Portal blood flow and net absorption of ammonia-nitrogen, urea-nitrogen, and glucose in nonlactating Holstein cows.

Portal blood flow was measured in nonlactating Holstein cows by primed, continuous infusion of para-aminohippuric acid into a mesenteric vein and periodic collection of blood from the portal vein and femoral artery. Cows ranged in age from 2 to 4 yr; average body weight was 472 kg. They were fed ad libitum orchardgrass-red clover silage. Results of Experiment 1 in which portal blood flow was measured in four cows at 10-min intervals over 3 h indicate that flow can vary by a factor of three in a given cow. In Experiment 2, portal blood flow and net absorption of ammonia-nitrogen, urea-nitrogen, and glucose were measured in cows for 10 to 12 h. Mean portal blood flow (liters/h) and net absorption (mmoles/h) of ammonia-nitrogen, urea-nitrogen, and glucose were 836 +/- 32, 121 +/- 8, -52 +/- 14, and -39 +/- 16.