Nutrient fluxes in splanchnic tissue of dairy cows: influence of grass quality

A crossover design was used to investigate the effects of high (450 kg of N/ha) or low (150 kg of N/ha) N fertilization of ryegrass on fermentation and nutrient fluxes in splanchnic tissue of dairy cows fed those grasses. Grass that was fertilized with the high amount of N contained more N and less sugar than did grass that was fertilized with less N. In rumen fluid, the concentration of NH3 N was lower for ryegrass that was fertilized with the low amount of N. The NH3 release by portal-drained viscera and urea synthesis in the liver were higher for cows fed ryegrass that was fertilized with the high amount of N. The concentration of NH3 N in rumen fluid, NH3 N release in portal-drained viscera, urea synthesis in the liver, urea release from the liver, and urea concentrations in milk were highly correlated. The release of acetate and propionate in portal-drained viscera was similar for both grasses and was well correlated with the proportion of volatile fatty acids in rumen fluid. The proportion of butyrate in rumen fluid was closely correlated with the release of butyrate and beta-hydroxybutyrate in portal-drained viscera. Glucose synthesis in the liver indicated gluconeogenesis from amino acids, which corresponded well with urea synthesis in the liver. For the grass fertilized with more N, availability of energy sources for rumen microbes was low, and, therefore, cows did not use the N in that grass efficiently.