Differential rumen responses in neonatal ruminants to volatile fatty acids, glucose, and physical stimulation: insights into the drivers of early rumen development

INTRODUCTION: The transition from milk to solid feed is a critical stage for rumen development in young ruminants; however, the relative contributions of chemical stimuli (such as volatile fatty acids, VFAs) and physical stimulation remain poorly understood. OBJECTIVES: This study aimed to disentangle these influential factors during the preruminant phase, particularly evaluating the roles of chemical signals (VFAs), simple energy provision (glucose [GLU]), and physical stimuli in initiating functional rumen maturation. METHODS: Using a novel, noninvasive oral tube delivery technique and 28 newborn male lambs, we evaluated rumen development, host tissue transcriptomics and proteomics, and the gastric microbiota in response to a mixed VFA solution, an isoenergetic GLU solution, and a sponge (a physical stimulus). RESULTS: VFA gavage proved to be the most effective treatment. This treatment significantly increased rumen and reticulum weights and increased papillae length and width. Transcriptomic analysis revealed upregulated expression of genes involved in epithelial development. These effects coincided with an improved host energy status, as evidenced by modulated hepatic gene expression, and a shift in the rumen microbiota toward VFA-producing and probiotic genera, such as Porphyromonas and Ligilactobacillus. By contrast, the isoenergetic GLU treatment induced a dysbiotic microbiota enriched with potential pathogens (e.g., Streptococcus) and triggered a host inflammatory response, despite partially stimulating papillae growth. Physical stimulation alone exerted a limited effect on papillae morphology and was associated with indicators of a negative energy balance, indicating that it is insufficient and potentially detrimental for initiating rumen development. CONCLUSION: Our findings provide clear evidence that chemical signaling by VFAs is the primary driver of functional, healthy rumen maturation and of the establishment of a beneficial host-microbe interplay in neonatal lambs. These results demonstrate that the chemical signals (VFAs), rather than physical abrasion or simple energy provision, are the key determinant guiding the transition toward a functional rumen system.