Farm Animal Metabolism and Nutrition

Chapter 15

Physiological and Metabolic

Aspects of Feed Intake Control

J.M. Forbes

Centre for Animal Sciences, School of Biology, University of Leeds, Leeds, UK

Introduction

The great majority of farm animals are fed
ad libitum, that is, with free access to food
for most of the time. High levels of
voluntary intake are required for efficient
production because, in general, the more
an animal eats the more it produces and
the more efficient it becomes. To a con-
siderable extent intake is driven by
nutrient requirements, and an approximate
estimate of how much food an animal will
eat can be calculated from what quantities
of energy, protein and nutrients it requires
for maintenance and production. However,
there are numerous factors which can
interfere with the concordance between
requirements and intake, and some of these
will become clear in this chapter. Briefly, it
is unlikely that the nutrients in the food(s)
on offer will be present in the same ratio as
required by the animal; perhaps protein
content will be too low in relation to
energy and the animal is then faced with
the decision as to whether to increase its
intake in order to satisfy its protein
requirements, thereby taking in an excess
of energy, or whether to reduce its intake in
order to avoid overconsumption of energy;

in the latter case, the quantity and/or quality

of product will be reduced due to lack of

dietary protein. The bulk of the food is

another major determinant of the level of

intake actually achieved, especially in

ruminant animals (see also Chapter 16).

It is generally considered that intake is

controlled by a series of negative feedback

signals from the digestive tract, liver and

other organs in response to the presence of

nutrients. In addition, animals learn the

metabolic consequences of eating foods with

particular sensory properties (appearance,

flavour, texture) and can then use ‘feed-

forward’ to choose preferentially or avoid

foods which they have experienced pre-

viously. The whole system can be viewed as

a cascade, depicted in Fig. 15.1. Briefly,

potentially ingestible material is selected by

sight and/or smell and a decision is made

regarding whether or not to eat it. Once in

the mouth, the food can be swallowed or

rejected, depending on its taste and texture.

After swallowing, the animal is committed

to dealing with the food, in terms of diges-

tion, absorption and metabolism, although if

it is strongly toxic it may well be vomited

(see Chapter 18). Following absorption, most

of the products of digestion go to the liver

© CAB International2000. Farm Animal Metabolism and Nutrition

(ed. J.P.F. D’Mello) 319