144. Impact of Weaning Stress, Maternal Age, and Genetic Selection on Gut Microbiome in Beef Steer Calves.

Abstract A multitude of factors influence the development and structure of the gut microbiome, including diet, host genetics, production system, environmental conditions, and stress. Through the bidirectional gut-brain axis, stress can initiate dysbiosis of the gut microbiome, which may lead to immune activation and/or disease, thus affecting animal health and well-being. Furthermore, maternal factors, including experience and environmental conditions, play a key role in early immune development and establishing the foundational structure of the gut microbiome. The study aimed to investigate the effects of weaning and associated stressors, as well as genotype, season, and maternal factors, on the gut microbiome during the preconditioning period. A subset of steer calves born in Spring (n = 29) or Fall (n = 34) were classified by sire expected progeny differences into four categories: high growth, high milk (HH; n = 18); high growth, low milk (HL; n = 9); moderate growth, high milk (MH; n = 18); and moderate growth, low milk (ML; n = 18). Dams were categorized by age: Heifer (2 yr; n = 15), Young (3–4 yr; n = 19), Mid (5–6 yr; n = 15), and Old (≥7 yr; n = 14). Body weights and fecal samples were collected at weaning and 14 (14DPW) and 60 days post-weaning (60DPW). Fecal microbiotas were analyzed using 16S rRNA sequencing to assess α-diversity, β-diversity, and the relative abundance of the top 15 phyla. Body weights were analyzed in R (version 4.4.3) using ANOVA with Tukey’s HSD, and microbiome α- and β-diversity were assessed in QIIME2 using diversity indices, Kruskal–Wallis tests with pairwise comparisons, and PERMANOVA. Weaning stress and associated dietary changes affected both α-diversity (P < 0.0001) and β-diversity (P < 0.01). After weaning, α-diversity (Shannon index) declined substantially at 14DPW and then increased, whereas β-diversity (Bray-Curtis Distance) showed significant differences in community composition across all time points. Dam age influenced both α- (P < 0.05) and β-diversity (P < 0.01). Calves from Heifer dams showed greater α-diversity (less efficient microbial community), and β-diversity was distinct from that of older dams but comparable to that of Young dams. Calves born to Mid- and Old-aged dams exhibited similar diversity profiles. Overall, genotype and season did not significantly affect α-diversity, but HL calves differed in β-diversity from all other genotypes (P < 0.01), and calving seasons exhibited distinct community compositions (P < 0.01), as expected. These results indicate that maternal age, genetic selection, season, and weaning stress jointly influence microbiome structure, with potential long-term implications for calf health and productivity.