from the consequences of eating a meal. If
the nutrients present are in the same ratio
as required, then food intake will be
optimal, i.e. it will meet the animal’s
requirements. However, few single foods
will meet these constraints for few animals
for a short part of their productive life.
Animals are therefore usually being offered
an imbalanced diet, and the extent of the
imbalance will determine the extent of the
difference between the amount of food
required and the amount actually eaten.
What the animal learns from eating a food
will also contribute to how much it eats on
subsequent occasions, i.e. will play a part
in the control of voluntary intake.
Where a choice between n foods is
available, the animal is in theory able to
control its intake of n constituents, e.g.
energy and protein when n= 2, but this
ability when n is >2 has been little
explored in farm animals. Choice feeding is
difficult to implement in practice and
animals sometimes make nutritionally
unwise choices, the reasons for which are
not always clear (Forbes and Kyriazakis,
1995).
To provide for the animals’ needs in
farming practice, the composition of the
food is usually changed at regular intervals
during growth, lactation or egg-laying, to
match the changes in requirements. With
ruminants, a basal forage is usually offered
ad libitum, with supplementary foods of
compositions and in amounts judged to
balance the diet.
There is no single factor responsible
for the control of voluntary food intake.
Rather, a multitude of influences are
coordinated by the CNS and the animal
eats that amount and mixture of food that it
learns give the minimum of discomfort.332 J.M. Forbes
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