Investigations of the influence of the content of crude plant protein in the ration on the utilisation of urea in dairy cattle. 3

The rations contained 10.7 (I), 13.7 (II) and 17.1 (III)% plant crude protein and, after the supplementation of 150 g urea per animal and day, a total of 13.8 (I), 16.7 (II) and 20.2 (III)% crude protein in the dry matter. The starch content and the dry matter was 25.6, 19.6 and 13.4% from I to III. The urea was intraruminally infused during the feeding in the morning and the evening. In the morning feeding of each 1st measuring day it was labelled with 27.5 atom-% 15N'. Samples were taken from the rumen up to 72 h after intake. 4, 8 and 12 hours after 15N intake 100 ml rumen fluid contained 2.5, 3.3 and 3.0 (I), 3.1, 3.0 and 2.8 (II) and 4.1, 4.4 and 4.2 (III) g dry matter of the bacteria and 7.9, 9.0 and 5.9 (I), 3.8, 3.2 and 3.1 (II) and 2.1, 3.1 and 1.7 (III) g dry matter of the protozoa. In comparison to I the concentration of microbes in III decreased to 60-67%. The N level of the ration did not influence the N and carbohydrate content of the microbes as well as the amino acid composition of the microbial protein. Level and dynamics of 15N incorporation differed between protozoa and bacteria. The latter were more intensively labelled and 15N frequency increased more quickly. From I to III the maximum labelling degrees were 3.27, 3.09 and 1.98 for bacteria and 0.93, 0.82 and 0.80 atom-% 15N excess. The apparent half life of 15N on 10...12 h in the bacteria N and of 34...59 h in the protozoa N showed that 15N metabolism in protozoa is distinctly slower. The 15N frequency in the amino acids (AA) of the microbial protein revaled that urea N was included in the synthesis of all the 16 AA investigated, with big differences in the relative 15N frequency in the AA both within the microbes and between the species of microbes. One can conclude that the decreasing utilization of 15N from urea in comparison with that of the total N is caused by the reduction of the concentration of microbes in the rumen fluid and the relatively lower quota of the incorporation of urea N into bacteria N. (author)