Determination of neonatal serum immunoglobulin G concentrations associated with mortality during the first 4 months of life in dairy heifer calves.

Colostral administration practices on dairy farms have significantly improved over the last 15-20 years resulting in prevalence of calves ingesting insufficient colostrum decreasing from 35-40% to 19%. Despite these improvements, the serum immunoglobulin G (IgG) concentration of ≥ 1000 g/dl and serum total protein (TP) concentrations of ≥ 5. 2 g/dl are considered indicative of adequate transfer of immunity. We hypothesised that the current serum IgG concentrations of ≥ 1000 mg/dl is too low to indicate adequate transfer of colostral immunity on modern dairies. The objective of this study was to determine the serum IgG and TP concentrations indicating adequate transfer of passive immunity in dairy heifer calves. A cohort study of 1290 heifers from a calf raising facility for 48 dairy farms was performed. Heifers were assigned into strata based on serum IgG and TP concentrations. Mortality events were recorded for the heifers for 4 months. Interval likelihood ratios for mortality were calculated for heifers in each stratum of serum IgG or TP concentrations. Logistic regression to predict probability of mortality events was performed. Estimates of probability of survival were evaluated using survival analysis. Serum strata of ≤ 1500, 1501-2000 or >2500 were not significant predictors of mortality during the 120 d of rearing. Serum IgG concentration was not a significant predictor of hazard for mortality. In contrast to previous studies, serum IgG and TP concentrations of 2001-2500 mg/dl and 5.8-6.3 g/dl respectively, were considered optimum for indicating adequate passive transfer of colostral immunity in dairy calves based on the likelihood ratios. On dairies with optimum colostral feeding practices, serum IgG and TP concentrations of 2001-2500 mg/dl and 5.8-6.3 g/dl are recommended as endpoints to indicate adequate passive immunity in dairy calves.