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 2 mo 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 h was used to indicate CMIR. To assess AMIR, the titer 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. Lay Summary Understanding the optimal time to immunophenotype beef calves is important for the accurate estimation of their genetic ability to resist disease. The compound stressors experienced by a calf during weaning may have a similar impact on the immune system as chronic stress. Therefore, the immune response phenotype of a calf immunophenotyped during the weaning period may not truly reflect the animal's genuine capacity for immune response. To accurately identify cattle with a superior capacity for immune response, with the goal of genetically selecting cattle for immunocompetence, immunophenotypes must be measured accurately. In this study, the effect of time of immunophenotyping in relation to weaning on immune responses of beef calves was determined. Calves immunophenotyped at weaning had lesser antibody-mediated and cell-mediated immune responses than calves immunophenotyped 2 mo post-weaning, this effect was influenced by sex. Sire affected immune responses when calves were immunophenotyped 2 mo post-weaning, but not when calves were immunophenotyped at weaning, indicating that when immunophenotyped post-weaning, the genetic component of a calf's immune response is quantified without being obscured by other environmental factors. Determining the most advantageous time to immunophenotype beef cattle is crucial to correctly assess an animal's true genetic ability to mount an immune response for the purpose of selecting cattle with superior immunity. This study demonstrated that the immune responses of crossbred beef cattle are diminished when immune response testing is performed on the day of weaning, suggesting that immunophenotyping of beef calves at the time of weaning is not representative of an animal's true immunocompetence.