by section of the vagus nerves (i.e. CCK
stimulates abdominal receptors which trans-
mit information to the CNS via the vagal
pathway), and when the above experiment
was repeated with chickens vagotomized at
the level of the proventriculus, no signifi-
cant colour preferences or aversions became
established.
Another example of learned associa-
tion, this time for ruminant animals, is the
way in which lambs learn to avoid
flavours associated with a deficiency or an
excess of nitrogen in the diet (Villalba and
Provenza, 1997). Lambs were conditioned
with wheat straw (deficient in nitrogen)
flavoured with two distinctive flavours. On
alternate days, flavours were switched and
with one of the flavours lambs received
capsules containing different amounts of
urea, ranging from 0.12 (insufficient) to
0.92 (excessive) g N day^1. When, after 8
days of conditioning, they were given a
choice between straws with each flavour,
they preferred the flavours associated with
urea at lower doses but avoided the
flavour associated with urea at the highest
dose. Thus, their preference was for the
flavour they had learnt to associate with
optimal nutrition, i.e. the most ‘metabolic
comfort’.
We can conclude then that animals
learn to associate the metabolic conse-
quences of eating a food with that food’s
sensory properties, and will discuss later
in this chapter how this ability is used in
diet selection and might be important in
the control of voluntary food intake.
Gastrointestinal receptorsThe ingestion of food causes changes in the
degree of fill and the chemical composition
of digesta which can be sensed by stretch
receptors and chemoreceptors in the wall
of the digestive tract.
Simple-stomached animals
Although it is not usually considered that
stomach fill is a predominant factor limit-
ing intake in pigs or poultry, extremes of
distension can certainly limit intake, and
stretch of stomach and intestine wall may
contribute to satiety. Inflation of a surgically
implanted balloon in the crop (a storage
sac at the base of the oesophagus)
depresses intake in chickens, while con-
sumption increases when material of low
or zero digestibility is added to a food, but
not to the extent of maintaining the same
rate of supply of nutrients to the animals as
a more concentrated food – this is often
ascribed to the bulky nature of the food
stimulating stretch receptors in one or
more parts of the digestive tract.
For example, duodenal infusion of
glucose solutions inhibits feeding in
chickens (Shurlock and Forbes, 1981a) but
potassium chloride and sorbitol solutions,
which are not absorbed, had a more
prolonged effect, suggesting that it is the
physical presence rather than the chemical
nature that is important. This information
is used by the brain, along with that from
many other abdominal receptors, to
determine whether feeding should stop or
proceed.
As stated above, CCK is secreted by the
wall of the duodenum in response to the
passage of digesta, particularly fat and
protein, and stimulates receptors locally
which relay their information to the CNS
where it results in a decrease in food
intake. It has been suggested that it is by
this means that food intake is controlled,
but it is clear rather that this is only one of
a large number of negative feedback signals
involved (see below).Ruminants
PHYSICAL LIMITS.Despite the fact that the
rumen has such a large capacity, the slow
rate of digestion of forage feeds means that
rumen capacity can be limiting to intake
(see Chapter 16). There are stretch receptors
in the rumen wall, especially in the
anterior dorsal part, and these signal the
degree of distension to the brain via the
vagus nerves (Leek and Harding, 1975). It
is unlikely that forage intake is controlled
only by distension, however, otherwise
ruminants would take a succession of
small meals as space became available by
digestion and onward passage of digesta. InPhysiological and Metabolic Aspects 323