318. Evaluation of High-forage and High-concentrate Diets During Gestation on Maternal and Offspring Respiratory Microbiome Development in Beef Cattle.

Abstract The respiratory microbiota plays a critical role in airway health and disease, and bovine respiratory disease (BRD) remains one of the costliest diseases in commercial feedlot systems. However, the impact of maternal diet on respiratory microbiota in dams and the developmental programming of the offspring airway microbiome remains poorly understood. While the overarching study aimed to determine how diet-induced alterations of the maternal gut microbiome during gestation influence offspring development within a DOHaD framework, the present analysis focuses on the respiratory microbiota. Specifically, we evaluated the effects of high-forage (HF) and high-concentrate (HC) diets during gestation on maternal and offspring nasopharyngeal microbiota and its development from birth to maturity in beef cattle. A total of 119 beef heifers were assigned to two dietary treatments. Heifers in the HF group (n = 24) received a diet containing 75% forage and 25% concentrate, whereas the HC group (n = 22) received 75% concentrate and 25% forage. Diets were administered from pre-breeding until calving. All heifers were bred using male-sexed semen from a single sire and managed to achieve a target gain of 0.45 kg/day in the first two-thirds of gestation and 0.79kg/day in the last third of gestation. Individual feed intake and feeding behavior were monitored. Deep nasopharyngeal swabs were collected from heifers at pre-breeding, post-breeding, calving, and 160 days postpartum. Steer calves born to these dams were monitored for growth performance and respiratory microbial development from birth through weaning to finishing. Deep nasopharyngeal swabs were collected from calves at birth and at 14, 120, and 160 days of age. Bacterial communities were profiled using 16S rRNA gene (V3–V4) sequencing. Diet had a significant effect (PERMANOVA: R² = 0.077, P < 0.001) on dam nasopharyngeal microbiota at d-2 (13 days after the final HF and HC diet), d56, d182 and d240 of pregnancy. There was no difference in the observed ASV richness in maternal nasopharyngeal microbiota throughout the study period. Maaslin3 identified genera in nasopharyngeal microbiota of dams that were significantly associated with the dietary treatment. Among such genera are corynebacteria, filobacterium and ornithinimicrobium. Calves born from HF vs. HC-fed dams exhibited significant (PERMANOVA: R² = 0.061, P < 0.001) nasopharyngeal microbial differences at birth, but difference in community structure between the two group calves was not observed at 14, 120 and 160 days of age (P > 0.05). Maternal diet during gestation significantly influenced the respiratory microbiota of dams, representing one of the first demonstrations that dietary modulation can shape the maternal respiratory microbiome in cattle. Differences observed in the nasopharyngeal microbiota of calves at birth between dietary groups suggest potential maternal programming of the offspring respiratory microbiome.