Glucose absorption and hepatic gluconeogenesis in dairy cows fed diets varying in forage content

One dry and four lactating Holstein cows were prepared with chronic catheters in the mesenteric, hepatic and portal veins and used to study glucose absorption and hepatic gluconeogenesis. The dry cow was used in a preliminary assessment of the methods. In the main experiment high concentrate and high forage diets, providing equal energy intake, were delivered in hourly portions from an automatic feeder as total mixed rations. A few data were collected with the high concentrate ration fed ad libitum in early lactation. A few additional observations were made with limited feed intake by using the same cows subsequent to the main experiment. Portal and hepatic vein blood flow rates were determined by using para-aminohippuric acid. Glucose entry rates were significantly higher during ad libitum feeding of the high concentrate diet than during feeding of the two diets with restricted intake. There was no difference between high concentrate and high forage diets when fed at equal energy intakes. Net glucose appearance in portal blood was demonstrated. Data from this study and from the literature were used to develop general relationships between digestible energy intake and 1) portal vein blood flow, 2) rate of portal appearance of glucose and 3) total splanchnic glucose entry rate.