Financial pressures associated with high pre-weaning rearing costs, result in dairy calves (Boulton et al., 2017) being weaned from milk much earlier than would occur naturally. This limits the time available for the digestive system to develop from a milk-fed pseudo-monogastric system to a functional ruminant system able to digest complex, fibre-based diets. Calves weaned with a poorly functioning digestive system may have lower growth rates post-weaning (Steele et al., 2017) and consequently may be older at first calving (Heinrichs et al., 2005). Strategies to predict sufficient digestive development for weaning to occur have been developed (e.g. concentrate intake or weaning weight); however, these are often impractical commercially. Identifying the precise point at which a calf is ready to be weaned is therefore challenging (Welk et al., 2024).This project will use already obtained gut tissue, content, and faecal samples from a time-series study on calves to identify molecules in the faeces that have potential to act as markers of digestive tract maturity.