Associations between subclinical bovine respiratory disease, growth patterns, and the nasal and fecal microbiota in dairy replacement heifers: a retrospective study.

Subclinical bovine respiratory disease (BRD) is difficult to detect using clinical signs alone, and its broader associations with growth and microbial communities remain poorly defined. This retrospective observational study classified twenty Holstein-Friesian and Jersey replacement heifers at weaning using clinical respiratory scoring and thoracic ultrasonography (TUS). Ten heifers had no clinical signs of BRD and no lung lesions, while ten showed no clinical signs but had ultrasonographic lung consolidation >1 cm in diameter. The study investigated (1) associations between subclinical BRD status at weaning and liveweight patterns from birth to early gestation, and (2) the nasal and fecal microbiota associated with BRD status at weaning and again at 8 mo of age. Heifers were clinically assessed from birth to 8 mo, with TUS, nasal swabs and fecal samples collected at weaning and 8 mo. Liveweights were recorded from birth to early gestation. Microbial community composition of nasal and fecal samples from weaning and 8 mowere characterized by 16S rRNA gene amplicon sequencing. Across the study period, heifers classified as healthy were heavier and had greater average daily gains than those with subclinical BRD. BRD status was associated with nasal microbiota composition at weaning and showed a tendency in fecal samples at weaning. No differences were detected at 8 mo. Dominant nasal genera included