B vitamins are co-enzymes involved in key metabolic reactions, and mathematical models can support the development of their better-defined adequate intake, supply and requirements for dairy cows. The objective of this thesis was to develop empirical models to describe B-vitamin apparent ruminal synthesis and supply to the cow and develop a mechanistic cobalamin sub-model to follow cobalamin (B12) in the rumen of the dairy cow. Both models were developed based on a unique database where B vita-mins were not supplemented, and B-vitamin concentrations were analyzed post-hoc. The empirical models identified similarities and differences between B vitamins and the key factors related to their apparent ruminal synthesis and post-ruminal flow (PRF). These models guided the development of a mechanistic B12 sub-model, which yielded mean B12 PRF predictions similar to the ob-served mean but cannot yet capture the full variation reported in the database. Ultimately, these models represent the first attempt at modelling B-vitamin supply in dairy cows.