Assessing adaptive immune response phenotypes in Australian Holstein-Friesian heifers in a pasture-based production system 1

The objective of this study was to determine the repeatability of ranking Holstein-Friesian heifers reared in an Australian pasture-based production system for immune responses (IR) when ranking was based on secondary versus tertiary IR. Further objectives were to investigate associations between IR and stress responsiveness, ADG and resistance to internal parasites. A total of 100 heifers were IR phenotyped at 5 to 6 mo of age and again at 12 to 13 mo of age using commercial vaccine antigens to induce measurable IR. Antibody production to tetanus toxoid (TT) was used to assess antibody-mediated IR (AMIR), and delayed-type hypersensitivity reactions to vaccine antigens were used to assess cell-mediated IR (CMIR). Changes in serum cortisol and haptoglobin were used to assess stress responsiveness and fecal egg counts used as a measure of resistance to internal parasites. Based on testing, animals were categorized as either average to above-average (High) or low responders for IR. Secondary and tertiary AMIR were well correlated (r = 0.651, adjusted R^sup 2^ = 0.418, P < 0.0001), whereas correlations between secondary and tertiary CMIR were poor (r = 0.078, R^sup 2^ = -0.004, P = 0.450). A Cohen kappa (κ) test of agreement was used to test the consistency of ranking of individual animal for IR and, therefore, the ability to consistently identify low immune responder animals within the herd across test periods. The consistency of ranking (High versus low) was moderately high for AMIR (κ = 0.445), poor for CMIR (κ = -0.055), and fair to moderate for combined IR (κ = 0.395). High AMIR phenotype animals had significantly higher serum cortisol concentrations than their low immune responder counterparts (P = 0.045). A similar relationship was observed in heifers categorized for CMIR, with High CMIR responders having higher serum cortisol concentrations than their low responder counterparts (P = 0.008). High AMIR calves had a higher ADG compared with low AMIR calves (0.72 ± 0.02 versus 0.66 ± 0.06 kg/d; P = 0.009). Serum haptoglobin concentrations and worm egg counts were very low and could not be used to investigate associations with immune responsiveness. It is concluded that secondary and tertiary antibody responses to TT were well correlated in the Holstein-Friesian heifers in this study and that by using the testing procedure described here, low antibody responders were able to be consistently identified in the herd.