animal the opportunity to fill her rumen to
a greater extent before the totalof all the
signals reaches a satiating level.
Diet Selection
So far we have considered only those situa-
tions in which animals have a single food
available so that they have no opportunity
to alter the nutritive value of the food other
than by eating more or less food. This is a
very artificial situation for most species
which evolved in rich environments in
which a wide variety of food types was
available. Although it might be thought
that a grazing animal might by chance eat
such a mixture of plant species so that it
would be unlikely to over- or undereat any
particular nutrient, there is ample evidence
that farm animals can make directed
decisions about which foods to eat in order
to match more closely their requirements
for various nutrients.
One of the clearest demonstrations of
‘nutritional wisdom’ is the ability of the
growing pig to select between foods with
protein contents higher and lower than that
required, so as to provide sufficient protein
for growth without overconsumption.
Kyriazakis and colleagues (1990) have
shown that choice-fed growing pigs select a
diet containing very close to 205 g protein
kg^1 which is very similar to the optimum
for growth under the conditions of the
experiment. They avoid an excess protein
intake as much as they avoid a deficiency,
presumably as both make them feel
metabolically uncomfortable and they learn
to avoid such discomfort by appropriate
diet selection. Note that if the two foods
were exactly similar in flavour, colour and
texture, the animals could not learn the
association with metabolic consequences.
Growing broiler chickens demonstrate
a clear ability to select a protein intake
appropriate to their requirements and, like
growing pigs, choose progressively less
protein, relative to energy, as they get older,
to match the declining requirements for
protein relative to energy (Shariatmadari
and Forbes, 1993). Figure 15.7 shows that
when given a choice between two foods,
both of which have a higher concentration
of protein (protein:energy ratio) than that
required for optimum growth, broilers
eat almost all of the lower protein food,
i.e. they avoid eating excess protein.
Conversely, with two low-protein foods,
birds eat more of the higher protein food.
When they can make a balanced diet from
foods of two protein contents they do
so, and the chosen protein:energy ratio
declines as the birds grow with time as the
dietary protein concentration for optimum
growth is known to decline. In the case of
the birds offered foods containing 280 and
65 g protein kg^1 , the mixture chosen pro-
vided 226 g of protein kg^1 at 27 days and
183 g kg^1 at 63 days after hatching, con-
tents quite close to those found to be opti-
mum for growth in experiments with single
foods.
Thus, growing pigs and poultry show
considerable ‘nutritional wisdom’ and there
is evidence that ruminants also do (e.g.
Kyriazakis and Oldham, 1993). However, in
addition to optimizing nutritional balance,
they also choose sufficient fibrous food to
stabilize rumen fermentation and prevent
rapid changes in fermentation and unstable
pH. For example, sheep changed from long
hay to a diet free of long fibre immediately
started to eat chopped polyethylene fibre
which they previously ignored (Campion
and Leek, 1997). The polythene intake
was reduced when a polyethylene fibre
pompom was introduced via a rumen
fistula. This suggests that ruminants have
evolved a ‘fibre appetite’ in order to reduce
the risk of disorders due to low intake of
inert fibre and that this might be a result of
the reduction of ruminal sensory input to
the brain normally provided by fibrous
reticulo-ruminal contents.Specific appetitesProtein supplies many amino acids, and
the above-mentioned ability to balance
protein and energy intakes independently
is not a clear example of a specific appetite.
However, poultry have been shown toPhysiological and Metabolic Aspects 329