the caecum is similar to that described for
the rumen. In rabbits, the VFA ratio is still
dominated by acetate, but butyrate
generally is increased and propionate
decreased.
Feeds such as maize that can provide a
source of readily fermentable carbohydrate
to the caecum without a supply of fibre to
maintain motility can have devastating
effects on rabbits. Rapid fermentation
increases the acidity of the caecum which
inhibits motility. Reduced motility allows
the fermentation to continue in the
caecum, producing more acids. The acids
cause irritation and damage to the epithe-
lium, allowing a large influx of tissue water
into the lumen. A severe and potentially
lethal diarrhoea ensues. This is an important
practical problem. Nearly 20% of all
rabbits born alive in commercial rabbit
production die as a result of this type of
situation. A second paradox of rabbit
physiology is that constipation induces
diarrhoea. The constipation can be
initiated by many factors: lack of water,
inadequate fibre, resistant starch in the
diet, genetics, stress and antibiotics.
CarnivoresCarnivores have not been a significant
farm animal raised for human food. Until
recently, commercial production of
carnivores was restricted to fur-bearing
animals. Recently there has been
increased interest in raising fish, some of
which are carnivores, alligators, and in
some parts of the world, dogs. Of these
species, only some fish and alligators are
true carnivores. The less strict carnivores
can obtain 30–50% of their glucose need
by the process described for direct
absorbers. Strict carnivores have very little
carbohydrate in their diet and lack the
capacity for significant digestion of carbo-
hydrate. Carnivores are direct absorbers,
but not of carbohydrates.
Protein must provide the carbon for
gluconeogenesis necessary to supply blood
glucose. Dietary protein is processed
analogously to carbohydrates as described
for direct absorbers. The enzymes, substrates
and products differ. Luminal proteases are
secreted as inactive zymogens that are
activated by specific cleavage. There also
are several brush border peptidases. Active
transporters similar to SGLT1 absorb free
amino acids. Many amino acids are
absorbed in the form of short peptides. The
mechanisms for peptide transport are less
well defined. Paracellular transport is also
probable.
The dietary protein hydrolysis
products that are absorbed are carried to
the liver by the hepatic portal vein. The
mechanisms that determine which and
how many amino acids are used as pre-
cursors of glucose synthesis, surprisingly,
have not been determined. It has been
observed that carnivores clear excesses of
dietary non-essential amino acids more
rapidly than dietary essential amino acids
(Coulson and Hernandez, 1983).
This does not do justice to carnivores,
but there appears to be very little known
about where their glucose carbon comes
from specifically. The area of fish nutrition
is fascinating, with much diversity in their
anatomy and feeding habits. Fish warrant a
separate chapter on their own.Concluding Comments
The purpose of this chapter was to trace
the routes of carbon from the diet to
glucose available to farm animals. It is
important for students to see beyond this
context to the overall challenge to agricul-
tural scientists. Examination of the human
population curve reveals that we as a
society must produce as much food for
human consumption during the next 40
years as we have produced since the begin-
ning of time. The driving force for all of
agriculture is solar radiation that must be
captured and converted to food for
humans. Of the biomass captured by
photosynthesis, most of which is carbo-
hydrate, 95% is unavailable as a direct
source of food for humans. Animals can
convert much of this non-food biomass
into high-quality food.144 R.W. Russell and S.A. Gahr