Studies on the nitrogen metabolism of the large intestine of ruminants. 1

The experiments were carried out on 3 bulls (body weight: 172, 229 and 193 kg), equipped with ileo-cecal cannulas and with catheters in the jugular veins on both sides. The offered pelleted ration consisted of straw 73%, molasses 12%, cereals 10%, ammonium hydrogen carbonate 3% and urea 2%. Feed intake amounted to about 3 kg per animal and day. During a preliminary period of 5 days 50% of ileal digesta were collected for 12 hours daily, deep-freezed and stored. In the main experiment 15N-urea was infused intravenously for 24 hours. During this period and during the following 6 hours ileal digesta were collected and replaced by precollected, unlabelled digesta. The urea metabolism was estimated by the 15N labelling of blood urea, by the 15N excretion via feces and urea, by the appearance of 15N in ileal digesta and by the 15N labelling of fecal NAN, NH3 and bacterial fraction. The time course of the 15N labelling of plasma urea during infusion can be described by an exponential function. The urea flux rate was estimated from the calculated plateau value. The flux rate for the 3 animals was 28.8, 30.7 and 34.8 μmol per minute per kg/sup 0.75/, respectively. During the infusion of 15N-urea 1.0-2.4% of the infused amount of 15N' appeared in ileal digesta, half of it in the TCA precipitable fraction. At the same time the 15N labelling of fecal NH3 increased sharply, however, the 15N labelling of the fecal bacterial fraction was smaller by one order of magnitude. Deficiency of fermentable substrates and problems of inhomogenity of the NH3 pool are supposed as reasons for this result. 30 to 50% of the urea flux entered the digestive tract, the direct entry of urea into the large intestine seems to be very low only. (author)