and 86.9%, respectively, with moderate specificity (51-57%). In contrast, serum Brix values showed a strong correlation with lamb serum IgG (r ≈ 0.67, P < 0.001), explaining 45.4% of the variability. Diagnostic performance for identifying inadequate passive transfer was excellent (AUC = 0.957 for < 10 g/L and 0.952 for < 15 g/L IgG). Brix thresholds of approximately 7.3–7.6% achieved sensitivities near 97% with moderate specificity (55-70%). These results demonstrate that while Brix refractometry has moderate predictive value for ewe colostrum IgG concentration from ewes in our sample, it provided an accurate and practical on-farm tool for assessing transfer of passive immunity in lambs using serum samples. Implementation of serum Brix refractometry may support rapid flock-level monitoring of passive transfer in prolific sheep production systems.