Advances in agronomy volume 21

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

ADVANCES IN

AGRONOMY

VOLUME 21

CONTRIBUTORS TO THIS VOLUME

ADVANCES IN

AGRONOMY

Prepared under the Auspices of the

AMERICAN SOCIETY O F AGRONOMY

COPYRIGHT ‘C 1969, BY ACADEMIC PRESS, INC

ALL RIGHTS RESERVED

N O PART OF T H I S BOOK MAY BE REPRODUCED IN ANY FORM, BY PHOTOSTAT, MICROFILM, RETRIEVAL SYSTEM, O R ANY O T H E R MEANS, W I T H O U T W R I T T E N PERMISSION FROM T H E PUBLISHERS

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LIBRARY OF C O N G R E S S C A T A L O G C A R D NUMBER 50-5598

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 Depart- ment 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 De-

partment 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

ARNOLD C 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

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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, inter- disciplinary 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

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CONTENTS

PREFACE vii THE NUTRITIVE VALUE OF FORAGE CROPS W F RAYMOND I I 1 111 IV V VI VII VIII IX X Introduction

The Components of Nutritive Value

The Digestibility of Forage Crops

The Digestibility of Different Forage Species

The Voluntary Intake of Forages

The Efficiency of Utilization of Digested Nutrients

The Relationship between Forage Quality and Forage Yield

Forage Breeding for Improved Nutritive Value

The Effects of Processing on the Components of Forage Nutritive Value

The Nutritive Value of Grazed Forage

References

POTENTIALLY ARABLE SOILS OF THE WORLD A N D CRITICAL MEASURES FOR THEIR USE CHARLES E KELLOGG A 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

GENETIC MODIFICATION OF PROTEIN QUALITY I N PLANTS OLIVER E NELSON I lntroduction

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

ix

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4

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CONTENTS

IV The Storage Proteins of Seeds 178

V Theopaque-2 andJloury-2 Mutations in Maize I80 VI The Prospects of Improvements in Other Plants I87 VI1 Summary I90 References 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 195

I l l Studies on Soil Polysaccharides . I99 IV Methods for the Analysis of Complex Polysaccharide Materials 222

V Summary and Conclusions 229

References 230

11 The Significance of Soil Polysaccharides 196

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 WILLEY A N D S B HEATH I Introduction 28 I 11 Relationships between Plant Density and Crop Yield 283

CONTENTS xi

I l l The Relationship between Plant Rectangularity and Crop Yield 3 14

AUTHOR I N D E X 323

SUBJECT INDEX 338

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THE NUTRITIVE VALUE OF FORAGE CROPS

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X

W F Raymond

The Grassland Research Institute, Hurley, England

Introduction

The Components of Nutritive Value

The Digestibility of Forage Crops

A The Measurement of Digestibility in

B The Prediction of Forage Digestibility from Chemical Composition

C D Estimation of Forage Digestibility by in Vitro Techniques

E The Relative Utility of Chemical and in Virro Estimations of Forage Digestibility _ _ _ _ _ _ _ _ _ _ , _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ , _ _. _ ._

The Digestibility of Different Forage Species _ _

A Basic Patterns of Digestibility

C The Effect of Environmental and Other Factors on Forage Digestibility , , , _ _ _ _ _ _ _ _ _ _ _ , ,

A The Factors Controlling Feed Intake

B Intrinsic Factors Determining Forage Intake _ ._ C The Nutritive Value Index

D The Crude Protein Content of Forage and Voluntary Intake

E The Effect of Supplementary Feeds on Forage Intake

A Methods of Expressing Energy Values , ,

B The Role of Volatile Fatty Acids in Ruminant D The Use of Nonprotein Nitrogen in Ruminant E The Mineral Nutrients in Forages

The Relationship between Forage Quality and Forage Yield

The Effects of Processing on the Components of Forage Nutritive Value , _ _, , _ , , _ _ _ ,

B

Improved Chemical Techniques

B The Digestibility of Different Plant Fractions _ _ _ The Voluntary Intake of Forages

The Efficiency of Utilization of Digested Nutrients C The Utilization of the Crude Protein in Forages

F Pharmacologically Active Components in Forage

Forage Breeding for Improved Nutritive Value

The Grinding and Pelleting of Dehydrated Forages

C The Ensiling of Forage Crops

The Nutritive Value of Grazed Forage

A Measurement of the Nutrient Intake by Grazing Animals

P a g e

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2 W F RAYMOND

B The Botanical Composition of Grazed Forage 90

C The Nutrient Intake of Grazing Livestock 91

D The Effect of Management on the Productivity of

Grazing Animals 94 References 97

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 pro- duce 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 con- fined 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 com- petition 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 agri- culture 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 popu- lations 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 O F 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 require- ment 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 kilo-

gram 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 concentra-

tion (starch equivalent, total digestible nutrients, or net energy) by in-

cluding feed intake as an integral component of nutritive value With the major economic feeds of earlier feeding systems (cereals and pulses, oil- seed 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 there- fore 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):

(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

Nutrient intake = intake of feed x digestibility of feed

X efficiency of utilization of digested feed

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 knowl- edge of these nutritional parameters Of these components, the digesti- bility of forage is considered first, because of the important influence which digestibility exerts on the other two components, intake and effi- ciency 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 nutri- tional potential of forage feeding systems, and the effectiveness with which soil-forage-ruminant systems can compete for the world’s in- creasingly scarce land resources

Ill The Digestibility of Forage Crops

A THE MEASUREMENT OF DIGESTIBILITY in V i V O

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 Cfeces is 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 com- plete 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, be-

cause 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 (Com- monwealth 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 diges-

tion within the rumen (Pigden et al., 1961), but slightly reduces the digestibility of the crude protein fraction (Raymond et al., 1953; Min-

son, 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 ex- periment The amounts of forage eaten and feces are summed over 5-

day subperiods, allowing a 2-day lag for passage of the feces, and digesti- bility coefficients are calculated on these subperiods, each of which serves as the preliminary (adaption) treatment for the succeeding sub- period 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 re- ported data on forage digestibility relate to such feeds Silage is gen- erally 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 “un- known” 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 de- velopment of improved laboratory techniques for predicting the nutri- tive value of forages, to replace wherever possible the laborious and expensive in vivo determination

B THE PREDICTION OF FORAGE DIGESTIBILITY

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 tor- rent 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 rela- tionship between fiber content and digestibility is found But this rela- tionship 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 digesti- bility To some extent this can be overcome by using tabulations of re-

lationships, 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 V A L U E 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), holocel- lulose (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 tech- niques more relevant to the digestion process, and of biological tech- niques that attempt to simulate the process of rumen digestion by a laboratory technique

Forage digestibility, Eq (2), is the summation C% content X % digesti- bility 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 com- pletely 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 dif- ferent 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 con- tent 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 cupriethylene- diamine (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 compre- hensive relationship between forage digestibility and composition:

(3) Digestibility of organic matter % = 0.37(C-19.19) - 5.51(L-5.58) - 0.51(H-18.10)

which includes the percent contents of cellulose (C), lignin (L), hemi- cellulose (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.):

+ 4 I I(U-3.80) + 65 I

(4)

An alternative approach, developed by Van Soest (1 967) and Terry and Tilley ( 1964a), emphasizes the contribution to total forage digesti- bility 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 constit- uents, 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 frac-

tion (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 frac- tion 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 un-

digested 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

Digestibility of organic matter % = 66.7 - 4.64(L-5.19) - 0.14(NDF-48.05)

+ 2.95(U-3.47)

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 cell- wall constituents (W=NDF) digested to an extent depending on the de- gree 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 ex- tended 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 ab- normally 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

(6)

x %N in A D F + 0.42

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 accu- racy of sampling has been discussed (Commonwealth Agricultural

Bureaux, 1961, p 135); until much larger freeze-driers become avail-

able, 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 suc-

cess 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 re- ducing 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 metab-

olizing 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) pro-

posed a modified form of Eq (5a):

(5b)

Apparent digestibility of dry matter % = 0.98s + W(1.473 - 0.789 log X)

As yet the evidence is restricted to relatively few forages, but further

study may clearly indicate the need for refinement of the biological con- cepts implicit in Eqs (5a) and (5b)

- 3.O(SiO2) - 12.9

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 chem- ical 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 V A L U E O F FORAGE CROPS 1 1

T h e inclusion of silica also illustrates a basic problem with chemical methods of evaluation, that a relationship such a s Eq (5a), which is found to be adequate with one population of forages, may give inaccu- rate 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 meas- ure the content of different components in forage samples; they do not measure the physical distribution and organization of these different com- ponents 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 a s a homogeneous material, an increase in lignin content, for instance, being visualized a s an increase

in lignification throughout the whole plant In practice the forage plant is more realistically considered a s 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 eval- uation, 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 digesti- bility, 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 incu- bation so that the fiber in the forage sample was digested to the same ex-

tent a s 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 cellu- lose from the sample (Lusk et al., 1962; Hopson et al., 1963) This tech- nique 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 How- ever 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 predic-

tion 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 micro- organisms 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 un-

digested 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-

THE NUTRITIVE VALUE O F FORAGE CROPS 13

amined several enzymes and concluded that a two-stage procedure com- prising digestion by rumen microorganisms followed by digestion by acid-pepsin gave the closest agreement with in vivo digestibility values for the dry matter and organic contents in forages This method showed a

correlation of 0.97 between in vitro and in vivo values when tested on a wide range of forages, including grasses fertilized with different levels of nitrogen, and legumes:

(8)

Similar high degrees of correlation have been found by O’Shea and Wil-

son ( I 965; r = 0.94), Wedin el al ( 1966; r = 0.996) and Ademosum et af

( 1 968; r = 0.96); Dent ( 1 963) found close agreement between two-stage

in vitro and in vivo digestibility results with brassicas and forage maize

In a number of studies the precision of prediction of digestibijity in vivo by this in vitro technique has been compared with the chemical methods already discussed Armstrong et al (1964a) found that the

metabolizable energy and net energy contents of a series of dried grass feeds were more accurately correlated with organic matter digestibility

in vitro than with cellulose or lignin contents; Bosman (1967) and

Ademosum ef al (1968) have reported the in vitro method to correlate more closely with in vivo digestibility than the chemical methods tested;

Engels and Van der Merwe (1967) have found the same result with low-

digestibility hays in South Africa However, to date no direct comparison has been reported with the improved chemical technique proposed by Van Soest ( 1 967, Eq 5 ) , and it is possible that these two techniques

differ little in the precision with which they allow prediction of forage digestibility in the laboratory

In fact the stage may be approaching at which little further improve- ment in precision can be expected In interpreting the error terms of these

relationships, it is important to recognize that this error does not arise

solely from deficiencies in the laboratory technique used (chemical or

in vitro), but that errors are also associated with the actual measurement

of the in vivo digestibility of the forages, and with the fact that digestibility

in vivo is not a constant parameter of a particular forage Thus digesti- bility determined in an animal experiment may depend on the amount of forage fed, digestibility decreasing a s the level of feeding increases (Moe

et al., 1963, and can be significantly reduced if the animal is parasitized with stomach worms-probably the rule rather than the exception with

sheep (Spedding, 1954; Shumard et al., 1957) T h e standard deviation of

an estimate of digestibility is between 1.0 and 1.3% (Raymond et al.,

Digestibility in vivo = 0.99 X digestibility in vitro - 1 .O I

(S.E = f 2.3 I )

14 W F RAYMOND

1953); as most digestibility determinations are made with only 2 or 3 sheep it is evident, even where the factors noted above are standardized, that much of the error in the relationships noted must result from errors

in the in vivo determination, rather than in the concept or precision of the laboratory determination

Probably the main area for improvement lies then in the better stand-

ardization of the in vivo digestibility experiments, of the preparation of

the forage samples for analysis, and of the conduct of the laboratory procedures

R L Reid et al (1 964) and Noller et al ( 1966) have both reported

significantly higher levels of dry-matter digestibility in vitro in forages

prepared by freeze-drying than by oven-drying, presumably because of the production of indigestible artifacts during oven-drying, as suggested

by Van Soest ( I965b) Tilley and Terry (personal communication), how-

ever, found no advantage of freeze drying compared with rapid oven-

drying at 1Oo”C., although the same authors (1963) had reported con- siderable depression of digestibility in vitro in samples dried at tempera-

ing forage samples in a ball mill before in vitro digestion However, within

the range of fineness of grinding found in forage samples ground by ham- mermill, Tilley and Terry (1 963) found no significant effect of particle size

The most serious problem arises, however, in the lack of standardiza-

tion of in vitro procedures between different laboratories Barnes (1 967)

reported the results of a collaborative study in which the in vitro digesti-

bilities of the dry matter and cellulose in three forages were measured

at 17 laboratories The mean values for cellulose digestibility after 24 hours ranged from 40.0 to 63.9 percent, reflecting the use of different techniques in terms of sample size, preparation of the rumen inoculum,

pH control, etc In contrast, Raymond and Terry (1966) have reported

close agreement between in vitro results from two laboratories using

identical procedures, and have stressed the importance of different laboratories using the same “standard” forage samples as an additional check on the reproducibility of the method

Tilley and Terry ( 1 963) found that rumen liquors taken from donor

animals fed on several contrasting forages were of similar digestive

efficiency in the two-stage in vitro system, and Troelsen and Hanel(l966)

THE NUTRITIVE VALUE O F FORAGE CROPS 15

have reported that the potency of liquors taken from different sheep differs less than the in vivo digestive efficiency between sheep I n gen- eral the most important consideration seems to be that the diet of the donor animals should contain at least 10 percent of crude protein (see below), that it should give a sample of rumen contents from which a strained liquor can readily be separated (i.e., the animals should be fed on a coarsely chopped hay rather than on a pelleted ground feed), and that the sample should be kept in an anaerobic condition and be pre- pared for inoculation of the digestion tubes as rapidly as possible

It must be accepted, however, that this in vitro procedure, developed with temperate grass and legume species, may not be directly applicable with other temperate species, or with tropical forage species, and Drew (1 966) has stressed that the system should wherever possible be checked with relevant samples of known in vivo digestibility Thus Raymond and

Terry (1 966) reported low in vitro digestibility levels when both the test forage (0.7 percent N ) and the feed of the donor animal were of low nitrogen content, as can often occur with tropical forage species The higher level of digestibility in vivo resulted from the animal’s ability to recycle urea via salivary and ruminal secretions, whereas digestibility

in vitro was limited by a deficiency of nitrogen in the combined sample

and inoculum Addition of 6 mg of N , as urea, to the in vitro system in- creased sample digestibility to the in vivo level

Engels and Van der Merwe (1967) found that the difference between

in vitro and in vivo digestibility values of veldt grasses became greater

as the nitrogen content of the test forages decreased Addition of 20 mg

of urea N to each digestion tube gave in vitro values in close agreement with those in vivo In a modification of the method of Tilley and Terry ( 1963), Alexander and McGowan ( 1966) have included ammonium sul- fate in the buffer added to each tube However in the author’s opinion this is advisable only where depressed levels of in vitro digestibility re-

sult from low nitrogen contents Engels and Van der Merwe (1967) showed a marked depression in digestibility when 60 mg of urea N was included in the digestion system, and a similar depression might occur when urea is added to an in vitro digestion of a forage sample already of high nitrogen content

Although some modification may be necessary in particular situations,

the study reported by Barnes (1967) does emphasize the importance of

the general adoption of standardized in vitro digestibility procedures, without which results reported by different laboratories cannot be directly comparable The two-stage procedure described by Tilley and Terry

(1963) is now used by many laboratories, and there seems a strong case

16 W F RAYMOND

that this should be adopted as a standard procedure in agronomic studies But such a technique, based on digestion in vitro with a mixed culture of rumen organisms, must always be sensitive to uncontrolled biological variation, and there is a clear need for the future for a technique that can

be more rigidly defined A promising development, reported by Dehority

et al (1968), is the use of a pure culture of cellulolytic bacteria to replace the mixed inoculum taken from the rumen of a donor animal Two of the strains of bacteria tested gave cellulose digestibility values in vitro in close agreement with the measured in vivo values; further development of this work could well lead to significant improvement in the standardiza- tion of laboratory in vitro techniques

E THE RELATIVE UTILITY OF CHEMICAL AND in Vitro

As has been indicated, the two-stage in vitro technique appears to give a better prediction of in vivo forage digestibility than any of the chemical methods yet investigated, and the use of this technique, re- ported below, has contributed greatly to our knowledge of forage digesti- bility However this is an integrative technique, the measured digestibility being the sum of the digestibilities of the many different chemical frac- tions within the forage; without additional chemical information it can only describe, rather than explain, the differences in digestibility observed among different forage samples This suggests that in vitro and chemical techniques should be considered as complementary, rather than competi- tive, methods of forage evaluation, the in vitro techniques being used

to establish that forages differ in digestibility, and the chemical tech- niques to study the probable reasons for these differences Such an under- standing is essential if further improvement of forage digestibility is to

be based on nonempirical concepts

IV The Digestibility of Different Forage Species

A BASIC PATTERNS OF DIGESTIBILITY

The use of these more precise laboratory techniques for estimating the digestibility of forages has led to considerable progress in extending in vivo studies that were started in the 1950’s It had long been recognized

that as a forage becomes more mature it also becomes less digestible; the possibility that this imprecise statement could be quantified was proposed

by Homb (1953), who showed a close relationship between the age

(maturity) of a timothy-clover mixture and its digestibility, and by J T

Reid et al (1959), who suggested that the digestibility of a range of

THE NUTRITIVE VALUE OF FORAGE CROPS 17

forage species harvested during first growth during the spring could be estimated:

% digestibility of dry matter = 85.0 - 0.48X (9)

where X = number of days to harvest from April 30

These reports stimulated further investigations that have greatly in- creased our understanding of forage digestibility Clearly the “date of

harvest” in Eq (9) can only be relevant in the area close to Cornell

(Ithaca, New York), where the forages used in these in vivo digestibility

determinations were harvested, but the basic principle, that digestibility

is closely related to forage maturity, with “date of cutting” being used

to describe stage of maturity in a particular location, was soon confirmed

by other workers (Kane and Moore, 1959, and others; reviewed by Blaser, 1964) However, several divergences from the original concept,

that all forages are of similar digestibility at a given date, became evident

Thus the digestibility of timothy (Phfeum pratense) (Mellin et a f , 1962;

Minson et a f , 1964), of lucerne (Medicago sativa) (Demarquilly, 1966a), and in particular of white clover (Trifofium repens) (Harkess, 1963)

were shown to decline less rapidly with advancing maturity than the 0.5

unit per day indicated in Eq (9) Small differences in the digestibility of a

given forage variety cut on the same date in different years may be due to the delayed onset of active spring growth in a “late” season (Minson

et a f , 1960), or to differences in leaf percentage in the forage grown in different years (R H Brown et a f , 1968) But the most important ob- servation has been that there can be large and consistent differences in digestibility between forage species and forage varieties In a detailed series of in vivo experiments, Minson et a f ( I 960, 1964) found that

certain species (Lofium spp and Festuca pratense) were considerably more digestible than others such as cocksfoot (Dactylis gfomerata) and tall fescue ( F arundinacea); within a species late-maturing varieties

(e.g., S.23 ryegrass) maintained a high level of digestibility to a later date than early-maturing varieties (e.g., S.24) These differences, illustrated

in Fig 1 , were considered large enough to be of agronomic and nutritional significance Figure 1 also shows that the digestibility of each grass

studied did not fall at a uniform rate a s it matured There was an initial period of almost constant digestibility before digestibility began to de-

crease In the case of Lofium and Dactylis this change to a more rapid fall

in digestibility, at a rate very similar to the 0.5 unit per day recorded by

Reid et a f ( 1 959), was closely associated with the first emergence of flowering heads; the digestibility of timothy (S.48), however, decreased

well before this stage, and this species also showed the much slower rate of fall in digestibility noted above

18 W F RAYMOND

In establishing these novel digestibility patterns, Minson et al (1 964) had two main advantages compared with their colleagues in North America-in the more general use of the genus Lofium in the United

I

date of first cuttinq

FIG I The percent digestibility of the organic matter in grass varieties during first growth in the spring S.23 and S.24, ryegrass; S.37 and Germinal, cocksfoot; S.215, meadow fescue; S.48, timothy; S 170, tall fescue; O.M., organic matter 0 indicates date of first ear emergence (Data from Minson et al., 1964.)

Kingdom than in North America; and in the availability of widely dif- fering maturity types, within this genus, developed at the Welsh Plant Breeding Station These results stimulated other in vivo studies, which

have in general confirmed the important conclusions implicit in Fig 1

Thus Castle et a f (1962) and Harkess (1963) have found consistently

higher digestibility of ryegrass than of cocksfoot, and Lowe et af (1962) reported that late-maturing varieties of grasses were more digestible than early-maturing at a given cutting date in the spring These in vivo

studies, initially on first spring growth, have also been extended to re- growths during the rest of the growing season, and consistent patterns have again emerged The digestibility of regrowth cocksfoot and tall fescue is always lower than that of the corresponding regrowth of rye- grass, and the rate of fall of digestibility with time is much less in these later, largely vegetative, regrowths than in the first, reproductive, growth

(Minson et af., 1960, 1964); a similar slower rate of fall in digestibility has been shown with regrowths of lucerne (Demarquilly, 1966a)

THE NUTRITIVE VALUE O F FORAGE CROPS 19 However, the digestibility of regrowth forage is likely to be less pre- dictable than that of first growth The latter comprises largely reproductive tillers which develop from the start of active growth in the spring; re- growths can contain both reproductive and vegetative tillers, the relative proportions of each depending on the management of earlier harvests Thus the regrowth from a tiller whose reproductive growing point (ear)

is harvested will comprise mainly leaf, which will decrease only slowly

in digestibility at ca 0.1 percent per day In contrast, a tiller which is harvested before the ear has reached the height of cutting or grazing will continue to develop, and its digestibility will decrease at the 0.5 percent per day characteristic of first growth Harvesting up to the time of first ear emergence will remove only some of the developing ears, and the di- gestibility of the regrowth will decrease at an intermediate rate, e.g.,

0.3 percent per day A later harvest, which removes most of the potential ears, will give a leafy regrowth which decreases more slowly in digestibility

Under a cutting regime regrowths are mainly leafy, and of relatively

predictable digestibility (Minson et al., 1960, 1964) Under a grazing

situation the prediction of the digestibility of regrowths is less precise The grazing animal generally leaves some of the herbage on offer, and this continues to decrease in digestibility, so that the combined regrowth and remaining herbage is of lower digestibility than the corresponding

regrowth from a cut sward (Tayler and Deriaz, 1963) As noted in Sec- tion IV, C , 2, the digestibility of a regrowth from grazing will also depend considerably on the moisture and nitrogen status of the sward, which determines the proportion of the next harvest that is composed of new growth of high digestibility

Limited information indicates that the digestibility of forage mixtures can be calculated from the proportions and digestibilities of the constitu- ent species at the time of harvest (Harkess, 1963) An exception may be the case where the digestibility of one species is low because it is de- ficient in protein content (as has been shown with some tropical forages; Smith, 1962) Such a species, grown with another species of higher pro-

tein content, could have an enhanced digestibility, so that the digestibility

of the “mixture” would be somewhat higher than predicted

The patterns of forage digestibility discussed above were all based on

in vivo experiments; they have been considerably extended by the use of laboratory in v i m techniques The validity of the two-stage in vitro

technique was indicated by Terry and Tilley (1964a), who showed that the basic patterns of digestibility shown in Fig 1, and in particular the nonlinear fall in digestibility with time and the consistently higher diges-

20 W F RAYMOND

tibility of ryegrass than of cocksfoot, could be closely reproduced by

in vitro measurements With this method many more forage samples

can be examined than would ever be possible by in vivo experiments,

but the need to confirm, in vivo, the more important conclusions indicated

from in vitro studies must be emphasized Thus Dent and Aldrich (1968)

have measured the digestibility, in both first growth and regrowths, of numbers of varieties within several forage species They have shown con- sistent differences in digestibility between species, and between maturity types within species, at different centers and in different years Their

work indicated that different varieties of the same maturity type within a

species might differ in digestibility Thus a small number of varieties of cocksfoot (e.g., Roskilde I1 and Scotia) appeared to be more digestible

than the varieties Germinal and S.37, shown in Fig 1, and Reveille

tetraploid ryegrass was more digestible than S.24 ryegrass These in

vitro results, subsequently confirmed in in vivo experiments (Osbourn, unpublished) have particular relevance to the possibility of breeding more digestible forage varieties, discussed in Section VIII

B THE DIGESTIBILITY OF DIFFERENT PLANT FRACTIONS

The use of in vitro techniques to measure the digestibility of different

plant fractions has also provided a more logical understanding of these patterns of forage digestibility These are in many cases contrary to what would have been predicted in terms of the earlier concepts of forage nutritive value, that leaf was more digestible than stem, and that forage fractions high in protein (N) content would be more digestible than those

of lower N content The results of Minson et al ( 1 960) showed that these

concepts might be incorrect Thus in first growth, S.24 ryegrass forage cut

in 1959 on May 1 , with a nitrogen content of 2.62 percent and 5 3 percent

of leaf lamina, was of exactly the same digestibility as that cut on April

20, which had contained 3.66 percent N and 77 percent leaf, S.37 cocks-

foot cut on May 5 , with 2.78 percent N and 58 percent leaf, was 4.5 units less digestible than the ryegrass cut on May 1 In later experiments Tayler and Rudman (1966) harvested a S.24 ryegrass sward in two horizons, a top fraction cut above 13.5 cm., and a bottom fraction from

6 to 13.5 cm The digestibilities of the organic matter in the two frac- tions, in vivo, were 84.0 and 82.6 percent, respectively

This indirect in vivo evidence has been followed up in detail by in vitro

digestibility determinations on forage samples separated into fractions

of leaf, leaf sheath, stem, inflorescences, and dead material Terry and

Tilley ( 1 964a) analyzed these fractions from the forages fed in vivo by Minson et al ( 1 960, 1964) (Fig 1) They showed that in all species the

THE NUTRITIVE VALUE OF FORAGE CROPS 21

digestibility of the leaf fraction fell only slowly with advancing maturity

(0.13 percent per day) whereas that of the leaf sheath (0.4 percent) and

stem fractions (0.7 percent) fell much more rapidly I n the immature

forages the stem was always more digestible than the other components

On any given date each fraction in S.24 ryegrass was more digestible than the corresponding fraction in S.37 cocksfoot, of equivalent maturity

type A typical set of results is shown in Fig 2 The digestibility of the

whole plant - leaf blade - -

leaf sheath stem -

FIG 2 The digestibility in vifro of the dry matter in the whole plant, and in the leaf blade, leaf sheath, and stem fractions of S.37 cocksfoot during first growth in the spring Figures

in parentheses are the percentage of stem in the whole plant (Data from Terry and Tilley,

I 964a )

whole forage material, calculated from the proportions and digestibilities

of the constituent fractions, changed with maturity just as in the in vivo

experiments-that is, with a slow fall in digestibility up to the time of ear emergence, followed by a more rapid fall as the stem and leaf sheath fractions, by now less digestible than the leaf, comprised an increasing proportion of the total forage The slower and more steady fall in the di-

gestibility of S.48 timothy could be explained by the much higher pro- portion of leaf sheath in this species than in ryegrass or cocksfoot

22 W F RAYMOND

These conclusions have been confirmed in considerable detail by

Pritchard et al (1963), Wedin et al (1966), Walters et al (1967), and

Dent and Aldrich (1968); Mowat et al (1965) found similar results with

timothy and bromegrass, but were unable to show higher digestibility

of cocksfoot stems than of leaves even in immature forage

Similar logical patterns of digestibility have been shown with legume forages (Terry and Tilley, 1964a; Mowat et al., 1965) With lucerne

( M sativa), red clover ( T pratense), and sainfoin (,Onobiychis vicii-

foliu) the older leaves tend to senesce and fall, so that the digestibility

of the leaf fraction decreases very little as the plant matures By sepa- rating the stem fraction into 6 inch subfractions, measured from the top

of the plant, it was shown that digestibility decreased down the stem, but that the digestibility of any given fraction changed relatively little with maturity A similar result has been found with the stems of brassicas and

forage maize (Zea mays), the stem tip of marrow-stem kale being highly

digestible (81.8 percent dry matter) and the stem base of lower digesti- bility (62.3 percent) (Dent, 1963)

Within a particular forage species, Walters et al (1967) and Dent and

Aldrich (1968) have shown, at a similar stage of morphological develop- ment, that “late” varieties tend to be less digestible than “early” varieties because both the stem and leaf fractions are somewhat less digestible, and because they contain a higher proportion of senescent and dead ma- terial Small differences in digestibility of a given variety at ear emergence

in different years can be attributed to differences in leafistem ratios, stem at this stage being less digestible than leaf (R H Brown et al., 1968)

Of particular importance is the observation, already noted, of differ- ences in digestibility between varieties of similar maturity type within a species (Dent and Aldrich, 1968) The higher digestibility of REVEILLE

ryegrass than S.24 was not accounted for in terms of leafstem ratio, but because both the leaf and stem fractions in REVEILLE were more digesti- ble than the same fractions in S.24 Differences in digestibility have also

been shown between the individual plants (genotypes) within a variety (Cooper et al., 1962; Walters et al., 1967; Mowat, 1969), resulting from differences in digestibility of the plant fractions rather than from dif- ferent leaf stem ratios

However, while the in virro techniques used in these studies can

describe the changes in digestibility within and between different forage species, they cannot explain them; for this the newer chemical tech- niques (Section 111, C ) are needed, to analyze digestibility measured

in vitro into its component parts In detailed studies by Tilley, Terry, and Outen (unpublished) the forage sample is separated into a cell con-

THE NUTRITIVE V A L U E O F FORAGE CROPS 23

tents fraction, soluble in acid pepsin, and a cell wall fraction (analogous

to the fraction soluble in neutral detergent and the cell wall residue of

Van Soest, 1967) The digestibility of the cell wall fraction is also meas-

ured by in vitro (rumen organism) digestion Consistent differences have been shown between S.24 ryegrass and S.37 cocksfoot, harvested at the

same stage of maturity: (a) the content of pepsin-soluble material is higher in the ryegrass than in the cocksfoot; (b) as a result there is a higher content of cell wall fraction in the cocksfoot; and (c) this fraction

in the cocksfoot is less digestible than in the ryegrass, so that the content

of “digestible cell wall material” in the two species is very similar As a result the digestibility of the ryegrass (cell contents X 0.98 + digestible cell wall fraction) is higher than that of the cocksfoot

Within different plant fractions, the small decrease in digestibility of the leaf fraction as the plant matures (Fig 2) is accounted for by the con- sistently high level of cell contents and high digestibility of the cell wall fraction in the leaf Young stem material contains an even higher propor- tion of cell contents than the leaf (hence its higher digestibility); but as the stem matures the cell content fraction decreases rapidly and is re- placed by cell wall material which becomes less digestible with increas- ing lignification, so that the digestibility of the stem decreases rapidly with advancing maturity

Extension of these more detailed studies of the components of digesti- bility to the genotypes within a species which have been found to be of higher digestibility (Section VIII) may provide a more objective basis for selection for improved digestibility than the in vitro techniques that have so far been used

c THE EFFECT OF ENVIRONMENTAL A N D OTHER FACTORS

O N FORAGE DIGESTIBILITY

1 Environmental Effects

The difference in digestibility between forages cut on the same date

at Cornell (J T Reid et al., 1959) and in Maryland (Kane and Moore, 1959) appeared to be due to the forages in these two locations being at different stages of physiological maturity It is of course possible that, even at the same stage of maturity, the digestibility of a forage may differ

between locations; thus Aldrich and Dent (1 967) have found an indication

of higher digestibility at a northern than at a southern latitude in the

United Kingdom in cocksfoot cut 10 days after 50 percent ear emergence

Deinum et al (1 968) measured the in vivo digestibility of perennial rye- grass grown under high and low light intensities, and with low and high levels of nitrogen manuring Considerable differences in chemical com-

24 W F RAYMOND

position of the grass were found between treatments, but these had no significant effect on dry matter digestibility at any one sampling These experiments showed lower levels of digestibility on all treatments during the summer when temperatures were higher, confirming earlier results of Deinum, based on chemical analysis of ryegrass grown in controlled en- vironment cabinets Deinum et al ( 1 968) postulated that this effect of

high temperature might partly account for the generally lower level of digestibility of tropical than of temperate forages Hiridoglou et al ( 1966)

have also shown that high summer temperatures were associated with low in vitro digestibility levels Forages growing in summer also tend to contain lower moisture contents than late season forages; however, the only report found on the effect of water intake on forage digestibility

(Thornton and Yates, 1968) has indicated a small increase in the digesti- bility by cattle of the dry matter and fiber in chaffed oat straw-lucerne hay when water intake was restricted

2 Fertilizers and Forage Digestibility

The effects of fertilizer nitrogen on forage digestibility have been studied in numerous experiments; most of these have reported an in- significant effect from the use of widely differing levels of application

(summarized by Blaser, 1964): thus Minson et al ( 1 960) found no effect

on the digestibility of ryegrass or cocksfoot from levels of nitrogen appli-

cation varying from 0 to 175 pounds/acre However, Raymond and Sped- ding (1 965) have indicated several situations in which fertilizer nitrogen

is likely to affect forage digestibility: (a) in a mixed grass-clover sward the use of this fertilizer may reduce the contribution, in the forage harvested, of the more digestible clover complement: (b) uneaten herbage left on a sward after stock have grazed will continue to decrease in digestibility, and by diluting the highly digestible new growth will depress the digestibility of herbage available at the next grazing; fertilizer nitro- gen will increase the proportion of new growth in this harvest, and so may increase its digestibility; (c) unfertilized herbage may contain an inade- quate level of nitrogen for the growth of rumen microorganisms: thus

Smith ( 1 962) found an increased level of digestibility after application of

nitrogen to such forage; 40 pounds of fertilizer nitrogen per acre increased

the crude protein content of late-cut veldt hay from 3.6 to 6.8 percent and

the digestibility of the forage dry matter from 5 1.7 to 59.5 percent

These effects of fertilizer nitrogen are consistent with the concepts

of forage digestibility already discussed McIlroy ( 1 967) has summarized

results showing that fertilizer nitrogen increases the crude protein con- tent and decreases the soluble carbohydrate content of herbage But

THE NUTRITIVE VALUE OF FORAGE CROPS 25

there is little net change in the content of crude protein plus soluble

carbohydrate, which largely comprises the cell contents fraction, or in the composition of the cell wall fraction Thus little change in the digesti- bility of the forage would be expected except when the digestibility of the cell-wall fraction is limited by the low content of protein in the forage

There is also the possibility that certain forages contain specific com-

ponents which reduce bacterial activity within the rumen Hawkins ( 1 959)

suggested that the low digestibility of the protein fraction in Sericea lespedeza and vetches might be due to the formation of insoluble protein complexes with the tannin in these forages, and Smart et al ( 1 96 I ) found

a depression of cellulose activity in vitro by an extract from Lespedeza

cuneata Schillinger and Elliott ( 1 966) observed differences of u p to 15

percent between the digestibilities in vitro of different lucerne plants

Low levels of digestibility could be raised by addition of amino acids

(glycine, aspartic acid, glutamine) to the in vitro system, and these

amino acids also increased the growth rate of voles fed on the lucerne forage These authors attributed these differences to the presence of water-soluble antimetabolites in the low digestibility plants

Such cases are, however, likely to be exceptional, and the digestibility

of most forages appears to be in line with the systems of evaluation pro- posed by Van Soest ( 1967) and Terry and Tilley ( 1964a)

3 The Effect of Feed Supplements on Forage Digestibility

These systems do not, however, predict adequately the digestibility

of forages fed in mixed rations Thus when carbohydrate (starch) sup- plements are fed with forages, there can be a significant decrease in the digestibility of the fiber (cell wall) fraction of the forage, unaccounted

for by any change in the composition of the cell wall (Eq 5 ) This has

been attributed to a preferential digestion of the starch by the rumen microorganisms, so that the extent of digestion of the plant fibers is re- duced, or alternatively that the amylolytic bacteria compete preferentially for ammonia against the cellulolytic bacteria, so reducing cellulose di-

gestion (El-Shazly et al., 1961)

More recent work has indicated an alternative explanation First, it is known that when a starch supplement is fed with a forage there is a re- duction in the pH of the rumen contents compared with that when the

forage is fed alone (Topps et al., 1965) Second, Tilley et al ( 1964) have shown a marked reduction in the rate and extent of digestion of dry

matter and cellulose in vitro when the pH of the in vitro system is reduced

(Table I); a decrease, with decrease in rumen pH, has also recently been

26 W F RAYMOND

indicated in the rate of “digestion” of cotton threads suspended within the rumen in vivo (Wilkins, unpublished) Tilley et al (1964) have there- fore suggested that the lower rumen pH when a starch supplement is fed

TABLE I The Effect of the pH of the in Vitro Digestion on the Extent of Dry Matter and Cellulose Digestibility of Samples of Cocksfoot, S.37, by the 2-Stage in Vitro Method“

Percent of forage dry matter digested

“From Tilley et al ( 1964)

*Figures in parentheses are percentages of digestible cellulose in the digestible dry matter

may provide a less favorable environment for the cellulolytic and other bacteria that are able to digest plant fiber (see also Head, 1961) In anthropomorphic terms, the natural rumen microflora has become adapted to the pH 6.6 to 6.8 characteristic of the rumen contents of the grazing herbivore; any marked divergence from this pH finds a micro- floral population increasingly unable to digest fiber This hypothesis, if substantiated, could lead to the development of feeding regimes, aimed

at optimizing rumen pH (and redox potential) to ensure maximum diges- tion of the cell wall fraction of forages

As already noted, forages may be of low digestibility because they are

of very low protein content (< 4% crude protein) The digestion of these forages can be increased by feeding protein supplements, and there has been much interest in the use of urea for this purpose Thus Campling

ef al (1962) measured an increase in organic matter digestibility from

41 to 50 percent when a urea supplement was fed with oat straw of 3.0 percent crude protein, and other examples have been reported (see M H Briggs, 1967) Within the rumen the urea is rapidly deaminated, the ammonia produced then being used by the celluloytic and other bacteria, whose digestive activity would have been limited by deficiency of pro- tein in the unsupplemented forage

The feeding of urea to increase the digestibility of forage is not often used in practice, because a crude protein level in forage of less than 4

T H E N U T R I T I V E V A L U E O F FORAGE CROPS 27 percent is uncommon The main use of urea is likely to be as a substitute

for protein in productive rations (Section VI, D)

V The Voluntary Intake of Forages

A THE FACTORS CONTROLLING FEED INTAKE

It was noted in the Introduction that the quantity of forage that rumi- nant animals eat is in practice seldom controlled in the same way that most other farm feeds are rationed Yet the amount of forage that animals eat is in many cases the major factor determining their level of nutrient intake and their output of useful products Increasing emphasis has been given in the last decade to the study of voluntary intake

Earlier studies of forage intake were undoubtedly hindered by the con-

fusion of “intake” with “palatability” (Blaxter et al., I96 1 ; Campling,

1964), but it is now accepted that forage intake is mainly controlled by

largely involuntary physiological reflexes within the animal, rather than

by its subjective liking for different feeds The development of some of the current concepts on voluntary intake have been reviewed by Balch and Campling (1962), Conrad ( 1 966), and L D Brown (1966) From these a broad distinction appears between the factors determining in- take by ruminants and by nonruminants Intake by nonruminants is con- trolled mainly by levels of blood metabolites, the animal ceasing to eat when these reach a threshold level Intake by ruminants depends much more on the capacity of the digestive tract, particularly the rumen, eat- ing ceasing when a certain degree of “fill” has been reached, and starting again when “fill” has been reduced by digestion and movement of food residues through the digestive tract: only on feeds of high energy concen- tration does blood metabolite level, rather than gastrointestinal fill, be- gin to control the amount of food that ruminants will eat (Conrad, 1966) However, as with other aspects of forage nutritive value it is essential

to recognize that the amount of forage that animals will eat is likely to be determined by a complex of factors Some earlier investigations may

have oversimplified the problem; the observation by Blaxter et al ( I96 1)

that the newer concepts related to digestibility and rate of passage of foods are “attributes which are hardly consonant with their acceptability

to the palate or taste” perhaps dismissed too lightly the possible signifi- cance of these other attributes It is evident also that the amount of forage that animals eat may depend as much on the amount of forage available as on any inherent characteristics of the forage itself Raymond (1966a) has proposed that the factors determining forage intake can be usefully divided into intrinsic factors (i.e., features inherent in the forage)