Biological Science: Research Developments and Innovations Vol. 3

The first hours of life establish a disproportionate share of the health trajectory of the dairy calf. Management has therefore centred appropriately on timely delivery of high-quality first-milking colostrum and successful transfer of passive immunity. Yet this focus has also encouraged an artificial nutritional boundary: once the first colostrum meals are completed, many calves move abruptly to whole milk or milk replacer even though mammary secretion changes progressively through transition milk before mature milk is reached. This critical narrative review evaluates the biological and practical significance of that post-colostral interval, integrating evidence on immunoglobulin transfer, transition-milk composition, intestinal development, local mucosal defence, growth, health, processing and precision feeding. Literature published from 1 January 1990 to 5 July 2026 was appraised, with earlier seminal studies retained where necessary. The evidence is strongest for a hierarchical interpretation. First-milking colostrum remains the irreplaceable intervention for systemic immunoglobulin G acquisition because absorptive efficiency declines rapidly after birth. Transition milk cannot reliably correct inadequate systemic passive immunity once macromolecular absorption has closed. Its potential value instead shifts towards nutrient density, luminal immunological activity and trophic effects on the developing intestine. Controlled studies generally support enhanced intestinal growth when colostrum-rich or transition-milk diets are extended for several days, while effects on preweaning growth and disease are less consistent and are frequently confounded by unequal energy, protein, immunoglobulin concentration, microbial exposure and comparator quality. Recent compositional work further shows that transition milk is a dynamic matrix containing changing concentrations of immunoglobulins, lactoferrin, insulin-like growth factor I, oligosaccharides, fatty acids and diverse proteins, but component abundance should not be equated with demonstrated causal benefit. A precision-nutrition framework should therefore begin with verified passive immunity, followed by measured transition-milk allocation, biosecurity and nutrient matching to calf size, environment and health. Closed-loop individualisation of transition-milk dose remains a research objective rather than an established standard. The most credible future progress will come from component-resolved, isoenergetic trials and multisite field studies that separate nutritional from bioactive effects.