52
Biological Values of the Proteins
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VOL. XVIII, PART I,
nitrogen-free diet and the total nitrogen in the excreta, uric acid nitrogen and ammonia N were
determined. The endogenous N and faecal metabolic nitrogen were calculated by Coulson and
Hughes' formula (Loc. cil).
Total N of dried excreta was determined by the Kjeldahl method. The ammonia N was estimated
by the Van Slyke aeration method using a mixture of methylene blue and methyl red as indicator
Cole [1933]. To 1 g. of the excreta 3-5 of potassium carbonate and a few c.c. of distilled water were
added and the aeration carried out for three hours. Parallel experiments with and without acidi-
fication of the excreta with HCl before adding potassim carbonate, showed that subsequent acid
treatment was not necessary when the excreta was dried after acidification with acetic acid. The
uric acid was estimated as by Bose [1944].
The urinary nitrogen was calculated on the basis that uric acid and ammonia N represented
80 per cent of total urinary nitrogen. The faecal nitrogen was obtained by subtracting the computed
value of urinary nitrogen from the total nitrogen. In the case of jowar, it was assumed, for the
reasons mentioned later on that the sum of uric acid and ammonia nitrogen formed 90 per cent
of the urinary nitrogen.
The results were calculated according to the following formulae Mitchell [1924].
(1) Food N-(Faecal N-Metabolic N) / Food N × 100 = True coefficient of digestibility.
(2) Food N-(Faecal N-Metabolic N)- (urinary N-endogenous N) × 100/Food N-(Faecal N-Met abol i c N) =Bi ol ogi cal value.
DISCUSSION
The metabolism data on nitrogen-free diet (Table I) show that the average endogenous N was
85 mg. per kg. body weight and the average metabolic faecal nitrogen was 0.0884 g. per 100 g. dry
matter in-take. In these studies each nitrogen feeding period was not preceded by a nitrogen-
free period. This step was omitted since according to Ackerson et al [1929]. Individual standardiz-
ation of the subject produces no greater accuracy of results than the use of an average value for the
endogenous nitrogen excretion in calculating biological values. It has been assumed that in birds
as in animals the metabolic faecal nitrogen is proportional to the dry matter in-take and that endo-
genous N is proportional to body weight. The average values thus calculated have accordingly
been used in the computation of endogenous and metabolic nitrogen excreted during the nitrogen
feeding periods for the calculation of biological values.
The compostion of the cereal grains in these experiments is given in Table II. The biological
values (Tables III-VI) are calculated on the assumption that the uric acid nitrogen and ammonia
nitrogen represent 80 per cent of the total urinary nitrogen. But, in the case of jowar (Table VII)
this assumption gave negative values for faecal nitrogen which strengthens the doubt of Van Landing-
ham, Clark and Schneider [1942] whether with all diets uric acid and ammonia N represent a constant
percentage of the total urinary N. We have therefore assumed, purely on an arbitrary basis, that
the sum of uric acid N and ammonia N forms 90 per cent of the total urinary nitrogen on the jowar
diet and thus obviated the difficulty of negative values for faecal nitrogen. Macdonald and Bose
determined the biological values of three mashes for which the 80 per cent basis might possibly hold
good as in the case of the remaining four feeds in the present observation, More extensive studies
with a larger number of feeds are required to settle this point. The results obtained for jowar show
that the uric acid N and ammonia N do not remain a constant percentage of the urinary nitrogen
with all diets.