Girard, C. L. and Matte, J. J. 2006. Impact of B-vitamin supply on major metabolic pathways of lactating dairy cows. Can. J. Anim. Sci. 86: 213-220. Knowledge of the major nutrient requirements of dairy cows has increased substantially during the past decades. Little is known, however, about the importance of the roles played by B vitamins. Since most of those vitamins act as essential cofactors in energy, protein and lipid metabolism, it is likely that as milk yield increases, the demand for these cofactors also increases. The supply of B vitamins from dietary sources and synthesis by the ruminal microflora is generally sufficient to avoid deficiency symptoms, but could be insufficient for optimizing metabolic efficiency, production, composition and the nutritional quality of milk in high-producing dairy cows. Results from recent experiments show how the supply of three B vitaminsfolic acid, biotin and vitamin B 12 -affects major metabolic pathways. Supplementary biotin has frequently been reported to increase milk yield but has a limited effect on milk composition. Folic acid supplements have been found to increase milk and milk protein yields in multiparous cows without affecting dry matter intake when vitamin B 12 supply was adequate. An insufficient vitamin B 12 supply blocked those effects but they can be restored through vitamin B 12 supplementation. Supplemental vitamin B 12 and biotin increased milk and milk protein yields without changing dry matter intake. Vitamin B 12 utilization by tissues increased in cows fed supplementary folic acid simultaneously; plasma glucose also increased in these cows but plasma biotin decreased. From these findings, it appears that, in high-producing dairy cows, especially in early lactation, the strong competition for nutrients that occurs between gluconeogenesis, methylneogenesis and protein synthesis increases the amount of folic acid, vitamin B 12 and biotin required to maintain metabolic efficiency, especially when the nutrient supply is limited. These observations emphasize the need to review the paradigm according to which B-vitamin supply by ruminal microflora cannot be limiting in dairy cow. Les connaissances sur les besoins des vaches laitières en nutriments majeurs ont augmentées substantiellement au cours des dernières décennies. Cependant, on connaît encore peu l'importance des rôles des vitamines B. La plupart de ces vitamines sont des cofacteurs essentiels dans les métabolismes énergétiques, protéiques et lipidiques, il est donc probable que la demande pour ces cofacteurs augmente avec la production laitière. Les apports en vitamines B provenant de l'alimentation et de la synthèse par la microflore du rumen suffisent généralement à éviter les symptômes de déficience mais sans nécessairement permettre d'optimiser l'efficacité métabolique, la production, la composition et la qualité nutritionnelle du lait chez les vaches hautes productrices. Les résultats d'expériences récentes mettent en évidence l'impact sur les voies métaboliques majeures des apports en tr...