[Determination of a prececal N-absorption from natural feed by 15N-labeled laboratory rats using the isotope dilution method].

60 Wistar rats (5 animals/group) received 12 different feedstuffs over a period of 7 days and were simultaneously labelled with 15N (orally by means of 15NH4Cl in the feed). In the subsequent 5 days faeces were collected in order to determine the apparent and true digestibility of crude protein. On the 13th experimental day the animals were killed 3 hours after the intake of half the daily ration and the atom-% 15N excess (15N') was determined in the TCA-soluble and TCA-precipitable fractions of the blood plasma and the digesta of the 2nd and 3rd thirds of the small intestine. Precaecal N-absorption was calculated with the help of the quotient [formula: see text] the blood plasma and the digesta The following values (in %) were registered in comparison to true N-digestibility (in the following in brackets): casein = 95.8 (99.2), whole egg = 92.1 (97.5), fish meal = 85.8 (93.4), dried skimmed milk = 98.4 (96.1), soybean meal = 79.6 (90.6), assay protein = 94.2 (98.9), wheat = 92.9 (90.7), barley = 84.3 (84.8), yeast, grown on molasses = 85.6 (87.2), yeast, grown on whey = 86.1 (88.5), biomass of liquid manure = 43.6 (68.9), activated sludge = 54.4 (64.1). One can conclude that the isotope dilution technique demonstrated here as an evaluation method is very well suited for the characterization of the N-digestibility of a feedstuff in the small intestine.