Advances in agronomy volume 21

ADVANCES IN

AGRONOMY
VOLUME 21


CONTRIBUTORS TO THIS VOLUME

F. J. CARLISLE
D. J. GREENLAND

R. B. GROSSMAN
S. B. HEATH
CHARLES
E. KELLOGG

OLIVER
E. NELSON
J. M. OADES
ARNOLD
C . ORVEDAL
W. F. RAYMOND

G. D. SWINCER
R. W. WILLEY


ADVANCES IN

AGRONOMY

Prepared under the Auspices of the

AMERICAN
SOCIETY

OF

AGRONOMY

VOLUME 21

Edifed by N. C. BRADY
Roberts Hall, Cornell University, ithaca, New York

ADVISORY BOARD

R. R. DAVIS
F. A. HASKINS
W. D. KEMPER

.I.P. MARTIN

J . W. PENDLETON
W. A. RANEY
1969

@

ACADEMIC PRESS 0 N e w York and London

COPYRIGHT
‘C 1969,

BY

ACADEMIC
PRESS, INC.

ALL RIGHTS RESERVED.
N O PART OF THIS BOOK MAY BE REPRODUCED IN ANY FORM, BY
PHOTOSTAT, MICROFILM, RETRIEVAL SYSTEM, OR ANY OTHER
MEANS, W I T H O U T WRITTEN PERMISSION FROM T H E PUBLISHERS.

ACADEMIC PRESS, INC.
1 1 1 Fifth Avenue, New York, New York 10003

United Kingdom Edition published by
ACADEMIC PRESS, INC. (LONDON) LTD.
Berkeley Square House, London W 1X 6BA

LIBRARY
OF

CONGRESS CATALOG C A R D

NUMBER50-5598

PRI N TED IN T H E UNITED STATES O F AMERICA

CONTRIBUTORS TO VOLUME 21
Numbers in parentheses indicate the pages on which the authors’ contributions begin.

F. J . CARLISLE
( 2 3 7 ) , Soil Conservation Service, United States Department of Agriculture, Hyattsville, Maryland
D. J . GREENLAND
(195), Depurtment of Agricultural Biochemistry and
Soil Science, Waite Agricultural Research Institute, University of
Adelaide, South Australia
R. B. GROSSMAN
( 2 3 7 ) , Soil Conservation Service, United States Department of Agriculture, Lincoln, Nebraska
S . B. HEATH(28 11, Department of Agriculture, University of Reading,
Reading Berkshire, England
CHARLES
E. KELLOGG( I09), Soil Survey, Soil Conservation Service,
United States Department of Agriculture, Washington, D.C.
OLIVER
E . NELSON*
(17 l ) , Purdue University, Lafayetre, Indiana
J . M . OADES(195), Department of Agricultural Biochemistry and Soil
Science, Waite Agricultural Research Institute, University of
Adelaide, South Australia
ARNOLDC. ORVEDAL
( 109), Soil Survey, Soil Conservation Service,
United States Department of Agriculture, Washington, D.C.
W. F . RAYMOND( I ) , The Grassland Research Institute, Hurley, England
G . D. SWINCER
(195), Department of Agricultural Biochemistry and
Soil Science, Waite Agricultural Research Institute, University of

Adelaide, South Australia
R. W. WILLEY
t (28 I ) , Department ofAgriculture, University of Reading,
Reading Berkshire, England

*Present address: University of Wisconsin, Madison, Wisconsin.
t Presenr address: Makerere University College, Kampala, Uganda.
V


This Page Intentionally Left Blank


PREFACE

This volume marks a significant milestone in the history of Advances
in Agronomy. The first 20 volumes were compiled under the very capable
editorship of Dr. A. G. Norman, now Vice-president for Research at the
University of Michigan. Mounting pressures of other responsibilities
have prompted Dr. Norman to ask to be relieved as editor of this serial
publication; this is the first volume that does not carry his name. It is
fitting that we reflect briefly on the contributions he has made, not only
to this work but to his profession as well.
As editor of Advances in Agronomy, Dr. Norman has given 20 years of
faithful service and leadership to agronomists and soil and crop scientists
throughout the world. His extraordinarily good judgment in the selection
of authors and of subject matter has been largely responsible for the
success of this publication. His guidance to authors has helped both them
and the quality of their papers. He has seen Advunces in Agronomy grow
from a struggling review journal of concern only to American scientists

to a prominent review series with contributors and subscribers in many
nations.
Dr. Norman has found other ways to benefit his profession. He has
contributed directly as an active researcher in soil microbiology and in
soil and plant biochemistry. He has served as director of a large, interdisciplinary research unit and has enriched the education and training of
many soil and crop scientists as well as biologists.
We are also indebted to Dr. Norman for his service in scientific
societies. He served as vice-president and later president of the American
Society of Agronomy during a very critical period in the Society’s history.
In addition, for a period of three years he served as chairman of the
Division of Biology and Agriculture of the National Research Council.
Even though Dr. Norman has resigned his editorial responsibilities,
Advances in Agronomy fortunately will reflect his influence for some
time to come. The challenge of maintaining Dr. Norman’s high standards
and the broad subject matter coverage he provided is materially aided by
the efforts of such men as the eleven who have contributed to this
Volume 2 I.

N. C. BRADY
Ithaca, New York
August, I969

vii


This Page Intentionally Left Blank


CONTENTS
CONTRIBUTORS TO VOLUME 21

.

.

.

.

.

.

.

.

.

.

.

PREFACE .

.

.

.

.

.

.

.

.

.

.

.

vii

2
3

.

.

.

.

V

THE NUTRITIVE VALUE OF FORAGE CROPS
W . F . RAYMOND

. . . . . . . . . . .
I . Introduction .
. . . . . .
I 1 . The Components of Nutritive Value
. . . . . .
111. The Digestibility of Forage Crops .
. . . .
IV . The Digestibility of Different Forage Species
. . . . . .
V . The Voluntary Intake of Forages .
. .
VI . The Efficiency of Utilization of Digested Nutrients .
VII . The Relationship between Forage Quality and Forage Yield .
. . .
VIII . Forage Breeding for Improved Nutritive Value .
IX . The Effects of Processing on the Components of Forage
. . . . . . . . . .
Nutritive Value .
X . The Nutritive Value of Grazed Forage . . . . . .
References .
. . . . . . . . . . .

.

.

.

.

.
.
.

.
.
.

.

.

.
.

.
.

65

4
16

27
38

68

.

.

.

.

69
80

.

.

97

.
.
.
.
.

.
.
.
.
.

POTENTIALLY ARABLE SOILS OF THE WORLD
A N D CRITICAL MEASURES FOR THEIR USE
CHARLES
E. KELLOGGA N D ARNOL.D
C . ORVEDAL

I . Introduction .
. . . . . . .
I 1 . The Principle of Interactions in Soil Use
.
111 . Higher Production from Existing Arable Soils
IV . New Potentially Arable Soils .
. . .
References .
. . . . . . .

.
.
.
.
.

.
.
.
.
.

.

.
.
.
.

.
.
.
.
.

109

I12
122
140
169

GENETIC MODIFICATION OF PROTEIN QUALITY I N PLANTS
OLIVERE . NELSON

I . lntroduction . . . . . . . . . .
. .
I I . The Genetic Control of Protein Structure .
111 . The Relative Constancy of Leaf Protein Composition

ix

.
.

.
.

.
.

.

.

.

.

.

.

171
173
177


X

CONTENTS

IV. The Storage Proteins of Seeds .
. . .

V. Theopaque-2 andJloury-2 Mutations in Maize
VI. The Prospects of Improvements in Other Plants
VI1. Summary
. . . . . . . .
References .
. . . . . . .

.
.
.
.
.

.
.
.
.
.

.
.
.
.
.

.
.
.
.
.

.
.
.
.
.

.
.
.
.
.

178
I80
I87
I90
191

THE EXTRACTION, CHARACTERIZATION, A N D SIGNIFICANCE
OF SOIL POLYSACCHARIDES
G. D. SWINCER,
J. M. OADES,A N D D. J. GREENLAND
I. Soil Carbohydrates . . . . . . . . . .
11. The Significance of Soil Polysaccharides
. . . . .
I l l . Studies on Soil Polysaccharides
. . . . . . .
IV. Methods for the Analysis of Complex Polysaccharide Materials
V. Summary and Conclusions.

. . . . . . . .
References .
. . . . . . . . . . .

.

.

.
.

.
.

.

.

.
.

.
.

195
196
I99
222
229
230

FRAGIPAN SOILS OF THE EASTERN UNITED STATES
R. B. GROSSMAN
A N D F. J. CARLISLE
1.
11.
111.

IV.
V.
VI.
VII.
VIII.
IX.

x.

Introduction .
. . . . .
Horizons of Fragipan Soils
. .
Occurrence of Fragipan Soils . .
Properties of Fragipans .
. .
Fragipans and the Soil Water Regime
. . .
Genesis of Fragipans .
Fragipans and Soil Use .
. .
Classification of Fragipan Soils.

.
Unresolved Problems.
. . .
Summary
. . . . . .
References .
. . . . .
Appendix
. . . . . .

.
.
.

.
.
.

.
.
.

.
.
.

.
.
.

.
.
.

.
.
.

.
.
.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.
.

.
.

.
.

.
.

.
.

.
.

.
.

.
.
.
.

.
.
.
.

.
.
.
.

.
.
.
.

.
.
.
.

.
.
.
.

.
.
.
.

.
.
.
.

.
.

231
240
244
246
254
256
263
265
269
27 I
272
276

THE QUANTITATIVE RELATIONSHIPS BETWEEN PLANT POPULATION
A N D CROP YIELD
R. W. WILLEYA N D S. B. HEATH

I.
11.

Introduction .
. . . . . . . . .
Relationships between Plant Density and Crop Yield.

.

.

.

.

.

.

.

.

28 I
283


xi

CONTENTS

I l l . The Relationship between Plant Rectangularity
IV. The Variation in Yield of the Individual Plant
. . . . . . .
V. Conclusions .
. . . . . . .
References .

AUTHORI N D E X .
SUBJECT

INDEX .

.
.

.
.

.
.

.
.

.
.

.
.

.
.

.
.

.
.
.

.
.
.

.
.
.

.
.
.

.
.
.
.

.

.
.
.

3 14
317
319
3 20

.
.

.
.

.
.

.
.

.
.

.
.

323

and Crop Yield

338


This Page Intentionally Left Blank


THE NUTRITIVE VALUE OF FORAGE CROPS
W. F. Raymond
The Grassland Research Institute, Hurley, England

I.
11.
111.

IV.

V.

VI.

VII.
v111.
IX.

X.

Page
Introduction.
......
...........................................................
2
The Components of Nutritive Value ..................................
....
3
The Digestibility of Forage Crops ...........
..................
4
A. The Measurement of Digestibility in
..................
4

B. The Prediction of Forage Digestibility from Chemical Composition .......
6
....
7
C. Improved Chemical Techniques ..... . . . . . ................... .....
D. Estimation of Forage Digestibility by in Vitro Techniques .................
10
E. The Relative Utility of Chemical and in Virro Estimations
of Forage Digestibility _ . _ _____
. _,_. .._ _ _ _
___._.__..__,
_ _ _ _ . .._....._.___.
._.......... 16
The Digestibility of Different Forage Species ..._...._....
... . .....
16
16
A. Basic Patterns of Digestibility .......................
B. The Digestibility of Different Plant Fractions ..
_........_____..._.
20
C. The Effect of Environmental and Other Factors on
...... .........
Forage Digestibility , . , . , . . . . . . . _ ._ . _ _ _ _ _._. ._. . _. . ,_..., __.............
23
..............................................
27
The Voluntary Intake of Forages
27
A. The Factors Controlling Feed Intake ...........................

28
B. Intrinsic Factors Determining Forage Intake ..._.. .........._
35
C. The Nutritive Value Index ...........................................................
D. The Crude Protein Content of Forage and Voluntary Intake ..............
36
E. The Effect of Supplementary Feeds on Forage Intake .....
37
The Efficiency of Utilization of Digested Nutrients.
.................. 38
A. Methods of Expressing Energy Values ....... . , , .. .. .. ................ . .. ........
38
B. The Role of Volatile Fatty Acids in Ruminant
40
C . The Utilization of the Crude Protein in Forages
..................
48
D. The Use of Nonprotein Nitrogen in Ruminant
51
E. The Mineral Nutrients in Forages .................................................
54
F. Pharmacologically Active Components in Forage
.................. 61
The Relationship between Forage Quality and Forage Yield ...... ..............
65
Forage Breeding for Improved Nutritive Value ....... . . ...........
.....
68
The Effects of Processing on the Components of
Forage Nutritive Value , . _.. .._,, . . . _ .. . . . . . , , _ _ _......

. . ..., . . . . . ............. ..............
69
..........................................
69
B. The Grinding and Pelleting of Dehydrated Forages ........
71
C . The Ensiling of Forage Crops ..........
...................................
74
The Nutritive Value of Grazed Forage ....
...................................
80
A. Measurement of the Nutrient Intake by Grazing Animals
.....
80


2

W. F. RAYMOND

B. The Botanical Composition of Grazed Forage .................................
C. The Nutrient Intake of Grazing Livestock ......................................
D. The Effect of Management on the Productivity of
Grazing Animals ........................................................................
References .......................................................................................

90
91
94

97

1. Introduction

Forages are grown for ruminant feeding, most ruminant animals eat
forages. Thus a review of the nutritive value of forages is essentially
a review of ruminant nutrition, yet with the difference that the nutritionist can treat the animal and the forage it eats in isolation, whereas
the agronomist must also consider the problems that arise when the animal and its feed are brought together in practical systems of forage production and utilization.
Ruminant feeding to date has been a nonintensive system of land use, in
comparison with crop farming or the feeding of nonruminant livestock.
This has been justified on the basis that forages are cheap to grow, and
that harvesting by grazing is a cheap method of utilization. However,
as Melville ( 1960) emphasized, present extensive pastoral systems produce very low outputs of human food per acre; as world demand for food
increases, pastures will either have to become markedly more productive,
or be replaced by crops that can be used by nonruminants, or directly
by humans. In the latter cases forage-ruminant systems would be confined to noncultivable agricultural areas, and so would contribute only
marginally to the nutrition of the world’s population.
At that time of apparent food surpluses the replacement of ruminant
products seemed remote; today analogue substitutes are already serious
competitors to meat and milk in North America, the most sophisticated
consumer market in the world. To date this competition has been in terms
of cost and convenience; in future it will increasingly be in terms of competition for land, as foreseen by Melville.
This means that the efficiency of soil-forage-ruminant systems must
be greatly increased if they are to continue as a significant sector of agriculture. Raymond (1968) has considered this problem in terms of (a)
the efficiency of use of incident light energy by the growing plant, (b)
the proportion of the energy in the plant which is actually eaten by the
ruminant animal, and (c) the efficiency with which different animal populations convert the energy they eat into products which can be used by
humans. In many cases it appears that stages (b) and (c) are the main
factors limiting the output of ruminant products per acre; until we can

THE NUTRITIVE VALUE OF FORAGE CROPS

3

ensure that a high proportion of the forage grown is eaten by efficient
animals, there may be little advantage in concentrating effort on growing
more forage.
Efficiency of feed conversion (c) depends on many factors, including
the structure of the animal population (the proportion of adult breeding
animals to productive offspring; Spedding, 1965) and the genetic potential
of the animals. But a dominant factor is the level of nutrient intake of the
animals being fed: the higher the level of nutrient intake, the higher the
level of productivity of the animals, and the lower the nutrient requirement for each unit of animal output. Thus, as the daily nutrient intake of
the 300-kg. steer increases from 15.3 to 20.3 Mcal. of metabolizable
energy, its daily rate of liveweight gain increases from 0.5 to 1.25 kg. per
day: the corresponding requirement of metabolizable energy per kilogram gain decreases markedly from 30.6 to 16.2 Mcal. (Raymond,
1968).
II. The Components of Nutritive Value

Thus the nutritive value of a forage should be considered not as a single
parameter, but as composed of a complex of parameters that determine
the nutrient intake of ruminant animals fed on that forage. In this it
differs from the classical concept of nutritive value as a feed concentration (starch equivalent, total digestible nutrients, or net energy) by including feed intake as an integral component of nutritive value. With the
major economic feeds of earlier feeding systems (cereals and pulses, oilseed residues, and industrial by-products) this was not necessary, as the
quantity eaten was controlled by rationing; with forages, on the other
hand, there is seldom any formal control of the amount eaten, which therefore depends on factors in the forage and in its method of presentation.
This review therefore considers the nutritive value of forages in terms
of the factors that determine the level of nutrient intake by ruminant
livestock. It has proved useful to treat nutrient intake as the product of

three parameters (Raymond, 1969b):
Nutrient intake

= intake
X

of feed x digestibility of feed
efficiency of utilization of digested feed

(1)

each of which can be investigated separately, before their interactions in
practical systems of ruminant feeding are considered. The importance of
this approach is indicated by the conclusion of lngalls er al. ( 1 965) that
70 percent of the variation in production potential between forages can
be accounted for in terms of differences in voluntary intake, compared
with 30 percent by differences in digestibility, the nutrient concentration


4

W. F. RAYMOND

measure of earlier systems. Equation (1) is also possibly more informative than the analogous nutritive value index (Crampton et al., 1960),
which combines intake and digestibility in one parameter, so that the
relative importance of these two factors is not immediately evident.
However, nutritive value and nutrient intake can have no real meaning
except in relation to the needs of ruminant animals. Different animals
have different nutritional requirements, depending on their species, sex,
physiological status, and level of production, and this means that the

nutritional adequacy of a forage diet can be assessed only in terms of the
nutritional needs of the particular animals to be fed. The requirements
by different classes of stock for energy, protein, minerals, and vitamins
have been tabulated (Morrison, 1957; Agricultural Research Council,
1965; National Academy of Sciences, 1966). The objective must then be
to establish relevant parameters to describe the nutritive value of forages
which can be equated with these nutrient needs. The components in
Eq. (1) provide a framework within which to assess our current knowledge of these nutritional parameters. Of these components, the digestibility of forage is considered first, because of the important influence
which digestibility exerts on the other two components, intake and efficiency of utilization. These components are discussed in relation to fresh
forages, but particular emphasis is then given to the effects of processing
methods, feed interactions, and methods of feeding, all of which can
markedly alter the basic nutritional features of forages. The practical
aim must be to exploit this new information so as to improve the nutritional potential of forage feeding systems, and the effectiveness with
which soil-forage-ruminant systems can compete for the world’s increasingly scarce land resources.
Ill. The Digestibility of Forage Crops

A. THEMEASUREMENT
OF DIGESTIBILITY in

ViVO

The digestibility of a feed is defined:
Digestibility = ‘Ieed

- cfses

X

100

cfeed

Where Cfed is the amount of feed or feed component eaten (organic
matter, cellulose, protein), and Cfecesis the corresponding amount of
fecal excretion. The measurement of digestibility requires a preliminary
feeding period during which the experimental animals adapt to the feed
under test, followed by a test period, during which feed eaten and fecal
output are measured. For precise measurement preliminary and test


THE NUTRITIVE VALUE OF FORAGE CROPS

5

periods of at least 10 days are recommended (Raymond et al., 1953); this
presents the particular difficulty with fresh forages that the forage must be
cut daily, and so may change in digestibility and chemical composition
during this experimental period.
In many studies this day-to-day variation in feed characteristics has
been overcome by cutting at one time sufficient fresh forage for the complete digestibility experiment, and preserving this forage so that it can
be fed over an extended period. Storage as hay (J. R. Jones and Hogue,
1963) or after artificial drying (Kivimae, 1959) has been used but, because of the changes in digestibility possible with these methods, cold
storage of forages has been adopted by some workers (Raymond et al.,
1953; Pigden et al., 196 1 ; Minson, 1966).
The technique of storage at 5°F. has been described in detail (Commonwealth Agricultural Bureaux, 1961, pp. 88 and 150); it has been
shown to have a negligible effect on the digestibility of the dry matter or
organic matter in forage (Raymond et al., 1953) or of the rate of digestion within the rumen (Pigden et al., 1961), but slightly reduces the
digestibility of the crude protein fraction (Raymond et al., 1953; Minson, 1966).
An alternative technique, the continuous digestion trial with fresh
forage, is now being increasingly widely used (Greenhalgh er al., 1960;

Commonwealth Agricultural Bureaux, 196 1 ; Ademosum et al., 1968).
Herbage is cut and fed daily over an extended period, and the amounts
of forage eaten and feces voided are measured daily throughout the experiment. The amounts of forage eaten and feces are summed over 5day subperiods, allowing a 2-day lag for passage of the feces, and digestibility coefficients are calculated on these subperiods, each of which
serves as the preliminary (adaption) treatment for the succeeding subperiod. This technique has proved of particular use in association with
grazing experiments (see Section X,A,2), but it is less accurate than the
cold-storage technique because of the shorter balance periods used.
The measurement of the digestibility of forages conserved by natural
or artificial dehydration presents no such problem, and most of the reported data on forage digestibility relate to such feeds. Silage is generally removed daily from silos for feeding, but cold storage of silage
(Harris and Raymond, 1963) requires less labor, and eliminates any risk
of day-to-day variation in silage quality.
The many thousands of recorded determinations of forage digestibility
have been collated at intervals and provide the broad background to our
present understanding of forage nutritive value (Schneider, 1952; Leitch,
1969; tropical forages, Butterworth, 1967). However, such compila-


6

W. F. RAYMOND

tions may be of limited value in indicating the digestibility of an “unknown” forage, because of the difficulty of identifying it with a particular
feed class. This problem, long recognized, led to the development of
techniques such as the Weende feed analysis for estimating nutritive
values; a major advance in the period under review has been in the development of improved laboratory techniques for predicting the nutritive value of forages, to replace wherever possible the laborious and
expensive in vivo determination.

B. THE PREDICTION
OF FORAGE
DIGESTIBILITY
FROM CHEMICAL

COMPOSITION
As Van Soest (1968) has noted, animal nutrition has had a history of
inertia and complacency, each further experiment carried out with old
techniques and old terminologies making it yet more difficult to adopt
new ones. But it is still difficult to create a logical pattern from the torrent of new analytical techniques and new parameters of nutritive value
that have recently been put forward to replace these older concepts.
The requirement is to establish a relationship between a nutritional
parameter (e.g., digestibility) of forages, measured in controlled in vivo
experiments, and the chemical composition of the same forages, from
which the nutritive value of other forages can be predicted. Digestion of
forage by the ruminant is a most complex process; yet for nearly a century
the attempt was made to predict the extent of forage digestion in terms of
its proximate analysis based on Weende crude fiber, crude protein and
nitrogen-free extractives. Sullivan (1 962) and Dijkstra ( 1 966) have both
shown that when these analyses are applied to a limited range of forages
close relationships between digestibility and chemical composition can
be established, but that these relationships become less precise as the
range of forages included is increased. As a forage crop matures its fiber
content increases and it becomes less digestible; a close negative relationship between fiber content and digestibility is found. But this relationship is likely to differ from that with a different forage species (in
particular, tropical forages; Butterworth, 1963) or from that with the
same forage species at a different time of year; in each case the forage
becomes less digestible as it becomes more fibrous, but at a given fiber
content different forages can have markedly different levels of digestibility. To some extent this can be overcome by using tabulations of relationships, each based on a limited feed class (Dijkstra, 1966). But again
these pose the problem of allocation to a particular feed class; more
seriously, they add little to our basic understanding of the factors that
determine forage digestibility.


T H E NUTRITIVE VALUE OF FORAGE CROPS

7

The inadequacy of crude fiber as a determinant of nutritive value was
clearly established by Norman ( 1 935). Tentative alternatives to crude
fiber were proposed: cellulose (Crampton and Maynard, 1938), holocellulose (Ely and Moore, 1955), modified acid-detergent fiber (Clancy and
Wilson, 1966). Each of these aimed to analyze a more precise chemical
grouping than crude fiber, but each perhaps reflected the same basic
thinking, that the complex process of forage digestion can be quantified
by a single chemical analysis. The relationships between these “fiber”
components and forage digestibility (reviewed by Miller, 196 1 ; Sullivan,
1962), are often more precise than those based on crude fiber; they are
still inadequate for predictive purposes.
This conclusion, which had become evident by 1960, stimulated the
two main developments discussed below: the study of chemical techniques more relevant to the digestion process, and of biological techniques that attempt to simulate the process of rumen digestion by a
laboratory technique.
C. IMPROVEDCHEMICAL
TECHNIQUES
Forage digestibility, Eq. (2), is the summation C% content X % digestibility of all the different chemical components in the forage. Some of
these components, such as soluble carbohydrates and organic acids, are
completely digested as the forage passes through the ruminant tract;
others, in particular the polysaccharides and lignin, are much less completely digested and comprise most of the feed residue excreted as feces.
The “classical” approach, discussed above, assumes that the extent to
which the fiber fraction is digested is directly related to the proportion
of that fraction in the forage. Detailed studies of the digestibility of different fiber fractions, based on in vivo experiments, have clearly shown
that this is not so. Thus Jarrige and Minson (1964) found that there was
no decrease in the digestibility of the cellulose in S.24 ryegrass as the
cellulose content increased from 14.1 to 19.0 percent of the dry matter in
early spring, while Gaillard (1962) and others showed that the cellulose
in alfalfa is much less digestible than that in grasses with the same content of cellulose.
This led to the development of techniques of graded extraction with

reagents of increasing concentration (Gaillard, 1958; Jarrige, 196 1 ;
Burdick and Sullivan, 1963) and of cellulose solubility in cupriethylenediamine (Dehority and Johnson, 1963) which take some account of the
chain length and resistance to digestion of the different polysaccharide
fractions. However, no single procedure is likely to give results relevant
to the wide range of polysaccharides and lignin that comprise the fiber


8

W. F. RAYMOND

fraction in forages, and Gaillard (1966) has developed a more comprehensive relationship between forage digestibility and composition:
Digestibility of organic matter % = 0.37(C-19.19) - 5.51(L-5.58) - 0.51(H-18.10)
(3)
+ 4. I I(U-3.80) 65. I

+

which includes the percent contents of cellulose (C), lignin (L), hemicellulose (H), and anhydrouronic acid (U). More recently Gaillard and
Nijkamp (1968) have proposed a less complex analytical system, which
replaces the separate determinations of cellulose and hemicellulose with
neutral-detergent fiber (N DF, v.i.):
Digestibility of organic matter % = 66.7 - 4.64(L-5.19)- 0.14(NDF-48.05)
+ 2.95(U-3.47)

(4)

An alternative approach, developed by Van Soest (1 967) and Terry
and Tilley ( 1964a), emphasizes the contribution to total forage digestibility of the highly digestible cell-contents fraction in forages. These
workers have considered forage to contain two main fractions, the cell

contents which are almost completely digested, and the cell-wall constituents, which are only partly digested, and they have proposed analytical
systems that (a) separate these two fractions and (b) indicate the extent
to which the cell-wall fraction would be digested in the ruminant tract.
In a series of papers (summarized by Van Soest, 1967) this author has
described methods for separating a forage sample into a cell-contents
fraction soluble in neutral detergent (S), and an insoluble cell-wall fraction (neutral-detergent fiber, NDF), as well as a fiber fraction insoluble
in acid detergent (acid-detergent fiber, ADF) and lignin (L). In a key
paper (Van Soest and Moore, 1966), the digestibility of the N D F fraction was shown to be negatively correlated with log X (r = -0.98**)
where X , the concentration of lignin in the A D F fraction, effectively
measures the extent of lignification of the cellulose in the forage (in that
paper X was denoted as L, which was confused with percent lignin).
The mechanism by which lignin reduces fiber digestibility probably
includes the effects of physical incrustation, of lignin-carbohydrate
complexes, and of molecular bonds. Van Soest (1967) also showed that
the cell-content fraction in forages is almost completely digested (98
percent) by the ruminant. However, a significant amount of material
soluble in neutral detergent occurs in ruminant feces. This is not undigested plant cell contents, but consists of endogenous materials
(mucus, salts, bile residues, and undigested bacteria) resulting from the
digestion process; digestibility as measured by Eq. (2) is not the “true”
digestibility of the forage material, but the “apparent” digestibility, (feed
- feces) measuring the amount of feed digested, less this inevitable


THE NUTRITIVE VALUE O F FORAGE CROPS

9

endogenous loss associated with the passage of the feed through the
tract. Based on in vivo results with a limited range of feeds, Van Soest
(1967) calculated this fecal loss to be 12.9 percent of the dry weight of

forage eaten.
Van Soest (1967) was then able to compute the apparent digestibility
of forage:
Apparent digestibility of dry matter % = 0.98s

+ W ( 1 . 4 7 3 - 0.789 log X ) - 12.9

(5a)

comprising the almost completely digested cell-contents (S),plus the cellwall constituents (W=NDF) digested to an extent depending on the degree of lignification of the A D F fraction ( X ) , and less the endogenous
excretion.
It has not yet been possible to test this relationship on a wider range
of forages than those studied by Van Soest. But by taking account of the
differing contents and digestibilities of the two main fractions in herbage,
the cell contents and the cell-wall material, Eq. (5a) clearly represents
an important advance over the more empirical methods summarized by
Miller ( 1 96 1) and Sullivan (1 962).
In the course of the development of the detergent-fiber methods,
Van Soest ( 1 965b) examined the effect of the method of drying herbage
samples before analysis on the measured levels of acid-detergent fiber
and lignin. Drying temperatures above 50”C., particularly over an extended period, significantly increased the levels of both these fractions;
this artifact “fiber” was shown to result from a nonenzymatic browning
reaction, in which protein polymerizes with products of carbohydrate
breakdown, so that the “lignin” fraction in particular contains an abnormally high percentage of nitrogen. In earlier work this had been
corrected by subtracting %N X 6.25 from the apparent lignin analysis.
However, Van Soest recognized that natural plant lignins may contain
some nitrogen, and derived a relationship that would correct only for
the nitrogenous matter which might be attributed to heat damage:
% corrected lignin (L,)

=

1.208 X % measured lignin (LA)- 10.75
x %N in A D F + 0.42

(6)

The acid-detergent fiber (ADF) content is then corrected:
% A D F corrected = % A D F observed - (LA- L,)

(7)

From Eqs. (6) and (7) the factor log X in Eq. (5a), based on corrected
values for A D F and lignin, can be calculated.
The need for this correction must reduce the utility (and precision) of
Eq. (5a) and emphasizes the importance of adequate drying methods for
preparing herbage samples for analysis. The method of choice must surely


10

W. F. RAYMOND

be freeze-drying (lyophilization). But the great majority of freeze-driers
in current laboratory use are of small capacity (< 1 kg. water/24 hours),
and this can introduce a source of error which is seldom recognized- that
the sample of forage which is dried by this ideal method may be so small
as to be quite unrepresentative of the material sampled. This possible
contradiction between the precision of the drying method and the accuracy of sampling has been discussed (Commonwealth Agricultural
Bureaux, 1961, p. 135); until much larger freeze-driers become available, the solution in many cases may be to dry forage samples of adequate

size as rapidly as possible at 100"C.,so as to minimize the time during
which nonenzymatic browning (which occurs only in the presence of
water) can take place. The individual investigator can then test the success of his own drying method by the application of Eq. (6) to analyses
on representative samples.
Recently Van Soest and Jones (1969) suggested a further refinement
to the concept summarized in Eq. (5a), by indicating that the silica
present in plant material may exert much the same effect as lignin in reducing the digestibility of the neutral-detergent cell-wall fraction. L. H. P.
Jones and Handreck (1967) discussed the forms and reactions of silica
in the food chain from soil to plant to animal. They pointed out that
silica absorbed by plant roots is carried in solution to the actively metabolizing tissues. As the transporting water is transpired, solid silica is
deposited on to the cell walls so that as these develop the polysaccharides
are intimately associated with encrusting silica as well as lignin. From
examination of the digestibility in vitro of forage samples of silica content
ranging from 0.5 percent to 5.4 percent, Van Soest and Jones (1969) proposed a modified form of Eq. (5a):
Apparent digestibility of dry matter % = 0.98s + W(1.473 - 0.789 log X)
- 3.O(SiO2)- 12.9

(5b)

As yet the evidence is restricted to relatively few forages, but further
study may clearly indicate the need for refinement of the biological concepts implicit in Eqs. (5a) and (5b).

D. ESTIMATION
OF FORAGE
DIGESTIBILITY
BY in Vitro TECHNIQUES
The inclusion of silica as a further component which may influence
forage digestibility illustrates the trend toward multicomponent chemical techniques for predicting digestibility. In effect, this accepts that no
single component can quantify the complex process of ruminant digestion,
and that this must be treated as a series of stages, each described by a

logical chemical evaluation, as in the decreasing digestibility of the N D F
fraction as the fiber becomes more lignified.


T H E NUTRITIVE VALUE OF FORAGE CROPS

11

The inclusion of silica also illustrates a basic problem with chemical
methods of evaluation, that a relationship such as Eq. (5a), which is
found to be adequate with one population of forages, may give inaccurate prediction of the digestibility of other forages-in this case, of
forages of unusually high silica content. This could arise from two causes:
(a) the original relationship did not include all the components that exert
a significant effect on forage digestibility and (b) chemical methods measure the content of different components in forage samples; they do not
measure the physical distribution and organization of these different components within the plant, which must to some extent determine how far
the plant fibers are digested by the microorganisms within the rumen. The
chemical approach tends to treat a forage as a homogeneous material, an
increase in lignin content, for instance, being visualized as an increase
in lignification throughout the whole plant. In practice the forage plant is
more realistically considered as made up of morphologically “distinct”
fractions, each of which can be changing in chemical composition and
digestibility in a way not necessarily related to the other fractions, so
that chemical analysis (an average of the whole plant material) may well
not describe the summation of the individual plant fractions that make up
the digestibility of the whole plant.
Thus, parallel to the development of chemical methods of forage evaluation, described in the previous section, has been the development of
biological methods of evaluation, the artificial rumen or in vitro digestion
methods. Essentially these have attempted to simulate the process of
ruminant digestion by methods that can take account both of the overall
chemical composition of the forage plant and of the distribution and

physical interrelations of the chemical components within the different
morphological parts of the plant.
With the recognition that the digestibility of the “fiber” fraction of
forages would be most affected by these physical characteristics the initial
investigations of biological methods were concerned with fiber digestibility, and in particular with the digestibility of the cellulose fraction in
forages. Although details of technique differed, all these methods were
based on the incubation, under controlled conditions, of a sample of the
test forage with a mixed culture of the microflora taken from the rumen
of a forage-fed animal; the aim was to standardize the conditions of incubation so that the fiber in the forage sample was digested to the same extent as in the same forage when fed in an in vivo experiment (Quicke et al.,
1959; Lefevre and Kamstra, 1960; Karn et al., 1967). These techniques
were also used to measure the extent to which the dry matter (Clark and
Mott, 1960), organic matter (R. L. Reid el al., 19601, or energy content
(R. L. Reid et al., 1960; Baumgardt el al., 1962; Naga and El-Shazly,


12

W. F. RAYMOND

1963) in forage was digested in vitro. In most cases the extent of digestion
in vitro was found to be less than that in vivo, and regression equations
were developed to allow prediction of in vivo values.
In an alternative system, a sample of dried forage is enclosed in a nylon
or dacron mesh bag suspended within the rumen in vivo, and digestibility
and rate of digestion are measured by the loss of dry matter or of cellulose from the sample (Lusk et al., 1962; Hopson et al., 1963). This technique could have the advantage that a normal microfloral population will
be maintained, although this will tend to be that characteristic of the
feed eaten by the host animal, rather than of the sample under test. However the technique does permit the comparison of large numbers of feed
samples under standard conditions, and it could be of use in ranking
forage samples in a breeding selection program.
This approach was analogous to that with the earlier chemical methods,

in attempting to predict the complex process of forage digestion by a
single procedure. As with the chemical methods, the accuracy of prediction was found to decrease as the range of forages examined was widened;
in particular marked divergences were found between results for grasses
and legumes (Shelton and Reid, 1960). Tilley and Terry (1963) suggested
that these discrepancies might be the result of correlating data from a
single digestion with rumen organisms with those from digestion within
the animal, which involves a mainly bacterial digestion within the rumen
followed by a mainly enzymatic digestion in the remainder of the digestive
tract. Within the rumen, the “digestible” polysaccharides, carbohydrates,
and protein in the feed are broken down by the action of the microorganisms there; some of the products of digestion are absorbed directly
through the lumen wall, but a considerable part serves as the substrate
for microbial growth, and is resynthesized into protein, polysaccharides,
and lipids within the proliferating bacterial and protozoal population.
These microorganisms, entrained in the residues of undigested fiber and
other feed components, then pass from the rumen to the abomasum and
duodenum. In these organs the digesta are acidified and further digested
by secreted enzymes that hydrolyze much of the bacterial and residual
plant proteins to amino acids. These are then absorbed as the main
source of amino acids for the metabolism of the host animal.
The undigested residue from the in vitro digestion of forage material
with rumen microorganisms is thus seen to contain, in addition to undigested feed, the rumen organisms which, in vivo, would be enzymatically
digested in the ruminant hind tract. Tilley and Terry ( 1 963) proposed that
this second stage should be simulated by subjecting the residue from the
in vitro bacterial digestion to a second enzymatic digestion. They ex-