142. Maternal Factors and Calving Season Affect Immune Phenotype of Beef Calves Sired by Bulls Selected for Moderate and High Growth.

Abstract Maternal factors play a critical role in shaping well-being and immune development of calves, particularly through the transfer of maternal antibodies. These factors can also be affected by genetics, previous experiences, and environmental interactions. However, repeated vaccination may alter vaccine responsiveness over time, potentially affecting the quality of passive immunity and subsequent adaptive immune function in calves. The objective of this study was to evaluate the effects of dam age (indicator of prior experience and lifetime BRD vaccinations), growth trait, and calving season on immune phenotype and stress responsiveness in beef calves after weaning. A subset of mixed-sex calves (steers, n = 79; heifers, n = 41) born in Spring (n = 58) or Fall (n = 62) were classified by sire expected progeny differences as high growth (HG; n = 58) or moderate growth (MG; n = 62). Dams were categorized by age: Heifer (2 yr; n = 33), Young (3-4 yr; n = 35), Mid (5-6 yr; n = 23), and Old (≥7 yr; n = 29). Body weights and blood samples were collected at weaning (6-7 mo.) and 14 days post-weaning. Immune assessments included stress-related biomarkers, lymphocyte proliferation, immunoglobulins, and complete blood counts. Data were analyzed using generalized linear mixed models (SAS 9.4). A dam age × growth × season interaction (P < 0.05) was observed for the cortisol-to-cortisone ratio; calves from Young and Old dams exhibited greater stress when born in Fall, whereas calves from Heifer and Mid dams showed season- and growth-dependent responses. T-cell proliferation was reduced in Fall-born calves and declined with increasing dam age (dam age × season, P < 0.05). Bcell proliferation was similarly reduced in Fallborn calves and in calves from Mid and Old dams (dam age × season, P < 0.01). While IgG1:IgG2 ratios were consistent for calves from Heifer and Young-aged dams, but varied by season and growth trait for calves from Mid- and Old-aged dams. Spring-born calves were heavier than Fall-born calves, and weight increased with dam age (dam age × season, P < 0.01). The HG-calves maintained similar weights across dam ages, whereas MG-calves from Heifer dams were lighter than their contemporaries (P < 0.01). These results suggest that the genetic growth potential is partially affected by maternal factors, especially dam age and calving season, which have varying impacts on stress responsiveness and immune phenotype following weaning. Future breeding strategies should incorporate other biological factors, such as maternal traits, to optimize calf health and overall production outcomes. This work was supported by the Agriculture and Food Research Initiative Competitive grant award no. 2022-67016-36201 from the U.S. Department of Agriculture.