Does selection of beef cattle for economically important traits impact post-weaning immunological phenotype and robustness traits?

Bovine respiratory disease (BRD) is a multifactorial complex disease that has created a billion dollar disparity annually and raised welfare concerns in the stocker and feedlot sectors. Therefore, the objectives of this dissertation were to evaluate the influence of calving season and growth and milk traits on: 1) variations in performance, feed efficiency, and carcass traits, 2) health and immune indicator traits, 3) changes in immune and growth related changes to the liver proteome, and 3) altered metabolite and metabolic pathways. Angus-cross cows from spring (n =120) and fall (n = 119) calving cowherds were randomly mated with Angus bulls selected for mean yearling weight expected progeny difference (YW EPD) with high growth (mean YW EPD = 72 ± 5.6 kg; HG) and moderate growth (mean YW EPD = 43 ± 3.2 kg; MG). and high (sire milk EPD = 15 ± 2.5 kg; HM) and low (sire milk EPD = 8 ± 2.6 kg; LM) genetic merit for the milk production of his daughters, creating a 2 × 2 factorial arrangement of sire genetic merit categories (HH, HL, MH, and ML). Weaned beef steers (n = 239; 224 ± 16.7 kg) from natural service and artificial insemination breeding were used over a 2-yr period. To evaluate the effects of growth, milk, birth season, and interactions four components were analyzed: 1) performance, feed efficiency, and carcass metrics, 2) immune phenotypes and health outcomes, 3) untargeted proteomics, and 4) untargeted metabolomics. Individual animal and phenotypic groups intakes were monitored from preconditioning to harvest. Immune phenotype data were collected during stressful events and individual animal health was monitored throughout the study. A bottom-up discovery approach to proteomic analysis was used to identify regulated protein pathways. Metabolomic discovery analysis uncovered increased abundance of glycolic and amino acid enriched pathways important for rapid growth of feedlot aged steers. Findings from the research has provided foundation for further investigation into the effects of growth traits on nutrient utilization, immune function, and regulation of key biologic processes and pathways. Expansion of the dataset to better understand the health outcomes observed in moderate growth sire EPD selection may shift the idea that growth traits and immune function are inversely correlated, and the role calving season plays on biological processes.