[Isobutylidene urea as NPN source for ruminants. 2. Digestion of 15N-labeled isobutylidene urea in lactating dairy cows].

Two cows with rumen cannulae and duodenal re-entrance cannulae received in the first experiment (Ia and IIa) a conventional diet on the basis of a mixture of maize silage, hay and concentrated feed and after a three-week adaptation to isobutylidene diurea (IBDU)--276 g per animal and day--138 g IBDU with 3.865 mg 15N-excess as a single supplementation to their first meal. In the second experiment (IIa and IIb) after a 6-week break the same cows served a repeated experiment without IBDU adaptation. Irrespective of the adaptation, a re-increase of the 15N-labelling in the TCA-soluble N in the rumen could be proved between the 6th and the 8th hour after the intake of the isotopes, which resulted from the backflow of 15N to the rumen. In the duodenal digesta the maximum labelling of the TCA--soluble N-fraction appeared 12 hours after the intake of the isotopes. At that moment a labelling plateau began in the protein fraction, which lasted to the 36th hour. On an average of all 4 cows approximately 30% of the 15N taken in the TCA-precipitable fraction and 55 and 60% in the TCA-soluble fraction had passed the duodenum up to the 72nd hour after the beginning of the experiment. Up to the 72nd hour after the beginning of the experiment, approximately 15% were excreted in urine, approximately 16% in feces and approximately 7% in the milk. This shows that roughly one half of the 90% 15N-amount measured at its passage in the re-entrance cannula (related to the intake) was metabolised in the rumen at least twice, resp. after the first passage through the duodenum it originated from the intermediary metabolism (e.g. as amino acids and their incorporation as digestion secretions). Negative correlations could be ascertained between the pH-value of the rumen fluid and the 15N incorporation in the rumen proteins as well as 15N-excretion through the TCA-soluble and -precipitable quota of feces. An adaptation to IBDU is obviously not necessary.