Factors associated with Apparent Efficiency of Absorption (AEA) of immunoglobulin G in dairy and beef-cross calves: A cohort analysis

Ensuring successful transfer of passive immunity (TPI) remains a major challenge in pasture-based block calving systems. This cohort study aimed to evaluate factors associated with apparent efficiency of absorption (AEA) of immunoglobulin G (IgG) in 213 dairy and beef-cross calves born on a spring block-calving pasture-based dairy herd in West Wales from February to May 2024. All calves were removed from the dam at birth and fed colostrum replacer and pooled colostrum/transition milk within the first 24 h of life. Serum IgG concentrations at 24–36 h and the IgG concentrations of the colostrum replacer and each pooled colostrum/transition-milk batch were measured by RID and used to calculate AEA. A multivariable linear regression model was then used to evaluate associations between AEA and potential explanatory calf- and management-level variables. Calves born during the morning (+5.49 percentage points; 95% CI: 1.56 to 9.41) and afternoon/evening (+5.35 percentage points; 95% CI: 1.46 to 9.24) had a higher AEA than those born overnight. Greater total IgG intake during the first 24 h was associated with lower AEA (−0.10 percentage point change in AEA for every 1 g of IgG fed, 95% CI: −0.14 to −0.05). There was no evidence of an association of AEA with breed, sex, birth weight or calving ease. Together, these results suggest that calving management and staffing patterns may influence AEA in pasture-based block-calving dairy herds.