Impact of B-vitamin supply on major metabolic pathways of lactating dairy cows

摘要

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 ruminai 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 vitamins - folic 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, vitaminB_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 ruminai microflora cannot be limiting in dairycow.