The Voluntary Food Intake of Farm Animals

207 Pages·1986·3.323 MB·English

by  J. M. Forbes (Auth.)

The Voluntary Food Intake of Farm Animals
J. M. Forbes BSC, PhD, DSC
Reader in Nutritional Physiology
Department of Animal Physiology and Nutrition, University of Leeds
Butterworths
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First published, 1986
© Butterworth & Co. (Publishers) Ltd, 1986
British Library Cataloguing in Publication Data
Forbes, J. M.
The voluntary food intake of farm animals.
1. Veterinary physiology 2. Digestion
I. Title
636.089'23 SF851
ISBN 0-408-11154-2
Library of Congress Cataloging in
Publication Data
Forbes, J. M. (Jeffrey M.)
The voluntary food intake of farm animals.
Bibliography: p.
Includes index.
1. Animal nutrition. 2. Veterinary physiology.
I. Title
SF95.F6741986 636.089'32 85-26890
ISBN 0-408-11154-2
Typeset by the University Printing Service at the University of Leeds
Printed and bound by Butler & Tanner Ltd London and Frome
Preface
'Voluntary intake can only be fully understood and manipulated when the control
factors and their integration mechanisms are known. Complex as this question may
be, it is basic to the biology of (ruminant) animals and to animal production, and
therefore worthy of greater effort.' (Moir, 1970, summing up in the session on
voluntary food intake at the Illrd International Congress on Ruminant Physiology,
p.291.)
This statement encompasses the two main reasons for my interest in the
voluntary food intake of farm animals: first, that it is a complex subject where a
wide range of interests is combined in order to gain greater understanding of the
biology; secondly, that animal production is dependent on adequate levels of
voluntary intake, especially in the case of ruminants which can utilise poor forages
and by-products which are of no direct nutritional value to man.
In attempting to cover the whole of the subject I have necessarily had to deal
with some areas in more detail than others and it is natural that these have tended
to be ones in which I have personal experience. Some readers will, therefore, find
areas in which they would have liked to have seen more depth and areas which are,
in their opinion, too detailed. In particular, I am conscious of not having discussed
fully the problems of low intake in tropical and arid parts of the world.
In many instances I have presented evidence which, in my view, points so clearly
to a conclusion that to state that conclusion is unnecessary; I hope that this style is
not too cryptic.
I wish to express my sincere thanks to the following who gave their time to
comment on sections of the draft: Dr Μ. H. Anil, Dr J. Hodgson, Dr C. L.
Johnson and Dr R. G. Rodway. Particular thanks are due to Professor W. Holmes
and Lindsay Caird of Wye College, London University, for their most careful and
valuable comments. A book depends heavily on its publishers and in this case it is a
pleasure to have worked with Sue Deeley of Butterworths. The text was prepared
using SCRIPT on the University of Leeds AMDAHL V7 and 580 computers with
permission of the University Computing Service. It was transferred direct to the
Linotron 202 typesetting system and printed by the University Printing Service,
where Mr H. Tolson, Mrs L. Robertson and Mr J. Graham played a significant part
in its smooth preparation.
J. M. Forbes
December 1985
V
1
Introduction
This chapter describes in general terms why studies of the control of voluntary food
intake in farm animals are important; it defines the terminology, and describes the
techniques used to measure food intake and feeding behaviour and also the major
features of domestic fowl and ruminants where they differ from the simple-
stomached mammal.
Voluntary food intake is the weight eaten by an animal or group of animals
during a given period of time during which they have free access to food. In this
book the words food and feed are used interchangeably.
Significance of voluntary food intake
If voluntary food intake is too low then the rate of production is likely to be
depressed, making the requirements of maintenance become a very large propor-
tion of the metabolisable energy in the food and so giving poor production and a
poor efficiency of food conversion. If there is too high a level of food intake then
excessive fat deposition may occur, in some species at least. Thus the aim must be
to match food consumption with the required level of production. This optimum
level of production depends to a large extent on the relative costs of different types
of feed and their nutrient values, and on the production response curve to changes
in feed quantity and quality. For example, depending on the circumstances, it may
be economically more efficient to feed a low level of compound feed to a dairy cow,
because forage intake increases and milk yield is not seriously depressed.
The quantitative importance of voluntary food intake is illustrated by the fact
that poultry, which are invariably fed ad libitum, consume 3.5 million tonnes of
food annually in the UK, accounting for some 70% of the total cost of poultry
production.
Animals compete with man for food and there is a need to reduce the amount of
grain-based feed, especially with ruminants which are able to make use of grass,
grass products and by-products of other agricultural and industrial activities;
cereals can thus be saved for human consumption and for pigs and poultry which,
unlike ruminants, cannot digest cellulose. Because the bulk of a food may depress
intake, knowledge of the effects of changes in the type of food offered is essential.
The scientific study of voluntary food intake is important, therefore, and
ι
2 Introduction
demands a multidisciplinary approach. The nutritionist, the physiologist, the
psychologist, the pharmacologist and, in the agricultural context, the animal and
crop scientist all have to be involved to unravel the complexities of the subject.
Problems of overeating
While the problems of human obesity are obvious, those concerning farm animals
are less so. That proverbial glutton, the pig, is often prevented from eating his fill
by restricted feeding of a daily measured amount of food which is expensive in
labour. Other species also become obese, however; cattle, sheep and broiler
chickens fed ad libitum continue to deposit fat until they become grossly
Overweight'. For example, Friesian dairy cows offered a feed low in roughage ad
lib-1itum and not remated were seen to increase in weight at the rate of about 1 kg
d and to show no sign of slowing down after 70 weeks, when they weighed
700 kg (Monteiro, 1972).
Do these animals get fat through overeating or because their energy output is too
low? There has been much interest in the possibility that brown fat might 'burn off
excess energy intake (Stock and Rothwell, 1982; Hervey and Tobin, 1983), but
brown fat appears to be absent in farm animals after the first few weeks of life. This
then might be why cattle, sheep and pigs do not control their body fat content as
well as the rat. Cattle, sheep, pigs and broilers seem to be more prone to obesity
than other species, possibly as a result of selection by man for fast growth even if
much of the weight gain was in the form of fat.
Consumer preference has been to reduce amounts of fat in meat over the last 50
years or so due, at least in part, to the general decrease in physical exertion of the
human population. It is in the interests of the producer to reduce the amount of fat
his animals deposit because of its low sale value and also because the high energy
content and low water content of adipose tissue make it very expensive to produce.
Problems of undereating
Animals normally eat that amount of food which satisfies their energy require-
ments including continued fat deposition in the adult. There are some circum-
stances, however, when insufficient is eaten, resulting in loss of body weight or a
decrease in a productive process, such as growth or milk secretion. Undereating
can occur, in humans at least, even in the presence of adequate availability of food
(e.g. the condition of anorexia nervosa). More commonly it occurs when there is a
shortage of food (famine). In farm animals the problem of undereating is most
often seen with ruminants where highly fibrous, bulky food is offered. This is
digested slowly and its disappearance from the rumen sets a limit to the rate at
which more food can be eaten; the mechanisms are dealt with in Chapters 2 and 3.
This problem of undereating is at its most acute when other abdominal organs are
competing for space (uterus, fat) or when the energy requirements are very high, as
in early lactation.
Food intake may be depressed also when the food is deficient in an essential
component such as protein, a mineral, a vitamin or an amino acid (see Chapter 6).
Figure 1.1 shows the contrast between two cattle; the animal on the left was offered
a diet which was deficient in cobalt while the one on the right was given a balanced
diet, the major effect being on voluntary intake. Such deficiencies depress intake
Significance of voluntary food intake 3
Figure LI Effect of cobalt deficiency on the growth of cattle fed ad libitum. The animal on the left was
on a cobalt-deficient diet (From A. MacPherson, personal communication; reproduced with permission)
due to a specific effect (on the brain) or a more general depression of metabolism
leading to a decrease in energy requirements.
When the amount of herbage available for grazing is very sparse and each
mouthful is small there may not be enough time in the day for the animal to eat
enough to satisfy its nutrient demands. When snow, cold wind or hot weather
prevent grazing there will again be inadequate food intake.
Matching diet with appetite
Under natural conditions animals such as ruminants, pigs and poultry are 'general'
feeders; that is, they eat from a wide range of foods. Initially they sample most
potential foods, but as they learn the nutritive (or aversive) properties of each type
of material they become more selective. Although energy is probably the main
controller of food intake there are other appetites, some innate, others learned,
which influence the animal's choice of food and its total intake. The aim of the
animal nutritionist is to match the quantity and quality of the diet with the nutrient
requirements of the animal. If the diet is offered ad libitum this implies that the
composition of the food should be such as to allow enough to be eaten to meet the
animal's nutrient needs, but not to overconsume. In practice this means offering a
highly digestible nutrient-dense food (or foods) when high production is required
(growth, late pregnancy, early lactation, egg production) but reducing the nutrient
density of the feed at other times so as to prevent excessive fat deposition. This
control of diet quality is often achieved with ruminants by varying the amount of a
4 Introduction
cereal-based compound food while allowing free access to a more fibrous
roughage. With pigs and poultry, the greater degree of dilution with inert or poorly
digestible 'fillers' which is necessary to depress intake of digestible nutrients usually
renders this approach impractical.
With grazing animals, intake is influenced by varying the stocking rate, herbage
height or time available for grazing. The composition of the available herbage is
not so amenable to manipulation and the changes in digestibility and composition
which occur at different stages of the growing cycle must be understood if optimum
use of grass is to be made. The formulation of diets has become increasingly
sophisticated, especially for non-ruminants, so that they meet as closely as possible
the requirements of the animals for which they are intended. Because requirements
of an animal are for a given amount of a nutrient to be taken in a day, rather than
per unit of feed, assessment of optimum levels of inclusion of nutrients depends on
a knowledge or prediction of voluntary intake if it is intended that the feed should
be given ad libitum, as is usually the case with poultry. It is unnecessary and
impractical to control exactly the composition of ruminant feeds because they are
going to be modified by rumen fermentation which precedes the normal processes
of mammalian digestion (see later).
Main features of eating
Definitions
Animals eat for discrete periods of time, each period being a meal; given free
access to food of good quality, individuals of many species may eat from ten to
fifteen meals per day. Very often the distribution of meals through a 24 h period is
not uniform, with more frequent, larger meals being taken during the photophase
in those species, including the common farm animals, which are more active during
that period. Dulphy et al. (1980) have reviewed the feeding behaviour of ruminants
in detail. Feeding patterns through a 24 h period are shown in Figure 1.2 for a pig, a
chicken, a cow and a sheep.
Distinct eating periods, which may include short breaks but which are separated
by longer intervals, are called meals and the short within-meal periods of eating are
called feeding bouts. In analysing feeding behaviour a minimum inter-meal interval
is often adopted, meals separated by intervals of less than this value being
considered as part of the same continuing meal. This critical inter-meal interval, if
it is to be applied, should not be selected arbritrarily but only after study of a
frequency plot of inter-meal intervals (Savory, 1979). In several species of bird, for
example, intervals between feeding bouts are distributed in the form of negative
exponentials, implying that there is a constant probability of a meal starting.
However, the shortest intervals between meals do not follow this pattern and can
be regarded as breaks in a meal, rather than true intervals between meals. Duncan
et al. (1970) show the frequency distribution of inter-meal intervals for domestic
fowl. There were far more intervals of 2 min or less than would be predicted from
the negative exponential of the rest of the data, and 2 min was therefore adopted as
the minimum inter-meal interval. Slater (1974) used the same principle but in a
form in which the critical inter-meal interval can be found more accurately.
Metz (1975) derived survivorship curves for lengths of feeding bouts and
Main features of eating 5
100
al)
ot
t
y
ail
d
of
%
(
e
k
a
nt
e i
v
ati
ul
m
u
C
Time of day (mins from midnight)
Figure 1.2 Meal patterns of a chicken, sheep, cow and pig (unpublished observations). Note the smaller,
less frequent meals at night (A, chicken, 31 meals; B, sheep, 14 meals; C, cow, 18 meals; D, pig, 9
meals)
inter-bout intervals for cows; bout length shows a distribution which is close to
random. The bout-to-bout interval changes its frequency distribution at about 4
min, which is interpreted to mean that intervals of less than 4 min are within a meal
(Figure 1.3).
Metz also analysed rumination patterns in a similar manner and a thorough
reading of his work is recommended to anyone who is planning to study feeding
and ruminating behaviour.
When an animal starts to eat it is said to do so because it is in a state of hunger,
when it stops eating it does so because it is satiated. These two terms have no
precise physiological meaning.
Derived from measurements of meal size and inter-meal interval are the hunger
ratio, i.e. weight of meal divided by pre-meal interval, and the satiety ratio, i.e.
weight of meal divided by post-meal interval. Appetite is used to describe a drive to
eat a specific nutrient, rather than to eat food in general. The overall sensory
impression the animal receives from its food is the palatability.
The weight of food eaten per unit of time is the rate of eating. When it is possible
to measure intermediate weights of the food during the course of a meal it can be
seen that rate of eating sometimes declines towards the end of a meal (cattle,
Suzuki et al., 1969; pig, Auffray and Marcilloux, 1983).
The total amount eaten during a given period of time (usually 1 d) is usually
called the voluntary intake', this is often lower than the potential intake (the weight
of food required to fulfil all of the animal's nutrient requirements) due to the
physical or chemical constraints within the animal, or environmental limitations.
6 Introduction
100
0 2 4 6 8 10 12
Bout length (min)
Figure 1.3 Cumulative frequency of inter-meal intervals (FF, dashed line) and meal lengths (F,
continuous line) for two cows; dotted line represents the exponential model (From Metz, 1975;
reproduced with permission)
Methods of measuring food intake
In order to study factors affecting voluntary food intake and to develop methods of
prediction, we need to be able to measure intake in a variety of experimental and
farm situations. Much of the more applied experimental work covered in later
chapters has relied for measurement of voluntary food intake on single weighings
of food at intervals of 24 h, but often shorter and occasionally longer periods. It is
sometimes difficult to interpret data collected in this way because animals eat
numerous meals during the day and a knowledge of the size and frequency of these
meals is useful, as will be seen in later chapters. The method of 24 h weighings is
not applicable to the grazing situation, nor to the estimation of individual intakes of
animals kept in a group.
When fresh food is offered only once per day it is important to offer sufficient so
that at least 15% remains; excessive allowance may, however, enable selection of
the more palatable parts of feed mixtures (e.g. Moore and Dolling, 1961) which
aggravates the difficulty of assessing the consumption of nutrients.
Sufficient time should be allowed for animals to become accustomed to new food
before voluntary intake is recorded. For ruminants, at least 10 d is required
because of the slow rate of passage and adaptation of the rumen micropopulation,
but prolonged standardisation appears to be unnecessary (Heaney and Pigden,
1972).
In view of the variability between individuals, it is necessary to use a sufficient
number of animals in order to get a reliable estimate of intake. Variability between
animals in a group does not differ greatly between different feeds and is less when
they are penned individually than when they are penned together (Heaney et al.,
1968). These authors observed that '. . . while intake is unquestionably an
Methods of measuring food intake 7
important index of forage value, high variability of estimates (coefficient of
variation, 0.16) renders it not so useful as it might appear at first sight'.
Recording feeding behaviour of individually penned animals
This has traditionally been carried out by many hours of patient observation,
noting at regular intervals whether or not each animal is eating, ruminating,
standing, etc. While there is no substitute for this in terms of learning about animal
behaviour, and certain types of behaviour are not amenable to quantification in any
other way, this method suffers from two major drawbacks — it is very time-
consuming (this can be overcome to some degree by video-recording and observing
the playback at a fast speed) and it does not provide information on the weight of
food eaten, but only on the timing of meals.
A more sophisticated system for recording the length of meals was developed by
Wangsness et al. (1976) who used a light beam and photocell to detect when a sheep
had its head in the feeder and used this to move a new container of food into the
feeding area automatically if at least 20 min had elapsed since the previous meal
(i.e. a minimum inter-meal interval defined as 20 min). This equipment, slightly
modified, was also used to study the feeding behaviour of growing cattle (Chase et
al, 1976).
More comprehensive data can be collected by continuous automatic recording of
the weight of the food container (e.g. Suzuki et al, 1969; Figure 1.4). When the
weight is static the animal is not eating. The duration of a meal is signalled by
Strain gauge
Figure 1.4 Apparatus for recording meals of cows tethered individually (After Suzuki et al., 1969)