Farm Animal Metabolism and Nutrition

and thence to the general circulation. There
are several types of receptor in the stomach,
intestines and liver which can inform the
central nervous system (CNS) about the vol-
ume, osmolality, pH and concentration of
some specific types of chemical in digesta
and portal blood. Once in the general circu-
lation, the metabolites are available for sup-
porting the various metabolic activities. Any
major imbalance between entry of a material
into the circulation and its rate of removal is
postulated to cause ‘metabolic discomfort’
which will become associated with the
sensory properties of the recently eaten food
and tend to induce avoidance of that food
when it is next encountered by the animal.
Such learned associations are particularly
well illustrated in situations in which
animals have a choice between two or more
foods with different flavours and different
patterns of nutrient supply. Of course, the
whole system is coordinated by the CNS in a
diverse set of pathways that includes the
hindbrain and the hypothalamus as
important components. The complexities of
feeding behaviour, as observed in
spontaneously feeding animals (see Chapter
17), are unlikely to be fully explained, there-
fore, only by a knowledge of physiology and
metabolism, however complete that might
become.
Each of the aspects outlined in this
Introduction will be considered in turn.
Further information on all aspects covered
in this chapter can be found in Forbes
(1995).

Mechanisms of Intake Control

Central nervous system

The brain collects information from the

special senses and receptors in the diges-

tive tract wall and metabolizing tissues.

This information is integrated and used to

determine what food to eat and whether

feeding should start or stop. Extensive

studies, mostly with laboratory animals,

have been carried out to determine the

nature of the anatomical sites and neural

transmitters involved, and the relatively

few studies with farm animals provide

confirmation that they are, in general,

similar to rats. Electrolytic lesions of the

ventromedial hypothalamus cause over-

eating and obesity while in the lateral

hypothalamus they cause undereating,

often to the extent that animals die if not

kept alive by force feeding. These are not

the only parts of the brain involved in

intake control, however, and the para-

ventricular nuclei of the hypothalamus,

just ventral to the ventromedial nuclei, are

particularly sensitive to the effects of trans-

mitter chemicals, including noradrenaline

and neural peptide Y (NPY) (Parrott et al.,

1986). Figure 15.2 shows the effect of

several doses of NPY injected into the

lateral ventricles of the brain in pigs

trained to work for food by pressing a

panel in the wall of their pen (operant

conditioning); with a dose of 100 μg, the

pigs worked to obtain 60 rewards of food

320 J.M. Forbes

Eating Digestion Metabolism

Sight

Odour

Taste

Texture

GI tract

Mechanoreceptors

Chemoreceptors

Blood-borne

Liver

Hindbrain

Body

Adipose

Muscle

FOOD DIGESTA NUTRIENTS STORES

Time

CNS

Fig. 15.1.Satiety cascade (after Blundell and Halford, 1994). See text for explanation.