Gluconeogenesis in ruminants: propionic acid production from a high-grain diet fed to cattle.

Effects of amount of feed intake on in vivo remen propionate production and the reproducibility of measurements of propionate production rates were investigated in five dairy steers weighing 142 to 228 kg. Each steer received 275 g of an 80%-grain diet every 2 hours, and three of the five steers later received 500 g/2 hours. Rumen propionate pool sizes and production rates were determined at both intakes by administering a single dose of [1-14C] propionate via rumen fistula and observing decreasing specific activity of rumen propionate for 4 hours. A linear regression equation was calculated from the natural logs of decreasing specific activity. Propionate pool sizes averaged 30.6 and 47.4 g, and production rates averaged 579 and 1032 g/day at feed intakes of 275 and 500 g/2 hours. Thus, propionate production changed in almost direct proportion to feed intake. Acetate to propionate ratios were about 3:1 in rumen fluid. Considerable variability in estimates of propionate pool sizes and production rates was found both within and among steers. The results indicate propionate production is nearly equivalent to blood glucose turnover determined previously; all propionate produced in the rumen, however, is not used for glucose synthesis.