Metabolic changes associated with intake by cows of complete diets containing straw and concentrates in different proportions.

1. Pelleted diets containing concentrates and 0, 200, 400 or 600 g chopped straw/kg were fed ad lib, for 5 h daily to four cows. Concentrations of various energy-yielding metabolites were measured in samples of rumen fluid and jugular blood taken before feeding and at intervals after food was offered. 2. After feeding, the pH of rumen fluid decreased rapidly and the total concentration of volatile fatty acids (VFA) increased; the changes were greatest in the diet containing no roughage. The changes were essentially complete by 135 min at approximately the time feeding stopped. Rumen lactate concentrations were always low and increased significantly only in cows given no roughage. 3. There were highly significant relationships between the peak rumen acetate concentration after a meal and the apparent digestibility of the dry matter of that meal, and the amount of material in the rumen at the end of a meal. The latter relationship resulted in there being no difference between treatments in the total amount of acetate present in the rumen after feeding. 4. In the blood, concentrations of acetate, propionate, beta-hydroxybutyrate (BHB) and lactate all increased after feeding. The increase was prolonged and maximum values were rarely reached before 4-5 h. The highest concentrations of acetate and BHB were found in cows given 20 g roughage/kg and were twice as great as those in cows given no roughage; lower concentrations were found in cows given 400 or 600 g roughage/kg. This information, together with the rumen concentrations of acetate and butyrate, was interpreted as indicating an inhibition of VFA absorption from the rumen of cows given no roughage. 5. Plasma glucose concentrations decreased rapidly for 4 h after feeding, the decrease being greatest in cows given 200 g roughage/kg. Non-esterified fatty acid concentrations also decreased after feeding 200 and 0 g roughage/kg rations, but concentrations were not high at any time. 6. It is concluded that chemical changes in the rumen correspond to feeding behaviour much more closely than changes in blood and therefore any chemostatic regulation of food intake probably occurs at the former site, with acetate playing a major role. Monitoring energy balance at tissue level is likely to be mediated hormonally, with insulin an important factor.