[Nitrogen metabolism in the large intestine of ruminants. 2. Metabolism of i.v. infused 15N-urea by additional supply of fermentable material to the large intestine of bulls].

The experiment was carried out on 3 bulls with body weights of 201, 168 and 190 kg, respectively. The animals were equipped with a ileo caecal re-entrant cannula and with catheters in the jugular veins on both sides. The pelleted ration was composed of straw 70-72%, cereals 10%, molasses 12-41%, ammoniumhydrogencarbonate 3%, urea 2% and mineral mixture 1%. During a preliminary period ileal digesta were collected, deep-freezed and stored. During the main experiment 15N-urea was infused intravenously for 24 hours. In this period and during the following 6 hours outflowing ideal digesta were collected quantitatively. At the same time precollected, unlabelled digesta together with a supplement of partly hydrolysed straw meal were reintroduced into the caecal part of the cannula. Plasma urea-N, urinary-N as well as several N-fractions of faeces and digesta were analysed for 15N abundance. A urea flux rate of 27.9 +/- 3.4 mumol per minute per kg 0.75 was estimated. It was calculated that 52% of this amount of urea was transferred into the digestive tract. In both, digesta and faeces NH3-N was highest 15N-labelled indicating a direct urea entry and degradation in both segments of the digestive tract. The amounts of 15N-excess found during the period of digesta replacement were in faeces 0.25 and in ileal digesta 4.02% of the infused amount of 15N. Although the microbial utilization of endogenous urea-N was generally low in the large intestine there was a clear stimulation of this process due to the additional supply of the large intestine with a fermentable source.