The effects of timing of High Immune Response phenotyping in relation to weaning on immune responses of crossbred beef calves.

Genetic selection for immune response has the potential to increase the sustainability of the beef industry by breeding cattle that are productive yet with an increased capacity to resist disease. Determining the optimal time to immunophenotype beef cattle is crucial for the accurate prediction of an animal's immune response. The objective of this study was to determine the effect of time of immunophenotyping in relation to weaning on immune responses of beef calves. Antibody- (AMIR) and cell-mediated (CMIR) immune responses were measured on 97 calves on the day of weaning (WEANING, N = 56) or two months post-weaning (POST-WEANING, N = 41). Within each period of immunophenotyping, on day 0, blood was collected, and calves received a 1.0 mL intramuscular injection of type-1 and -2 test antigens. On day 14, blood was collected, and baseline skinfold thickness (SFT) was measured. Calves received an intradermal injection of 0.1 mg of the type-1 antigen suspended in 0.1 mL phosphate buffered saline (PBS) in the right tail fold, and 0.1 mL of PBS in the left. Changes in SFT at 24 hours was used to indicate CMIR. To assess AMIR, the titre of type-2 antigen-specific bovine immunoglobulin G in serum from blood collected on day 14 was determined by measuring optical density (OD) using an enzyme-linked immunosorbent assay (ELISA). Among heifers, AMIR was greater for the POST-WEANING group than for the WEANING group (P < 0.01). Among steers, AMIR was not different between the POST-WEANING group and the WEANING group (P = 1.0). Therefore, the AMIR of heifers may be more negatively affected by immunophenotyping at weaning than the AMIR of steers. For steers, CMIR was greater in the POST-WEANING group than the WEANING group (P < 0.001). For heifers, CMIR was not different between the POST-WEANING group and the WEANING group (P = 0.22). The CMIR of steers may be more negatively affected by immunophenotyping at weaning than the CMIR of heifers. Calf age was not associated with AMIR or CMIR for calves phenotyped at weaning or post-weaning. The effect of sire nested within dam age was significant for CMIR for calves in the POST-WEANING group (P < 0.01), but not for calves in the WEANING group (P = 0.67). The results suggest that measuring immunocompetence at weaning may not be representative of a calf's genetic ability to mount an effective immune response, and immunophenotyping should be performed outside the weaning period.