18 Impact of nutritional management strategies on the gastrointestinal development, barrier function and metabolism of the calf and cow.

Abstract Gut health is a critical determinant of overall performance, disease resistance, and productivity in calves and cows. The gastrointestinal epithelium is tasked with nutrient absorption while maintaining a barrier against harmful luminal contents, making it a highly metabolically active and organ system. In fact, the gastrointestinal tract (GIT), accounts for up to 20% of total oxygen consumption in ruminants, highlighting its substantial energy demand. Key developmental transitions—such as birth, weaning, and the onset of lactation—require major structural and functional adaptations in both the upper and lower GIT. These transitions, however, also expose animals to periods of physiological stress and compromised gut barrier function. In calves, the preweaning period presents a high risk for gastrointestinal disorders and diseases, often exacerbated by microbial and gut barrier dysfunction. Weaning introduces significant stress as the digestive system shifts from milk-based to solid diets, often leading to transient inflammation, altered microbiota, and increased gut permeability, particularly in the lower gut. In adult cows, the transition to highly fermentable, energy-dense diets during early lactation poses another critical challenge. This abrupt dietary shift can disrupt ruminal and intestinal integrity, leading to ruminal acidosis and systemic inflammation via translocation of microbial endotoxins. Nutrients, microbial metabolites, such as butyrate, and gut hormones, including glucagon-like peptide 2 and insulin-like growth factor, play vital roles in stimulating epithelial growth and repair. Understanding how these factors interact to influence gut proliferation and barrier function is crucial, especially when balanced against the energetic cost of gut tissue expansion. Ultimately, optimizing gut development and resilience during key life stages holds promise for improving health, feed efficiency, and lifetime productivity in cattle.