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The Nutritive Value of Alkali-treated Cereal Straws
General discussion of the results of digestibility and metabolism experiments
(a) The digestibility of crude fibre and nitrogen-free extracts.—It is now
well known that, though the digestive enzymes are lacking in the gastric
secretions for higher carbohydrates such as cellulose, these latter are never-
theless subject to breakdown in the body of the animal by the action of micro-
organisms which are present in some parts of the alimentary canal. Such
microbial action occurs extensively in the rumen of cattle, sheep and goats
and in the caeca of non-ruminants such as the horse and the rabbit. The
digestion of cellulose by herbivorous animals is thus indirectly performed
through the agency of micro-organisms inhabiting the rumen or caecum.
Pure cellulose has been found to be well digested by herbivora [Fries, 1920],
whereas the digestibility of crude fibre, which is mainly composed of cellu-
lose, is low. The low digestibility of fibre has been ascribed to its lignin
content which forms a protective covering against the cellulose-splitting
micro-organisms. Crampton [1939], experimenting on pasture herbage, cut
periodically throughout the season, has shown that the lowest digestibility
of the fibre of a sample is associated with its highest lignin content. Accord-
ing to Woodman and Stewart [1932], however, the indigestibility of crude
fibre may not be so much dependent upon the lignin which it may contain
as upon the manner of deposition of the lignin within the cell wall. In the
light of the work carried out by Norman [1935], one could reasonably
anticipate that the strength of the caustic soda solution used for the treat-
ment would adversely affect the lignin coat, even though the lignin itself
may not be quantitatively removed. An attempt was made in this laboratory
to estimate the lignin content in the untreated and the alkali-treated wheat
straw, and it was found that the alkali treatment followed by the water
wash had removed only about 16 per cent of the lignin originally present in
the straw. Treating wheat straw with 1.5 per cent caustic soda solution
in the cold, Pringsheim [1919] has recorded a loss of incrustants to the extent
of only 12.4 per cent. These experimental results thus tend to suggest
that the increased digestibility of fibre is due to the adverse effect of the alkali
on the physical nature of the lignin crust.
Crampton and Maynard [1938] have shown that the coefficient of digesti-
bility of lignin in ruminants is very low. As lignin forms a part of the nitro-
gen-free extracts, the increased digestibility of N.F.E. in treated straw may
have been due to the partial removal of poorly digestible lignin from the
straws during the alkali treatment.
From the value of S.E. actually determined in the present series of ex-
periments with untreated and alkali-treated wheat and paddy straws, it may
be observed that the experimental animals were receiving a considerably
higher level of S.E. in periods 2 and 4 than during periods 1 and 3. The level
of intake of S. E., in fact, was 65 per cent and 35 per cent higher from treated
wheat and paddy straws than that from the respective untreated straws.
The extra energy intake which is automatically achieved can serve two bene-
ficial purposes : (a It may help, by its protein sparing action, to maintain
the animals at a comparatively low protein intake. That this is possible
has been proved by the nitrogen balance data obtained in experiment 2
by feeding the animals with a lower quantity of cake and alkali-treated paddy