Voluntary food intake and diet selection in farm animals

461 Pages·2007·2.683 MB·English

by  J M Forbes

V F I
OLUNTARY OOD NTAKE AND
D S F A
IET ELECTION IN ARM NIMALS
Second Edition
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V F I
OLUNTARY OOD NTAKE
D S
AND IET ELECTION IN
F A
ARM NIMALS
Second Edition
J. Michael Forbes
Emeritus Professor
Institute of Integrative and Comparative Biology
University of Leeds
Leeds LS2 9JT
UK
Correspondence:
1, The Woodlands
Arthington Lane
Pool in Wharfedale
Otley LS21 1JY
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© J.M. Forbes 2007. All rights reserved. No part of this publication may be
reproduced in any form or by any means, electronically, mechanically, by
photocopying, recording or otherwise, without the prior permission of the
copyright owners.
A catalogue record for this book is available from the British Library,
London, UK.
ISBN-13: 978 1 84593 279 4
Library of Congress Cataloging-in-Publication Data
Forbes, J. M. (John Michael), 1940–
Voluntary food intake and diet selection in farm animals / J. Michael Forbes.
-- 2nd ed.
p. cm.
Includes bibliographical references and index.
ISBN 978-1-84593-279-4 (alk. paper) -- ISBN 978-1-84593-280-0 (ebook)
1. Animal nutrition. 2. Food preferences. 3. Ingestion--Regulation.
I. Title.
SF95.F675 2007
636.08’4--dc22
2007006773
Typeset by Columns Design Ltd, Reading, UK
Printed and bound in the UK by Biddles Ltd, King’s Lynn
Contents
Preface vii
Part I Basics
1 Introduction 1
2 Feeding Behaviour 12
Part II Mechanisms
3 Feedbacks from the Gastrointestinal Tract 41
4 Metabolites and Hormones 69
5 Central Nervous System and the Special Senses 91
6 Learning about Food: Conditioned Preferences 117
and Aversions
7 Diet Selection: Principles 144
8 Diet Selection: Practicalities 171
Part III Theories
9 Integrative Theories of Food Intake Control 188
10 Minimal Total Discomfort 203
Part IV Food Composition
11 Diet Digestibility, Energy Concentration and 219
Supplementation of Forages
v
vi Contents
12 Protein and Other Nutrients 243
13 Appetites for Specific Nutrients 261
14 Silage 287
Part V Animal and Environmental Factors
15 Growth and Fattening 310
16 Reproduction and Lactation 341
17 Environmental Factors Affecting Intake 365
18 Epilogue 391
References 407
Index 449
Preface
The importance of the subject of this book encouraged me to prepare a second
edition both to provide up-to-date coverage and to change the emphasis of
some sections. The first edition (Forbes, 1995), as with its predecessor (Forbes,
1986b), has been well received, so a broadly similar approach has been
adopted for this edition.
The preface to the first edition states that: ‘My approach throughout has
been to provide examples …I have given many references to the literature’.
This not only cluttered the text and might have interfered with ease of reading,
it also resulted in a very long reference list (over 1300 references!) occupying a
great many pages. In this edition I have not sought to justify my every
statement but rather to provide references to reviews; in many cases only the
most recent of a batch of papers is referenced and older literature can be traced
through this. Nevertheless, there still remain many references to what I consider
to be key original papers. The list of references of the first edition has been
made available at http://www.fbs.leeds.ac.uk/ForbesRef/RefsForFirstEdition to
provide access to some of the information for which references have been
omitted from this second edition.
Where I have felt that the text and/or diagrams from the first edition are still
appropriate, I have left them in rather than paraphrase for the sake of it.
However, there are many new sections and much reorganized material.
Chapters have been added and grouped into common themes, and the chapter
on intake at grazing has been removed, as that in the first edition did not do
justice to the subject. The material on prediction of intake, previously in its own
chapter, has now been distributed to relevant sections in appropriate chapters;
in this way most of the material on, say, lactation is together rather than being
divided between two or more chapters. Prediction of food intake and
understanding of how it is controlled should go hand in hand and not be
artificially separated as they were in the first edition.
viii Preface
Some areas have seen consolidation but not a great deal of development,
especially in physiology, as few are working with farm animals in that area now.
There have been expansions in modelling and in molecular biology, but little of
the latter has been applied to farm animals yet. It would be logical to include
information on laboratory animals where it is missing for farm animals, but this
would make an already long book far too long. The classic book of Le Magnen
(1985) is an excellent starting point, while that of Dockray and Smith (2006)
brings the subject up to date. For those with an interest in fish the book edited
by Houlihanet al. (2001) provides comprehensive coverage.
I have attempted to provide more structure to the book by using the
Minimal Total Discomfort (MTD) concept as a linking strategy. Do not mistake
this for a belief that this theory explains everything (or even anything). Simply,
the first edition was not as strong as it should have been in structure and
direction, and MTD provides a simple way of incorporating several factors
affecting intake in a semi-quantitative manner and thereby acting as a
framework around which many of the experimental results and ideas can be
integrated.
In covering the subject from so many different angles it is unavoidable that
there will be some overlap between chapters; in most cases I have cross-
referenced between chapters in order to minimize overlap. Nevertheless, it has
at times been necessary to present similar information in more than one place.
It seems to me that the most obvious such case is in the consideration of diet
selection and appetites for protein. Thus, this subject appears in Chapter 7 in
the consideration of the principles of diet selection and its methodology, in
Chapter 8 where more practical aspects of diet selection are being discussed
and in Chapter 13 where the appetite for protein comes under scrutiny. To
have put these three sections together in one chapter would have left obvious
gaps in the two chapters from which the material was removed.
In describing results of experiments I have not usually given the level of
statistical significance; if I say there is an effect then it is significant at P< 0.05.
If a difference or a relationship is not quite significant then I make this clear.
I am very pleased to acknowledge the advice of Dr Mike Wilkinson on
Chapter 14, to my academic colleagues for discussions and collaboration, and
to my wife Joan for her patience and understanding.
J. Michael Forbes
February 2007
1
Introduction
The majority of farm animals are fed ad libitum; that is, they have food
available almost all of the time. This is true whether they are kept intensively –
the supply of food being under direct control of their keeper – or extensively,
where the herbage available varies in quantity and quality according to the
time of year, but is rarely completely unavailable. Despite this freedom of
access to food there are many situations in which animals over- or undereat,
and ‘voluntary food intake’ and the factors which control and influence it are of
great importance to agriculturists across the world. The ramifications of the
subject are so great that it is impossible to cover them all in one book, and I
have tried to give an even coverage of all aspects although, no doubt, some of
my particular interests and prejudices will show through.
The subject of grazing and herbage intake is not covered and readers are
referred to books by Hodgson (1990) and Vallentine (2001) and to conference
proceedings edited by Hodgson and Illius (1998). Vallentine’s book also
includes details of the anatomical and metabolic adaptations of ruminants.
In this brief introductory chapter, we discuss in general terms why studies
of the control of voluntary food intake in farm animals are important, the
terminology is defined and a little of the history of the subject is described. The
techniques used to measure food intake are briefly described while methods of
monitoring feeding behaviour are dealt with in Chapter 2.
The Significance of Voluntary Food Intake
If voluntary intake is too low then the rate of production is likely to be
depressed, meaning that the requirements for maintenance become a very
large proportion of the metabolizable energy in the food and so giving a poor
efficiency of food conversion. If there is too high a level of intake then excessive
fat deposition may occur, in some species at least. Thus the aim must be to
©J.M. Forbes 2007.Voluntary Food Intake and Diet Selection 1
in Farm Animals 2nd Edition (J.M. Forbes)
2 Chapter 1
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 food and their nutritive values, and on the production response curve
to changes in food quantity and quality. For example, depending on the
circumstances, it may be economically more efficient to feed a low rather than
a high level of compound supplement 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 mostly fed ad libitum, consume some 3.5 million t
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 food, especially in 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 cannot digest cellulose, unlike the ruminant. Because the bulk of
a food as well as its nutritive value may limit 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
demands a multidisciplinary approach. The nutritionist, the physiologist, the
psychologist, the pharmacologist, the modeller and, in the agricultural context,
the animal and crop scientist all have to be involved in unravelling 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’ (see Chapter 15). For example, Friesian dairy cows offered
a food low in roughage ad libitum and not remated were seen to increase in
weight at the rate of about 1 kg/day and to show no sign of slowing down after
70 weeks, when they weighed 700 kg, some 100 kg above the ‘normal’ weight
for such cattle (Monteiro, 1972).
Do these animals become 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, 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 does 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 (up to the middle
of the last century), even if much of the rapid weight gain was in the form of fat.
Consumer preference has been to reduce amounts of fat in meat over the
last 60 years or so due, at least in part, to the general decrease in physical