Method: Factors associated with plasma immunoglobulin G and adequate transfer of passive immunity in dairy calves using different cut-offs

This observational study evaluated factors associated with calf plasma immunoglobulin G ( IgG ) and adequate transfer of passive immunity ( TPI ) based on different cut-offs. Calves (n = 251) in 48 dairy herds were blood sampled between d 1 and 7 of age for plasma total protein ( TP ) determination by refractometry and IgG concentration by ELISA. Information on calving and colostrum management was recorded by dairy producers. Colostrum from the first milking was collected and analyzed for IgG concentration. Three cut-offs determined adequate TPI for plasma samples: 5.5, 5.6, and 5.8 g of TP/dL. Adequate TPI were 58.4, 56.9, and 46.7% according to the 5.5, 5.6, and 5.8 g /dL thresholds, respectively. Two independent variables remained in all models explaining factors related to adequate TPI and calf plasma IgG, which were the quantity of IgG ingested by the calf at the first meal following birth and calf plasma non-esterified fatty acids. Unexpectedly, with the three cut-offs, the odds of adequate TPI for male calves were 1.6–1.8 times higher than for females. In conclusion, evaluating factors associated with continuous calf plasma IgG and adequate TPI using different thresholds yielded to similar inclusion of independent variables in final models. Only the sex of the calf differed between using binary models based on three different thresholds and the continuous calf plasma IgG concentration. However, the strength of the significance for each independent variable varied. For instance, P -values varied from 0.009 to 0.07 regarding the independent variable calf plasma non-esterified fatty acid concentration, which changed from a significant result to a tendency. Hence, the interpretation of results by researchers might change based on the threshold used to define a condition. This can also have implications in meta-analyses where data from several studies are put together to obtain a consensus in the literature. This should be accounted for when interpreting results from a study.