Investigations of the influence of the content of crude plant protein in the ration on the utilisation of urea in dairy cattle. 2

The metabolism of 15N-urea in dairy cows was investigated in dependence on the crude protein content of the rations. With the energy concentration remaining unchanged, 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, 16.7 and 20.2% crude protein in the dry matter. 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-excess (15N'). The degree of labelling with 15N' of the N fraction of rumen fluid, contents of the duodenum, feces and milk, precipitable with trichloric acetic acid (TCA) decreased with the rising protein level of the ration. This effect was bigger than could be expected considering the low 15N' quota in the total N of the ration. In the sequence I..III, 52.7, 32.2 and 30.6% of the 15N' amount taken in passed the duodenal re-entrant cannula in TCA-precipitable form within 72 hours after 15N application. 33.3, 21.9 and 22.6% were apparently absorbed in the intestines as TCA-precipitable N within 120 h after the 15N' application. In the same period 31.7, 43.1 and 72.8% of the 15N' taken in were excreted in urine. 12.3, 9.6 and 5.8% of the applied 15N' were found in milk protein. One can conclude that the utilization of urea N decreases with the rising level of crude protein in the ration and that, however, urea N is still biochemically utilized when there is an excess of plant N in the ration. (author)