The
animal and its food
Food is material which, after
ingestion by animals, is capable of being digested, absorbed and utilised. In a
more general sense we use the term 'food' to describe edible material. Grass
and hay, for example, are described as foods, but not all their components are
digestible. Where the term 'food' is used in the general sense, as in this
book, then those components capable of being utilised by animals are described
as nutrients.
The animals associated with man
cover the spectrum from herbivores, the plant eaters (ruminants, horses and
small animals such as rabbits and guinea pigs); omnivores, which eat all types
of foods (pigs and poultry); to carnivores, which eat chiefly meat (dogs and
cats). Under the control of man these major classes of animal still pertain,
but the range of foods that animals are now offered is far greater than they
might normally consume in the wild (for example, ruminants are given plant
by-products of various human food industries and some dog foods contain
appreciable amounts of cereals). Nevertheless, plant and plant products form
the major source of nutrients in animal nutrition.
The diet of farm animals in
particular consists of plants and plant products, although some foods of animal
origin such as fishmeal and milk are used in limited amounts. Animals depend
upon plants for their existence and consequently a study of animal nutrition
must necessarily begin with the plant itself.
Plants are able to synthesise complex materials
from simple substances such as carbon dioxide from the air, and water and
inorganic elements from the soil. By means of photosynthesis, energy from
sunlight is trapped and used in these synthetic processes. The greater part of
the energy, however, is stored as chemical energy within the plant itself and
it is this energy that is used by the animal for the maintenance of life and
synthesis of its own body tissues. Plants and animals contain similar types of
chemical substances, and we can group these into classes according to
constitution, properties and function.
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