-—
5.4 0.7 5.0 0.3 4.6 0.3 5.0 0.3
8.0 0.1 7.9 0.1 7.9 0.1 8.1 0.1
13.6 0.3 15.1 0.5 14.9 0.2 14.0 0.3
4.18 0.02 4.20 0.01 4.19 0.04 4.21 0.01
3.98 0.03 3.99 0.05 4.05 0.02 4.10 0.03
3.7. Analytical methods and evaluation of their accuracy Chemical analyses in feed, faeces and urine
In the previous section 3.4 (p. 27) the sampling and preparation of feed, faeces and urine have been described. In a part of the original feed sample the content of dry matter was determined, while the rest of the sample was milled and used for the determination of the percentage of dry matter, nitrogen, crude fat (ether extract), crude fibre and ash together with carbon and energy. By means of the values for dry matter content in the original and in the milled sample, the chemical composition determined in the milled sample was, recal-culated in order to express the content in the original sample.
Immediately after concluding a balance period the content of dry matter and nitrogen in the collected sample of fresh faeces were determined. Then a part was dried, milled and aired as described, and this sample was used for chemical determinations and calculations according to the methods described above for feed stuffs.
The sample of urine was filtered and then the nitrogen content was determi-ned. The carbon content was determined in freeze-dried samples of urine, while for determination of energy content the urine was dried on cellulose blocks over sulphuric acid in a vacuum desiccator. All chemical analyses, except for the carbon determination, were carried out as described by Weidner
& Jakobsen (1962).
The carbon content was determined by means of a Wösthoff instrument measuring the difference of potential gradient caused by the decrease of electric conductivety in a measuring cell containing NaOH-solution, after absorption of CO2 from a complete combustion of the sample in question. The instrument and the procedure as well as a comparison with the gravimetric method were described by Neergaard, Petersen & Thorbek (1969).
Gas analyses
For determining the concentration of carbon dioxide and oxygen in the aliquot samples of outgoing air an Uras 1 and a Magnos 2 together with a scale galvanometer were used as previously described. Details of the function and calibration of the instruments are given by Thorbek (1969 a).
Evaluation of accuracy in determination of nitrogen, carbon, energy and gas analyses
It is a common practice to conduct a special series of analyses in order to determine the accuracy of a method. Such a procedure may result in errors that are much less than they would be in the daily routine. This fact was pointed out by Rasch, Ludvigsen & Thorbek (1958) at the First Symposium on Energy Metabolism, and it was demonstrated that it is possible by means of a fairly simple cumulative statistical device to use the daily routine work with duplicate analyses to estimate the standard deviation of the method. This method of calculation has been used to evaluate the accuracy in the determination of nitrogen, carbon and energy and the composition of the outgoing air being the principal analyses, when measuring energy metabolism.
The accuracies obtained in the nitrogen determination in feed, faeces and urine in the four experimental series are shown in Table 9. For reasons of comparison the average nitrogen content in the different feedstuffs, samples of faeces and urine is stated together with the S.D., the relative standard deviation being below 1%. As the nitrogen excreted in faeces constituted 12-22% of the intake of nitrogen, while the nitrogen in urine varied from 35-50% of the nitro-gen intake, the accuracy obtained for the determinations of nitronitro-gen in the diffe-rent samples of feed, faeces and urine are considered to be satisfactory.
The accuracies obtained in the determinations of carbon and energy are shown in Table 10 and 11. The accuracy obtained in the determination of carbon and energy in feed and faeces was even higher than that obtained in case
Table 9. Accuracy in determination of nitrogen in feed, faeces and urine. Series C-D-E-F Tabel 9. Analytisk nøjagtighed ved bestemmelse af kvælstof i foder, gødning og urin.
Serie C-D-E-F
Materials Series
no. d.f. Mean
%
C.V.
% Barley C-D-E-F
Maize C-E Sorghum D-F Sk.milk powder C-D-E-F Protein mixture C-D-E-F Faeces, fresh C
» » D
» » E
» » F •
Urine C
» D
of nitrogen. The relative standard deviation being fairly constant around 0.30%. The accuracy in determining carbon and energy in urine was lower than in the case of feed and faeces, the relative standard deviation being about ten times as high, partly caused by inaccuracy in the technical process of drying the
Table 10. Accuracy in determination of carbon in feed, faeces and urine. Series C-D-E-F Tabel 10. Analytisk nøjagtighed ved bestemmelse af kulstof i foder, gødning og urin.
Serie C-D-E-F
Materials d.f. Mean
%
S.D. C.V.
% Barley C-D-E-F
Maize C-E Sorghum D-F Sk.milk powder C-D-E-F Protein mixture C-D-E-F Faeces, dried C
» » D
» E
» » F Urine C
» D
Table 11. Accuracy in determination of energy in feed, faeces and urine. Series C-D-E-F Tabel 11. Analytisk nøjagtighed ved bestemmelse af energi i foder, gødning og urin.
Serie C-D-E-F
Materials Series d.f. Mean S.D. C.V.
no. kcal/kg % Barley C-D-E-F
Maize C-E Sorghum D-F Sk.milk powder C-D-E-F Protein mixture ,. C-D-E-F Faeces, dried C
» » D
» » E
» » F Urine C
» D
» E
» F
urine before the determination. However, with the very low content of carbon and energy in urine influencing the carbon-and energy balances by only 2-3%, the validity of the balances is not influenced by the lower accuracy obtained in the determination öf carbon and energy in the urine.
For the gas analyses two independent working recipients are used for collec-ting aliquot samples of the outgoing air. The evaluation of accuracy in the determinations of CO2 and O2 are made by comparing the results obtained from the two recipients and the figures are shown in Table 12.
The Uras instrument is designed to measure from zero to 1.5% CO2, while the Magnos instrument measures in the range from 19.0 to 21.0%, or at a maximum difference of 2.0% O2. The results obtained indicate that the
stan-Table 12. Accuracy in determination of CO2 and Ch-concentration in outgoing air by means of URAS and MAGNOS instruments. Series C-D-E-F
Tabel 12. Analytisk nøjagtighed ved bestemmelse afCOi og Oi-koncentration i udgåen-de luft ved hjælp af URAS og MAGNOS instrumenter. Serie C-D-E-F
31 15 16 33 33 105 92 92 91 101 87 88 88
3857 3974 3949 4193 4032 4119 4124 4352
• 4411 45.6 48.5 46.3 48.3
10.6 10.1 8.9 10.0 12.6 10.2 11.1 11.0 14.9 1.06 1.02 1.33 1.38
0.27 0.25 0.23 0.24 0.31 0.25 0.27 0.25 0.34 2.32 2.10 2.87 2.86
Series no.
c
D E F
Determination of CO2 d.f.
107 96 96 . . . . 96
Mean
% 0.696 0.794 0.755 0.715
(URAS) S.D.
0.0052 0.0040 0.0030 0.0028
Determination of O2 (MAGNOS) d.f.
107 96 96 96
Mean
20.313 20.232 20.264 20.259
S.D.
0.0033 0.0031 0.0026 0.0019
dard deviation was of the same magnitude for both instruments, the values for CCh-determination ranging from 0.003 to 0.005% (abs.) and for O2-determina-tion from 0.002 to 0.003%. (abs.). As the carbon loss in CO2 is about 50% of the carbon intake, the accuracy obtained in the determination of CO2 in the out-going air is considered to be high and in correspondance with the accuracy obtained in the determination of carbon in feed.