The aim of this study was to demonstrate the effectiveness of lavender (Lavandula angustifolia) volatile compounds (LVCs) when supplemented to calf milk to assess their effect on performance, ruminal metagenomics, fermentation end-products and immune variables. Calves received milk, starter feed and wheat straw until weaning at 65 days of age. Milk was supplemented with 0 µl/day (control), 60 µl/day (LAV60), or 120 µl/day (LAV120) of LVCs per calf. Feed intake, body weight at days 40 and 65, ruminal concentrations of acetic acid (AcA), propionic acid (PA) and ammonia-N, and the relative abundance of Prevotella_7, Erysipelotrichaceae_UCG_002, Succiniclasticum, Ruminococcus and Bifidobacterium were all increased by LVCs (P < 0.05). In contrast, serum amyloid-A (SAA), tumour necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6) concentrations decreased linearly with LVC supplementation (P < 0.05). Overall, LVCs improved feed intake and body weight during the liquid and solid feeding period, and reduced pro-inflammatory cytokines at weaning. The active components of LVCs appear to promote the formation of a bacterial ecosystem required for solid feed digestion and development of rumen functions characteristic of adult ruminant.