Title: Evaluation of kinetic models of ruminant intake and digestibility utilizing tropical grasses /
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Title: Evaluation of kinetic models of ruminant intake and digestibility utilizing tropical grasses /
Physical Description: xii, 118 leaves : graphs ; 28 cm.
Language: English
Creator: Abrams, Stephen M., 1944-
Publication Date: 1980
Copyright Date: 1980
 Subjects
Subject: Sheep -- Feed utilization efficiency   ( lcsh )
Ruminants -- Feed utilization efficiency   ( lcsh )
Forage plants -- Quality   ( lcsh )
Grasses -- Tropics   ( lcsh )
Animal Science thesis Ph. D
Dissertations, Academic -- Animal Science -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis--University of Florida.
Bibliography: Bibliography: leaves 110-117.
Statement of Responsibility: by Stephen M. Abrams.
General Note: Typescript.
General Note: Vita.
 Record Information
Bibliographic ID: UF00098906
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000014234
oclc - 06324825
notis - AAB7433

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EVALUATION OF KINETIC MODELS OF
RUMINANT INTAKE AND DIGESTIBILITY
UTILIZING TROPICAL GRASSES









By



Stephen M. Abrams


A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY





UNIVERSITY OF FLORIDA


























For Lillian, Michael, Joshua and Sara



(They packed.)















ACKNOWLEDGEMENTS

The author would like to sincerely thank the chairman of his

supervisory committee, Dr. John E. Moore, for sharing his knowledge, for

his critical review of this dissertation, and for the understanding of

when a student needed to work independently and when that student needed

direction. Appreciation is also extended to the members of the supervisory

committee, Dr. C. B. Ammerman, Dr. G. 0. Mott and Dr. C. J. Wilcox, and

to Dr. R. L. Shirley, for their willing assistance and availability, and

for reviewing this dissertation.

The author would also like to thank the technical staff at the

Nutrition Laboratory for their willingness to extend aid to the author

during the course of his research. Particular appreciation is extended to

John Funk and Debbie Ray. The author would also like to thank his fellow

graduate students over the years for the friendship and humor that makes

life more bearable: David Creswell, Jorge Beltran, Mike Richter, Carlos

Chaves and Joe Harris.

The author would also like to extend special appreciation to

Dr. Hal Wallace for the continuing financial support he provided over the

course of the author's graduate studies.














TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS. . . . . . . . . . . . ... iii

LIST OF TABLES. ... . . . . . . . . . . vi

LIST OF FIGURES . . . . . . . . . . . viii

ABSTRACT . . . . . . . . . . . . . x

CHAPTER

I INTRODUCTION . . . . . . . . . . 1

II REVIEW OF THE LITERATURE ... . . . . .. . .. 5

Determinants of Forage Quality .. .. . . . ... 5
Fill . . . . . . . . . . . . 7
Potentially digestible and indigestible pools . 10
Initial particle size distribution . . . .. 14
Rate of particle size reduction . . . .... 16
Rate of passage . . . . .. . . . . 17
Rate of digestion . . . . . . . . . 18

Within Animal Variation in the Determinants of Forage
Quality. . . . . . . . . ... . . . 20

Integrated Models of Rumen Digesta Disappearance .... . 21

III DIGESTIBILITY AND INTAKE OF CULTIVARS OF BERMUDAGRASS,
DIGITGRASS AND BAHIAGRASS FED TO SHEEP--EXPERIMENT 1 . 34

Introduction . . . . . ... . . . . . 34

Materials and Methods. . . . . . . . . ... 34

Results and Discussion . . .. .. . . 37

Determination of intake expressions . . . ... 37
Cultivar comparisons. . . . . . . . 39
Interrelationships among animal measurements. ... . 55

Summary. ................ . . . . . . . . 56








Page
IV ESTIMATION OF CELL WALL DIGESTION RATES AND POTENTIALLY
DIGESTIBLE AND INDIGESTIBLE CELL WALL IN FORAGE AND
FECES BY IN VITRO AND IN SITU DIGESTION--EXPERIMENT 2. .. 57

Introduction . . . . . . . .... . . 57

Materials and Methods. . . . . . . . . .. 58
Digestion and intake trial . . . . . . 58
Rate study. . . . . . . . . ... . 58
Chemical analyses . . . . . . . . . 60
Statistical analysis and estimation of model
parameters. . . . . . . . .... 60

Results and Discussion . . . . . . . .. .. 61
Digestion and intake trial. . . . . . . 61
Comparison of hays and methods of digestion . .. 61
Comparison of hays, orts and feces. . . . ... 66
Digestibility of the indigestible fraction. .... . 68
Effect of method of determination on the NDF
content of five hays .... . . . . . 69

Summary. . . . . . . . . . . . . 71

V EVALUATION OF A MODEL OF RUMEN CELL WALL
DISAPPEARANCE--EXPERIMENT 3. . . . . . . ... 72

Introduction . . . . . . . . ... .. . . 72

Materials and Methods. . . . . ....... . 73
In vitro rate study . . . . . . . 73
Estimation of digestion rate constant, and
potentially digestible and indigestible pools . .. 74
Estimation of rumen NDF digestibility . . ... 75

Results and Discussion . . . . . . . 76

Summary. . . . . . . . . . . . . 87

VI GENERAL DISCUSSION . . . . . . . ... . .. 89

APPENDIX. . . . . . . . .. .. ... . .. 93

LITERATURE CITED. .... . . . . . . . . 110

BIOGRAPHICAL SKETCH . . . ... . . . . . . 118















LIST OF TABLES


ABBREVIATIONS AND SYMBOLS USED IN TEXT, TABLES AND
FIGURES. .... .. . . . . .


2 HAY HARVESTING SCHEDULE. ..

3 LEAST SQUARES MEANS OF INTAKE
TWO-WEEK REGROWTHS .. ...

4 LEAST SQUARES MEANS OF INTAKE
FOUR-WEEK REGROWTHS ....

5 LEASTSQUARES MEANS OF INTAKE
SIX-WEEK REGROWTHS .. ...

6 LEAST SQUARES MEANS OF INTAKE
EIGHT-WEEK REGROWTHS ..

7 CORRELATION MATRIX OF INTAKE


AND DIGESTIBILITY


AND DIGESTIBILITY


AND DIGESTIBILITY


AND DIGESTIBILITY


AND DIGESTIBILITY


IN VIVO AND CHEMICAL CHARACTERISTICS OF HQ AND

RATE PARAMETERS OF HAY, ORTS AND FECES .

SELECTED CONTRASTS . . . . . . .


11 EFFECT OF METHOD OF DETERMINATION ON NDF CONTENT OF
FIVE HAYS . . . . . . . . . . . . .

12 IN VIVO AND CHEMICAL CHARACTERISTICS OF HAYS .. .....

13 MODEL PARAMETER RANGES . . . . . . . . . .

14 CORRELATIONS BETWEEN MODEL PARAMETERS AND CHEMICAL
COMPOSITION. .... . . . . . . . ... . .


Table
1


Page

. . 32


OF


OF


OF


OF




LQ HAYS . .

















Table Page
15 ANALYSIS OF.VARIANCE OF ORGANIC MATTER INTAKE AND
DIGESTIBILITY FOR EXPERIMENT . . . . . . .. 93

16 ANALYSIS OF VARIANCE OF CELL WALL INTAKE AND
DIGESTIBILITY FOR EXPERIMENT 1. . . . . ... . 94

17 LEASTSQUARES MEANS OF INTAKE AND DIGESTIBILITY BY
CULTIVAR, REPLICATION (REF.) AND REGROWTH FOR EXPERIMENT 1 95

18 ANALYSIS OF VARIANCE OF DIGESTIBILITY AND INTAKE
OF HQ AND LQ HAYS FOR EXPERIMENT 2. ... . ....... 98

19 ANALYSIS OF VARIANCE OF NDF RESIDUE OF HQ AND LQ HAY
SAMPLES WHEN DIGESTED BY IN VITRO AND IN SITU METHODS
FOR EXPERIMENT 2. . . .. . . . . . .. .99

20 ANALYSIS OF VARIANCE OF RATE PARAMETERS OF HQ AND LQ HAY
SAMPLES WHEN DIGESTED BY IN VITRO AND IN SITU METHODS
FOR EXPERIMENT 2. ... . . . . . . . . 100

21 ANALYSIS OF VARIANCE OF NDF RESIDUE OF HAYS, ORTS AND
FECES FOR EXPERIMENT 2. . . ... .. . . . 101

22 ANALYSIS OF VARIANCE OF RATE PARAMETERS OF HAYS, LQ ORTS
AND FECES WHEN DIGESTED BY IN VITRO AND IN SITU METHODS
FOR EXPERIMENT 2. . . ..... ...... 1.02

23 ANALYSIS OF VARIANCE OF RATE PARAMETERS OF HAYS, HQ ORTS
AND HQ FECES WHEN DIGESTED BY IN VITRO AND IN SITU
METHODS FOR EXPERIMENT 2 .. . . . . . . . 103

24 ANALYSIS OF VARIANCE OF METHOD OF DETERMINATION ON NDF
CONTENT OF FIVE HAYS FOR EXPERIMENT 2 .. . ...... 104

25 IN VITRO CELL WALL RESIDUES AS A PERCENT OF INITIAL
UIRY MATTER BY CULTIVAR, REPLICATION(REP.) AND
REGROWTH FOR EXPERIMENT 3 . . . . . . . . 105

26 MEASURED AND ESTIMATED MODEL PARAMETERS BY CULTIVAR,
REPLICATION (REP.) AND REGROWTH FOR EXPERIMENT 3 .... .. 107

27 CORRELATIONS BETWEEN IN VITRO MEASUREMENTS AND IN VIVO
CHARACTERISTICS OF HAYS FOR EXPERIMENT 3 .. ....... 09














LIST OF FIGURES
Figure Page
1 Organic matter digestibility of digitgrasses from
area A versus regrowth . . . . . . . .. 45

2 Organic matter digestibility of digitgrasses and
bahiagrasses from area C versus regrowth .. . .... 46

3 Organic matter digestibility of Coastcross-1 bermudagrass versus
regrowth. .. .... . . . . . . . . .. ... 47

4 Organic matter intake of digitgrasses from area A
versus regrowth . . . . . . . . . . 48

5 Organic matter intake of digitgrasses and
bahiagrasses from area C versus regrowth .. . ..... 49

6 Organic matter intake of Coastcross-1 bermudagrass versus
regrowth . . . . . . . . . . . .. 50

7 Digestible organic matter intake of digitgrasses
from area A versus regrowth .. . . . .. ... 51

8 Digestible organic matter intake of digitgrasses
and bahiagrasses from area C versus regrowth .. ... 52

9 Digestible organic matter intake of Coastcross-1
bermudagrass versus regrowth. . . . . . . 53

10 In vitro and in situ disappearance of NDF of HQ and LQ hays. 63

11 In situ disappearance of NDF of HQ hay and HQ orts. ... 67

12 Relationship between in vivo and twelve-hour in vitro
neutral detergent fiber digestibility . .......... 79

13 Relationship between in vivo and twenty-four-hour
in vitro neutral detergent fiber digestibility. . . ... 80

14 Relationship between in vivo and thirty-six-hour in vitro
neutral detergent fiber digestibility . . . .... 81

15 Relationship between in vivo and forty-eight-hour in vitro
neutral detergent fiber Tigestibility . . . . .. 82









Figure Page
16 Relationship between in vivo and sixty-hour in vitro
neutral detergent fiber digestibility .... . .... 83

17 Relationship between in vivo and ninety-six-hour
in vitro neutral detergent fiber digestibility ...... 84














Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


EVALUATION OF KINETIC MODELS OF
RUMINANT INTAKE AND DIGESTIBILITY
UTILIZING TROPICAL FORAGES

By

Stephen M. Abrams

March, 1980

Chairman: Dr. John E. Moore
Major Department: Animal Science

Three studies were conducted with the following goals: 1) to compare

ten cultivars of tropical grasses, harvested at different maturities, as

to their intake and digestibility by sheep, and to provide a set of forage

samples with known in vivo values, for the evaluation of laboratory

methods to predict forage quality; 2) to evaluate a model of forage cell

wall digestion and to compare in vitro estimates of the parameters of the

model with in situ derived estimates; and 3) to evaluate an integrated

model of ruminal cell wall digestion and intake.

In Experiment 1, six cultivars of digitgrass (Digitaria decumbens),

three cultivars of bahiagrass (Paspalum notatum) and one cultivar of

bermudagrass (Cynodon dactylon) were harvested at different stages of re-

growth (two to eight weeks), artificially dried, and fed to sheep in a

digestibility and intake trial. Animal variation in organic matter intake

(OMI) and neutral detergent fiber intake (NDFI) was best removed by powers

of body weight that were closer to 1.0 than to .75. Digitgrasses grown in








the same area as the bahiagrasses were superior in OMD (P<.0l), with a mean

digestibility of 63.4 versus 58.7 percent. Digestible organic matter in-

take per metabolic weight (DOMI75), g/kg"'5, for the three genera varied

from 26.4 to 50.2. No consistent regrowth effects, either within genera

or within species, were noted. This may have been because there was

severe insect damage to some of the grasses between the two and four-week

cuttings. The experimental digitgrass X124-4 and 'Slenderstem' digitgrass

maintained higher DOMI75 across all regrowths. The overall correlation

(r) between organic matter intake and digestibility of these grasses was

.47. Digestibilities of organic matter and neutral detergent fiber were

closely related (r = .96) as were OMI and NDFI (r = .94).

In Experiment 2, two bermudagrass hays, one high quality (HQ) and one

low quality (LQ), were fed ad libitum to wethers in a digestibility and intake

trial. Samples of hays, orts and feces from this trial were digested

in vitro and in situ, in two fistulated steers fed these hays in a crossover

design. Samples were removed at 16 times ranging from 2 to 129 hours and

analyzed for neutral detergent fiber (NDF). NDF residue over time was

fitted to an exponential decay curve, generating estimates of potentially

digestible cell wall (D, %), indigestible cell wall (U, %), digestion rate

constant (kj, hr-') and digestion lag time (L, hr). All differences

between hays and method of digestion occurred in the estimates of D and U.

The HQ hay had a higher fraction of D than did the LQ hay. In situ

digestion provided higher estimates of D than did in vitro digestion.

Application of the estimates of U in hay, orts, and feces to the intake

and excretion data of the digestion trial resulted in positive digestibilities

of U in both hays, irrespective of method of digestion, with a range of 14.3

to 18.1 percent.









In Experiment 3, sixty hay samples from Experiment 1, were subjected

to in vitro NDF digestion for 12, 24, 36, 48, 60 and 96 hours. Parameters

D, U, ki and L were determined. The rumen NDF passage rate constant (k2)

was estimated using these parameter estimates, an assumed rumen NDF fill

of 16.03 g per kg body weight, and in vivo NDFI per kg of body weight,

under two assumptions: that digestion lag observed in vitro does not occur

in vivo, and that lag does occur in vivo. Two predicted rumen NDF digesti-

bilities were then generated:

1) NDFDP, = D(kl/(kl+k2)

2) NDFDP2 = D[e-k2L-((k2e-k2L)/(kl+k2))]

Correlations (r) of NDFDPI and NDFDP2 with in vivo NDF digestibility were

.75 and .34, respectively. In vivo NDF digestibility was more closely

related to D (r = .89), which is the 96-hour in vitro NDF digestibility,

than to NDFDPI, NDFDP2, or any of the other model parameters.













CHAPTER I
INTRODUCTION

A local radio personality in Gainesville, who provides fine music for

a very few appreciative listeners, has been known to refer to his program

as "Gainesville's best kept secret." The role of forages in providing

sustenance to the human population is perhaps one of this country's best

kept secrets.

This ignorance is bewildering in light of the scope of forage pro-

duction in the United States alone (Hodgson, 1976). More land is devoted

to forage production than all other crops combined. The dollar value of

forages in terms of their contribution to human food of animal origin

exceeds the value of any other crop. In 1974 forages supplied 60 percent

of all the feed units fed to all classes of livestock, and 82 percent of

the units fed to beef cattle. Some 20 to 25 percent of the food supply

of the average American is based on forage. And the lack of impact of

forages is not limited to the general public. In spite of their massive

contribution to food production, forages receive only 4 percent of the

funds devoted to agricultural research in the United States.

As energy becomes increasingly expensive, the role of forages in

animal production will increase. They represent one of the least energy

intensive methods of fulfilling the human requirement for protein. With

increasing energy costs, cereal grain production will be diverted to human

use or to the production of alcohol as fuel. Beef cattle producers,

raising and marketing the least efficient domestic species in converting

high quality feed to protein,will come under increasing economic pressure

to reduce feed costs. Forages grown on land that is not amenable to the







production of cereal grains, in combination with ruminants that are capable

of converting cellulose to human food, will of necessity play an even larger

role in the nation's agriculture.

Many factors are involved in the lack of attention to forages. One

is that as less people are employed in the agricultural production sector

of the economy, fewer people have any knowledge of how food is brought to

their table. An additional factor is the nature of forage production

itself. It is more fragmented than grain production, and a relatively

small fraction (20 percent) is marketed on a cash basis (Rohweder. et al.,

1976). Unlike grains, which have a large export market, little forage

finds its way abroad. Finally, for animal nutritionists, forage as a feed-

stuff presents challenges that are more demanding than other feedstuffs.

A classification of corn grain into four categories defines it fairly

precisely in terms of expected animal production. No such classification

of forages is possible. Some 35 species are considered important forage

crops, and some 100 species are grown in the United States alone. The

ability of these forages to be converted to milk or meat varies with species,

plant maturity, soil fertility, rainfall, sunlight, processing and grazing

intensity. This diversity is increased many times by the activities of

plant breeders seeking improvement in forage quality. Nutritionists are

thus presented with a large and heterogeneous assortment of feedstuffs,

the evaluation of which in animal trials is economically prohibitive.

Laboratory evaluation of this material has identified no single method that

is adequate for predicting the feed value of forages, undoubtedly due to

their extreme diversity in chemical and physical structure.

The complexities of agricultural production in general, and the quality

of forages in particular, have increasingly been studied with tools recently

made available by the discipline of systems analysis. The use of systems






3
analysis and modeling in agriculi ral research has risen dramatically in

the past decade, spawning hundreds of articles and at least one journal

devoted entirely to systems research in agriculture. Concommittantly,

there has been some resistance to the use of modeling. The blame for

this must be shared by those working in this area. Some have shown

excessive zeal for the advantages of systems analysis and scorn for

traditional scientific thinking. Dillon (1976), for example, suggests

that this approach represents a "technological change in our mode of

thinking of such magnitude as to imply that we are moving from one socio-

technical age to another." Models have been presented with inadequate

notation. Rice et al. (1974) presents a diagramatic representation of a

model of the grazing system that is virtually unreadable. Nutritionists

often do not have a good grasp of the mathematics employed in modeling.

These barriers have led to attacks on systems modeling as overly

theoretical, when in fact modeling is, at its best, a formal methodology

that permits researchers to simplify and order complex relationships.

The advantages of modeling have been described by several authors.

Mertens (1977) states that modeling helps to organize information, cry-

stalize thinking, identify new research areas, and test research and

hypotheses. Rountree (1977) emphasizes that the whole is greater than

the parts and that interactions among the parts precludes studying them

without reference to the whole, since it is the interactions themselves

that produce the whole's organizational integrity and identity. He also

sees systems analysis as a bridge between pure mathematics and the empirical

sciences. Dillon (1976) states that systems thinking will lead to more

efficient and purposeful research. Perhaps Baldwin (1976) put it most

succinctly when he wrote that modeling becomes an extremely useful tool

when complex interactions between elements that determine output (animal








performance, e.g.) cannot be fea ibly evaluated in a quantitative or dynamic

fashion by the human mind or traditional research methods.

The purposes of the research described in this dissertation were: 1)

to compare ten cultivars of tropical grasses, harvested at different

maturities, as to their intake and digestibility by sheep; 2) to evaluate

a model of forage cell wall digestion, and to compare in vitro estimates

of the parameters of the model with in situ estimates; and 3) to evaluate

an integrated model of ruminal cell wall digestion and intake.













CHAPTER II
REVIEW OF THE LITERATURE

Determinants of Forage Quality

Forage quality has been defined as output per animal when forage

availability is not limiting and when animal potential does not vary

between treatments (Moore and Mott, 1973). Given no other nutritionally

limiting factors, animal output will be determined by net energy intake.

Due to the expense of measuring either animal performance or net energy

intake, other measurements, highly correlated with animal output, have

been used: digestible energy intake, digestible dry matter intake and

digestible organic matter intake. The choice of method is dependent on

the equipment available, expense and local conditions. At the University

of Florida, digestible organic matter intake is the method of choice.

In contrast to forages, highly digestible feeds, such as cereal grains,

are evaluated on the basis of their content of digestible, metabolizable

or net energy alone. Why then do we require two separate definitions of

feed quality? Ruminants consuming highly digestible feeds are able to

adjust their intake to provide sufficient net energy to meet their maximum

potential for output under 'chemostatic' regulation, the nature of which

has been reviewed by Capote (1975). Ruminants consuming most forages are

unable to consume sufficient quantities to meet this potential.

Conrad et al. (1964) noted that as forage quality increases, there is

a specific point at which voluntary intake comes under the control of

chemostatic mechanisms. Below this point, dry matter intake increases with

increasing quality; above it, intake decreases with increasing quality.








Conrad's use of digestibility as he independent variable was probably

influenced by the traditional use of digestibility, or total digestible

nutrients, to evaluate highly digestible feeds. But this study did

recognize the existence of two kinds of intake control. Control of forage

intake has been predicated on the assumption that there exists neural

sensors in the gastro-intestinal tract that respond to a certain level of

fill, causing the cessation of consumption (Campling, 1970). This has

been referred to as the distensionn' mechanism.

Fiber, lignin, cellulose, hemicellulose, physical form, tensile

strength and many other characteristics have been shown to affect forage

quality (Raymond, 1969; Moore and Mott, 1973). A complicating element is

that these characteristics often appear to affect forage quality

differently in different forages. This has made the accurate prediction of

forage quality, from one or more of these, an elusive goal. Success in

achieving this goal depends on the understanding of the intermediate

mechanisms that link input to output:

Forage chemistry and structure

Intermediate mechanisms
4-
Digestible organic matter intake

Evidence has accumulated that six interrelated factors determine digestible

organic matter intake when control is by distension:

1) Fill

2) Potentially digestible and indigestible pool sizes

3) Initial particle size distribution

4) Rate of particle size reduction

5) Rate of passage

6) Rate of digestion

Collectively, control of digestion and passage by these factors determine





7

forage quality. If quantifiable 1ir a given forage-animal combination, and

their relation to each other understood, measurement of these factors will

enhance our ability to evaluate forage quality.

Fill. The structure of the gastro-intestinal tract alone suggests that

under certain conditions, it will limit feed intake. It is not infinitely

elastic nor is it an open tube through which digesta can flow at any rate

(Church, 1976). With respect to fill, in effect three compartments exist

in the ruminant digestive tract: the reticulo-rumen (rumen), the abomasum

and the cecum-colon (large intestine). Exit from these compartments is

limited. The omasum, a spherical structure filled with muscular laminae,

lies between the rumen and the abomasum, and the reticulo-omasal orifice

restricts exit from the rumen. A sphincter, the pylorus, is located at the

junction of the abomasum and the small intestine. The anal sphincter

restricts movement from the large intestine. Thus, even in the absence of

neural sensors to fill, which have yet to be identified (Church, 1979), the

physical structure of the digestive tract will act as a limit to intake.

Evidence has accumulated that the rumen is the structure that is limiting

in terms of fill. Sawdust of a large particle size introduced into the rumen

decreased voluntary intake by 15 percent (Weston, 1966), while particles of

polyvinyl chloride, smaller than sawdust, decreased intake by only 7 to

9 percent. The introduction of polypropylene fibers 15 cm long into

the rumen via stomach tube depressed intake for 60-65 days (Welch, 1965).

Thirty cm long fibers were found unaltered in the rumen some 28 days after

administration, while feeding ground fibers had no effect on intake. The

introduction of water-filled bladders into the rumen of cattle receiving ad

libitum quantities of hay reduced intake (Campling and Balch. 1961). Weston

(1966) introduced feed into the rumens of fistulated sheep receiving hay

and straw diets. Voluntary intake declined by a quantity equal to 90-110 percent

of that introduced. The introduction of sawdust at a rate of 17 percent of






8

the voluntary feed consumption during a control period, reduced voluntary

feed intake by 15 percent.

That the other organs do not materially contribute to intake res-

triction has been shown (Grovum and Phillips, 1978; Grovum and Williams,

1973b). The rate constant for passage of 144Pr, a heavy metal solid phase

marker, was shown to be some twenty times larger from the abomasum than

from the rumen. The rate constant for the large intestine was of a

similar magnitude to that of the rumen. However, artificially increasing

the fill of the large intestine had little effect on intake. Utilizing

abomasally cannulated animals maintained on alfalfa (Medicago sativa) hay,

a solution of methylcellulose, a bulk laxative, was injected into the

abomasum to determine if the intestines are capable of limiting intake.

Infusion of 2.95 kg per day had no effect on voluntary intake, although

wet fecal output increased from 2.4 to 4.6 kg per day. Only with extremely

high levels of infusate (5.4 kg/day) did voluntary intake significantly

decline, and it was hypothesized that this was a mechanism to protect the

intestines from damage. Furthermore, intestinal transit time was not

altered by this treatment.

The component of rumen digesta that constitutes rumen fill has not

been as yet identified with certainty. Most workers have reported no

relationship between fill (expressed as total digesta. dry matter or

organic matter) and voluntary intake (Ulyatt et al., 1967; Thornton and

Minson, 1972; Ingalls, et al. 1966). This is an indication that ruminants

eat to a constant fill. In the latter study, cell wall constituents

constituted the least variable expression of fill. This is in agreement

with Van Soest's (1975) "hotel theory," in which plant cell walls, rather

than dry matter, constitute the essential fill qualities. Under this








assumption, digestion of cell cortLents has no effect in reducing fill,

just as removal of furniture from a hotel does not reduce the space that

the building occupies. Only when cell walls are digested or move to the

lower part of the intestinal tract is fill reduced.

Variation in fill can occur in three ways: man induced changes,

animal to animal variation and within animal variation. Since within

animal variation has only been observed indirectly, and this may be due

to rate of passage changes rather than alterations of fill, it will be

discussed in a later section. Within a species of animal, the size of

the rumen will vary with the size of animal, and thus fill will vary

accordingly. Although intake is often expressed as a function of

metabolic weight, the volume of different containers will vary directly

with the weight of their contents. Capote (1975), utilizing four successive

digestibility trials with a group of 24 wethers fed pelleted forages,

found that cell wall intake between animals varied least when expressed as

a function of body weight to the .96 power. Ivins (1959) and MacLusky (1955)

reported that the intake by cattle of forage was closely related to body

weight. Other workers have found a wide variation in the relationship of

forage intake to body weight (Colburn and Evans, 1968; Karue et al., 1971).

Some of the forages used in these studies were high quality temperate

forages, where chemostatic regulation rather than fill may be actively

limiting intake. An additional source of variation is that even if fill

is, theoretically, a function of rumen cell wall content, which in turn

is a function of rumen volume, animal-to-animal variation in rumen volume

may not be perfectly explained by body weight variation.

Man may also reduce fill by reducing animal intake. In theory this

should increase digestibility, since a reduction in fill reduces rate of

passage to the abomasum. This has been born out by many researchers.






10

Blaxter et al. (1956) varied th_ intake of long, medium-ground and fine-

ground hay fed to sheep from 600 to 1500 g daily. Digestibility of dry

matter declined, on the average, seven percentage points. Rate of pass-

age, as measured by the appearance of stained particles in the feces,

increased with increasing feed intake. Campling et al. (1961) fed hay at

4.5 and 6.8 kg per day, and ad libitum, and observed a decline in

digestibility of four percentage points at the higher intake level. Grovum

and Hecker (1973) observed increased quantities of digesta in the digestive

tract and decreased retention times as sheep intake of alfalfa was

increased from 400 to 1200 g per day.

Potentially digestible and indigestible pools. Rates of digestion of

cell wall in the rumen are dependent on the nature of the pools upon which

they act. If forage cell wall is uniform in chemical and physical

structure, then the rate of digestion of cell walls will be quantified by

a single rate constant. In the introduction to this dissertation the

diversity of forage chemistry and structure was emphasized. There is

considerable evidence that forage cell wall is not of a uniform nature.

Blaxter et al. (1956) suggested that there may be a maximum limit of

forage dry matter digestibility that was less than complete digestion.

Using stains as markers to measure retention time, they altered retention

times by varying particle size and the amount of hay fed to sheep. A plot

of digestibility against mean time in the digestive tract suggested the

exponential equation, digestibility = D(1-e-kt ), where D is the potentially

digestible dry matter (estimated at 81 percent of dry matter for the hay fed

in these trials), k, is the rate constant for digestion and t is time. This

equation is known as the first order reaction curve. This paper was ahead

of its time and recognition of the importance of the concept of potentially

digestible and indigestible pools did not occur until much later. Lucas








(1958) realized that there was a ieed for a rigorous definition of the

chemical components of feeds with respect to their nutritional value.

He proposed the basic outline of the concept of a "nutritive entity," and

later expanded on this concept (Lucas, 1962). A nutritive entity was

defined as a "nutritionally ideal chemical fraction with invariant

digestibility properties;" i.e.,it will have a constant true digestibility.

The Lucas definition did not take account of retention time; a nutritive

entity will vary in its true digestibility, depending on the length of

time it spends in the digestive tract. The constant should be the

potential digestibility of the nutritive entity; it remains the same,

regardless of retention time. Additionally, given no other limiting

factors, and a uniform rumen environment, the digestion rate constant of

a nutritive entity should be constant. This latter concept is of course

an idealized one; nutrient deficiencies (e.g. nitrogen) have been shown to

affect rates of digestion, and a decrease in cell wall digestibility with

increasing levels of concentrate in the diet is a commonly observed

phenomenon. Lucas placed these effects under the umbrella term, "associative

effects."

In 1969 Wilkins defined the potential digestibility of cellulose as

the "maximum digestibility attainable when the conditions and duration of

digestion are not limiting factors." Cellulose digestibility in vitro

did not increase with an incubation period of greater than five days, and

the use of two incubations of six days each did not result in a different

digestibility than 12 days continuous fermentation. Using four grasses

at several stages of growth, the potential digestibilities varied from 54

to 88 percent, with an observed decrease in the size of the potentially

digestible pool of an individual cultivar with the increasing maturity of

that plant. Similar values were found when samples were suspended in nylon








bags (20u) for equivalent duration. A subsequent study found potential

cellulose digestibility to be correlated with the percent of schlerenchyma,

vascular tissue and lignified tissue in five forage species (Wilkins, 1972).

Leaf blade was found to have the highest potential cellulose digestibility

followed by the inflorescence, sheath and stem in oats (Avena sativa) and

tall fescue (Festuca arundinacea).

The concept of an indigestible fraction is supported by the work of Akin

and coworkers who have examined the fate of different forage microstructures

when subject to in vitro digesiton. An initial study (Akin et al., 1973)

demonstrated histologically that the lignified tissues of Coastal bermuda-

grass (Cynodon dactylon) appeared to be completely intact after 72 hours of

in vitro digestion. However, the lignified tissue of a temperate species,

tall fescue, showed some signs of digestion of the lignified portion. This

was indicative of an indigestible pool, but one that varied in chemical com-

position among forage species. A more extensive study of the leaf tissue of

one bahiagrass (Paspalum notatum) and two bermudagrasses (Akin et al., 1974)

showed that after 72 hours, lignified vascular tissue was undegraded, small

vascular bundles, mesophyll and outer bundle sheaths were completely degraded,

while epidermis was at various stages of digestion. Histological studies of

sections after six and twelve hours of digestion suggested different rates of

digestion, or at least different lag times, of different potentially digestible

pools. A difference between tropical and temperate species, utilizing 10

different grasses, was also observed (Akin and Burdick, 1975). After 72

hours of digestion the schlerenchyma and lignified vascular tissue of tropical

species were undegraded, while some temperate species showed a degree of

degradation of the lignified vascular bundles.

The work of Akin and associates suggests a structural or anatomical

classification of the potentially digestible pools and the indigestible pool,








at least in tropical species. Minson (1976) has taken a chemical approach

to these pools. According to his classification, the potentially digestible

pool contains the readily soluble carbohydrates and hydrolyzable poly-

saccharides (primarily cellulose and hemicellulose), and the indigestible

pool consists of lignin, silica, cuticle, and the polysaccharides that are

fully protected by lignin. Later, however, Minson had difficulty in

quantifying these fractions by in vitro digestion (Goto and Minson, 1977).

Using long term fermentation, with centrifugation, decantation and reinocu-

lation at four days, dry matter digestion was found to not be complete at

five days, as Wilkins had observed. Reinoculation resulted in a 10.2 percent

increase in dry matter digestibility and up to a 28 percent increase in

lignin solubility. A similar response was observed when sheep feces were

fermented. These authors suggested that 21 days was the minimum incubation

time to distinguish between the two pools. Dehority and Johnson (1961)

demonstrated a wide variation in the asymptotic peak of in vitro cellulose

digestion of mature timothy (Phleum pratense) as particle size was varied

with ball milling. Material milled for 6, 24 and 72 hours appeared to have

maximum cellulose digestibilities of 58, 65 and 75 percent, respectively.

In another phase of this experiment, samples of timothy, ground through a

40 mesh screen, were subjected to digestion for 48 hours. One half of the

samples were then ball milled for 72 hours. All samples were reinoculated

and cellulose digestion determined at 6, 12, 24, 30 and 48 hours. The

samples that were not subject to ball milling showed an insignificant amount

of additional digestion, while the ball milled sample experienced loss of

67 percent of the cellulose it contained at the initiation of the second

fermentation. The authors suggest that lignin acts as a physical, rather

than chemical, barrier which is disrupted with ball milling. Minson (1976)

supports this viewpoint.






14

Inherent difficulties exist in the verification and quantification

of an indigestible cell wall pool. Outside of the closed environment of

the rumen, lignin and lignocellulose are digestible by microorganisms.

Under normal rumen conditions microorganisms which are capable of digesting

lignocellulose or of solubilizing lignin may be: 1) not present, due to

the nature of the rumen environment; 2) suppressed by the activity of

other microorganisms that utilize the more readily available sources of

energy, as cellulose digesters are suppressed in grain-fed ruminants; 3)

the same organisms which digest non-lignified cellulose, but do not

attack lignocellulose when the unlignified material is available as a

substrate. Under long term in vitro digestion, with reinoculation, the

readily available carbohydrates have been digested, decantation has

removed many toxic products, and organisms capable of solubilizing lignin

may proliferate. Thus, the transference of pool size derived in the

essentially static conditions of the test tube to the dynamic conditions

of the rumen may lead to erroneous conclusions. A second difficulty is

that the size of the indigestible pool may be a variable rather than a

constant pool. If the hypothesized physical barrier that lignin presents

can be broken down by ball milling, then to some extent this must also

occur with mastication and rumination. Thus, the effect of the lignin

barrier in vivo may vary with the duration and intensity of mastication,

rumination and the initial particle size, leading to variations in the

size of the indigestible pool.

Initial particle size distribution. If the omasum functions as a

barrier to the passage of solids out of the rumen, then particle size will

affect the rate at which solids move to the abomasum. Alternatively,

particle size may contribute to changes in effective rumen fill, for as

particle size decreases, density increases. An increase in fill due to






S5

particle size would also lead to increased rate of passage. There is

ample documentation that decreases in particle size of forage diets

result in increased passage, with a concomittant decline in digestibility.

In Blaxter's classic study of rates of passage in sheep (Blaxter

et al., 1956) three forms of a single hay were fed: finely ground and

cubed, medium-ground and cubed, and long. Intakes were held constant at

three levels: 600, 1200 and 1500 g per day. Rate of passage increased

as particle size decreased, irrespective of feeding level. Campling and

Freer (1966) found that the intake of ground, pelleted oat straw was 26

percent greater than that of long straw. Grinding of previously chopped

alfalfa and wheaten hays resulted in a 50 percent increase in voluntary

feed consumption in sheep (Weston, 1967; Weston and Hogan, 1967). A

similar response to the pelleting of ryegrass was observed by Greenhalgh

and Reid (1973). Intake in cattle, however, was increased by only 11

percent. Others have observed similar results with pelleting or grinding

of forages (Osbourn et al., 1976; Osuji et al., 1975). In view of the

mass of data with regard to the effect of particle size on intake, it is

unfortunate that in none of these studies were there any quantitative

measures of initial particle size distribution.

Alwash and Thomas (1974) fed hay diets at two levels of intake. The

hays were of four different particle size distributions with a mean

particlesize of .64, .54, .44 and .20 mm, having been ground through a

hammermill fitted with screens of 12.7, 4.75, 3.06 and 1.00 mm, respectively.

Differences in retention time among particle sizes were similar at both

levels of feeding. However, a doubling of the mean particle size did not

result in a doubling of the mean retention time, which suggests that

rumination and mastication will increase with increases in particle size

(Pearce and Moir, 1964). Additionally, mean particle size is probably an





'6

inadequate index to use in defining a particle distribution. A hypo-

thetical example will illustrate this. If the omasum permits particles

of 1.0 mm or less to pass through it and all particles are 1.0 mm in

size, they will have a mean particle size of 1.0 mm and all will pass to

the abomasum, independent of any particle size reduction. However,

given a particle distribution in which one half of the particles are 1.5

mm and the other half are .5 mm, the mean particle size will also be 1.0,

but only one half of these particles are capable of passing to the lower

tract unaided by mastication. Waldo et al. (1971) suggest that the use

of a log normal distribution might be a better method for describing

particle distributions.

Rate of particle size reduction. The rate of particle size reduction

as well as the initial distribution will affect rate of passage and other

components of nutritive value. Of all factors involved in the determina-

tion of nutritive value, it is particle size reduction that is least

understood. Measures of the variation in rate of particle size reduction

have been, for the most part, indirect.

Utilizing six varieties of temperate forages, Chenost (1966) measured

the electrical energy necessary to grind forage samples through a 1 mm

screen fitted to a laboratory mill. The correlation between the

'fibrousness" index and dry matter intake was a .90. However, correlation

with digestibility was also high, and intake and digestibility were closely

correlated. Troelson and Bigsby (1964) determined a "particle size index"

based on the particle size distribution after 10 minutes of artificial

mastication. Using four temperate forages cut at different stages of

maturity, for a total of 14 different hay samples, he observed that

voluntary feed intake was highly correlated (r = .94) with particle size

index. Omasal digesta from alfalfa hay had a greater relative particle








size than those from the grass hays at comparable maturity stages

(Troelson and Campbell, 1968). They suggested that this may be due to the

differences in particle shape between the species. Pearce (1967) measured

the change in particle distribution in the rumens of sheep fed a roughage

ration once daily. More particle reduction occurred during the evening hours

when rumination was more active. Pearce suggested that this was indicative

that rumination, rather than microbial activity, is the major factor

responsible for particle size reduction. This concept might be developed

into an extension of Van Soest's "hotel theory." If, in terms of space

occupying characteristics, the disappearance of cell contents is analogous

to the removal of furniture from the interior rooms of a hotel, cell wall

digestion may be like the removal of the hotel's windows. The structure

continues to occupy the same space, which can only be reduced when the outer

walls (cell walls) are demolished by a wrecking ball (rumination).

Rate of passage. The shape of cumulative fecal excretion curves over

time, irrespective of type of marker, has been shown to be sigmoidal. Some

early researchers (Balch, 1950) have attributed this behavior to lack of

instantaneous mixing upon administration of the marker substance, and have

ignored the first 5 percent of marker excretion for subsequent calculations.

However, both Blaxter et al. (1956) and Brandt and Thacker (1958) independ-

ently realized that a sequential, two-compartment model would fit the

resulting curves. If the amount of marker administered is normalized to 1,

then the cumulative excretion of marker is equal to:

1-(1/(k, -k,,)) (k21e-k22 (- -k22 e-k21(L-L)

In this model K and K are rate constants for the two compartments, L is
21 22
a time delay and t is time after marker administration. The model assumes

simple first order disappearance of marker from any one compartment.









Dual compartmental models will also resolve into sigmoidal curves, but they

would be difficult to relate to the actual physical structure of the

digestive tract.

An Australian group has intensively studied the behavior of various

sections of the gastrointestinal tract, utilizing a variety of markers and

abomasal and ruminal cannulation (Grovum and Williams, 1973a, b, c, 1977;

Grovum and Hecker, 1973; Grovum and Phillips, 1973, 1978). Elimination

of a particle phase marker, 14Pr, and a liquid phase marker, "Cr-EDTA,

in the rumen, abomasum and cecum-proximal colon, were well described in

each compartment by a first order kinetic model with sheep maintained on

an alfalfa hay diet. Passage rate constants for 44 Pr through the three

compartments were .020, .38 and .043 hr-I respectively, and for the "Cr-

EDTA wer .031, .80 and .042 hr-'. This indicated that, as had been

previously theorized, the rumen was the limiting compartment with respect

to passage, and that at least in the rumen and abomasum, water was capable

of passing independently of and at a higher rate than the particulate phase.

Ellis ahd Huston (1968) found similar differences between the two phases.

When excretion patterns were fitted to the two compartment model, a close

fit was observed for both phases (Grovum and Williams, 1973c). A subsequent

physical and computer simulation study utilizing reservoirs and flow lines,

illustrated that the two compartment model behaved as did the in vivo

results, although the rate constants were larger than those in sheep

(Grovum and Phillips, 1973). Ulyatt et al. (1976) found no abomasal

particles larger than 1 mm in sheep fed alfalfa hay. This suggests that

solid phase passage rate estimates apply only to particles less than some

critical size.

Rate of digestion. Although Blaxter et al. (1956) had observed that there

may be an upper limit of digestibility that was less than complete digestion,






19
prior to Wilkin's (1969) publication on potentially digestible cellulose,

rational methods of describing digestion curves were absent from the

literature. As late as 1968, Van Soest reported that the rate constant

for digestion of alfalfa and orchard grass (Dactyles glomesata) varied

with the duration of digestion, increasing during the first 12 hours and

then decreasing at a decreasing rate during the period after 12 hours.

The change in the rate constant after 12 hours occurred because cell wall

or cellulose was being used as the digestible pool, when only a portion of

either quantity was capable of being digested.

Since that time several studies, using multiple sampling times, have

determined rate constants for forages, both in vitro and in situ. Gill

et al. (1969) determined in vitro cellulose disappearance at six times,

using 48 hours digestibility as the end point of digestion. The rate

constants for alfalfa varied from .078 to .102 hr- Smith et al. (1972)

determined rate constants for cell wall digestion on 112 samples repre-

senting 15 species of temperate grass and legume hays, using 72 hours as

an end point. These values had a range of .040 to .309 hr-'. Digestion

rate constants were most highly correlated with the cell contents

fraction (r = .72), while the 72 hour digestibility was highly correlated

with the lignin fraction (r = .88), suggesting that lignin is associated

with pool size rather than rate of digestion. Separate correlations for

grasses and legumes improved the relationships involving the rate constants,

but not those involving 72 hour digestibility.

A lag time at the initiation of digestion has been observed in many

of these studies. The zero-time intercept of the semilog plots is almost

invariably above that of the measured zero-time fraction. This has also

been observed in in situ studies (Van Hellen and Ellis, 1977). Mertens

(1973) attempted to accommodate this lack of fit with the addition of






0
various mathematical description of lag time to the basic kinetic model.

A discrete lag time was adequate in describing this phenomenon, although

observations indicate that digestion during the lag period is increasing,

rather than not yet functioning.

Several researchers have noted a decline in forage intake when crude

protein levels are below approximately 7 percent (Milford and Minson,

1965; Blaxter and Wilson, 1963). Since protein deprivation affects both

microbial and host-animal metabolism, intake changes may be due to several

factors. However, there is evidence that rate of digestion may be

adversely affected. Houser (1970) observed that the in vitro rate of

digestion of low-protein digitgrass hay (4.2 percent crude protein), was

increased by the addition of urea, while digestion at 72 hours was

unaffected.

Within Animal Variation in the Determinants of Forage Quality

Under the principles thus far discussed, variation in digestible

organic matter intake may occur from three general sources: 1) animal to

animal variation, most of which will be removed by expressing intake per

unit weight, and the remainder will constitute natural variation; 2) man-

induced variation which can be controlled; 3) chemical and structural

forage factors which, given adequate understanding of their effect on

the parameters that determine forage quality, should be estimable. There

is also evidence that under certain physiological and environmental

conditions, animals can overcome distensive control.

Several experiments with dairy cattle have shown variation between

lactating and non-lactating cows. Hutton (1963) utilized six sets of

identical twin Jersey crossbred cows, each twin set comprised of one

lactating and one non-lactating member. The diets consisted of fresh

herbage cut once daily. Intake differences between twins increased over






',1

the 36 weeks post-calving period with the average difference being a 50

percent increase in favor of the lactating twin. Differences in digesti-

bility between twins were minimal (.7 percent), indicating that alterations

in fill, rather than the passage rate constant, were occurring. This

was confirmed in a second study by Tulloh (1966a) with lactating and non-

lactating twins. The water-filled capacity of the rumen was 29 percent

greater in lactating cattle. No significant differences between twins

was observed in the internal circumference of the duodenum, small

intestine or ileum. A 39 percent increase in the weight of digesta in

the rumen was observed in the lactating animals. This study also is

suggestive of the difficulties in expecting weight differences to account

completely for intake differences---the dry cows were heavier than the

lactating cows. Similar results were reported in a later study (Tulloh,

1966b).

Rate of passage alteration in the digestive tract of pregnant and

non-pregnant Merino ewes were determined by Graham and Williams (1962).

Passage in ewes that were 104 days pregnant was higher than that of ewes

that were 39 days pregnant. The mechanism responsible may have been an

increase in rumination, since the ewes were limit-fed. Exposure of shorn

sheep to cold has been shown to reduce the digestibility, rumen turnover

time and rumen fill (Kennedy and Milligan, 1978). Differences between

shorn sheep and unshorn sheep in intake varied with plant species (Minson

and Ternouth, 1971). Shorn sheep had greater intake of digitgrass, similar

intakes of Setaria and reduced intakes of alfalfa when compared to unshorn

sheep. No consistent differences in digestibility were observed.

Integrated Models of Rumen Digesta Disappearance

The six major factors identified in a previous section as deter-

minants of the quality of a forage have all been identified, directly or








indirectly, as being capable of producing alterations in the digestible

organic matter intake of a forage. Variations in forage structure and

chemistry will affect one, several, or more likely, all of these parameters.

It is not sufficient, for example, to state that the grinding of a forage

results in an increase in voluntary intake, a decrease in digestibility and

an overall increase in digestible organic matter intake. These six factors

do not operate independently; they interact with each other. Only by

understanding these interrelationships can the ultimate quantitative effect

on forage quality be understood. This effect will vary from forage to

forage and may, in a particular species, produce unexpected results

(Van Soest, 1975).

Reducing the initial particle size of a forage would have the following

effects based on our present knowledge of the way in which these parameters

behave, and on intuition based on theoretical principles:

1) an increase in the digestion rate constant as surface area

increased.

2) an increase in effective fill if greater packing occurred.

3) an increase in the potentially digestible pool if such

grinding results in a rupture of ligno-cellulose bonds.

4) a decrease in the rate of particle size reduction with a

decrease in rumination.

5) a decrease in the rate constant for passage, if rumination

is at least partially responsible for passage.

The net effect of these changes on the disappearance of digesta would be

the following:

1) an increase or a decrease in the rate of passage depending

on whether the net effect of the decreased initial particle






:'3
size distribution ind the increased effective fill is

greater than the effect of decreased rate constants for

particle size reduction and passage.

2) if passage rate is increased, an increase or a decrease in

the extent of digestion depending on whether the effect of

increased rate of passage in decreasing the retention time

is greater than the effect of increased digestion rate

constant.

3) if passage rate is decreased, an increase in the extent of

digestion.

Although the vast majority of studies show an increase in intake and a

decrease in digestibility with grinding, it is evident that without a

theoretical and quantitative understanding of these dynamic relation-

ships, precise predictions of such changes are not possible. Similarly,

efforts to predict intake from digestibility are doomed to failure,

since they are not causative factors, but the end result of these

processes.

Any given system contains within it component subsystems and is

itself part of a larger system. Therefore, selection of the level at

which to explore a system will depend on the specific goals of the

researcher. With respect to forages, one can attempt to model at the

level of: 1) the entire grazing ruminant system, including the agronomic

subsystems (climate, soil and herbage), animal digestive function and

animal metabolism (Rice et al., 1974); 2) the animal only, including both

distensive and chemostatic controls (Forbes, 1977); or 3) a component of

the diet subjected to only one type of control (Waldo et al., 1972). Once

this level is selected the degree of detail or complexity necessary to

fulfill objectives must be determined. Finally, the determination must








be made as to whether there is sufficient data available to validate the

model. At this station the major concern is ultimately with the evaluation

of forage quality. Therefore, this discussion will be limited to models

which offer the potential of predicting the quality of a given forage to

ruminant animals.

The foundation paper in this field published in 1956 by Blaxter and

his associates (Blaxter et al., 1956). Its importance was fourfold: 1)

it provided a kinetic explanation for the results of marker studies; 2) it

introduced the concept of potential digestibility; 3) it attempted to

integrate digestion and passage in one model; 4) it introduced the concepts

of fill and distension. Unfortunately, it took more than a decade for

other researchers to interpret, apply and extend this model. The two-

compartment model for passage has already been discussed. By varying the

form and amount of hay fed, they were able to change retention time. As

retention time increased, so did digestibility, in a manner suggesting the

exponential relation, digestibility = D(l-e-k t). Potential digestibility,

the rate of constant for digestion, the passage constants (k21 and k22) and

a time delay (L) were then integrated into a single model to predict in

vivo digestibility:

Digestibility = D-D(k2K.22/(k21-k22))(e-k22L/(k+k22)-ekL/

(k1+k21))

One assumption of the model is that one rate constant for digestion is

valid for the entire digestive tract. Using in vivo values to estimate

the digestion rate constant, BlaxLor noted that an increase in initial

particle size resulted in a higher digestion rate constant, and that an

increase in intake lowered the digestion rate constant. It is unlikely

that either would be valid.









In 1972 a simpler model to describe cellulose disappearance from the

rumen was proposed (Waldo et al., 1972), and has since been applied to

cell wall disappearance (Mertens, 1973) and organic matter disappearance

(Golding, 1976). Its use with respect to intake assumes that the rumen

is the limiting part of the gastrointestinal tract. It recognized that

previous attempts to resolve disappearance curves into exponential

functions failed because they used the total fraction rather than the

potentially digestible fraction as the digestion pool. The basic hypothe-

sis of the model is of two rumen pools, a potentially digestible pool

which can disappear by passage and digestion, and an indigestible pool

which can only escape the rumen by passage. Its simplifying aspects were

the assumption of the rumen as the key organ in limiting intake and that

an adequate description would be achieved by assuming steady state

conditions. The concentration of labeled cellulose (F) in the rumen at

5 hours post-administration is given as:

F = De-(kl+k2)t+Ue-k2t
Under steady state conditions, labeled cellulose fill (F) per unit intake

(I) is:

F/I = D/(k1+k2)+U/k2

where D = potentially digestible pool, U = indigestible pool, ki= digestion

rate constant and k2= passage rate constant.

The application of this model illustrates how modeling can aid in

resolving questions raised by experimental results and force the integra-

tion of concepts. Although it is now recognized that digestion and intake

are functions of the same dynamic processes, for many years they were

treated entirely separately. Various experimental methods have been used

to generate prediction equations of either or both of these in vivo

parameters. In the process, researchers observed that the coefficients









of variation for intake (for animals receiving the same forage) averaged

10-20 percent, and were much higher than those for digestibility, which

averaged 5-10 percent. An example, using the Waldo model, aids in

explaining these differences. Two equations, derived from this model,

define intake and digestibility under steady state conditions:

Intake = F/(D/(k1+k2)+U/k2)

Digestibility = D(kl/(ki+k2))

The same variables occur in both equations, with the exception that

fill appears only in the intake equation (U can be expressed as 1-D).

Thus, for example, a 25 percent change in F results in a 25 percent change

in intake and no change in digestibility. Fill contributes an additional

source of variation to intake that is not present for digestibility, and

the addition of a function of body weight to the equation will not

entirely remove this source of variation unless this function of body

weight is perfectly correlated with fill. There is an additional explana-

tion for the difference in observed variation between intake and digesti-

bility. First, we must assume that the digestion rate constant does not

vary among animals receiving the same forage, which is reasonable since

rumen environments are relatively constant and microbes tend to adapt to

their diets. Then let us examine what happens when two animals of the

same size and fill consume the same forage, but the passage constant, k,,

is 25 percent greater in one animal, perhaps due to differences in

rumination, size of the reticulo-omasal orifice, or chewing duration and

intensity. The parameters and expected intake and digestiblity are given

below.









F D U ki k2 CI CD DCI
Anima-l 1 500 .60 .40 .04 .020 401 40 160

Animal 2 500 .60 .40 .04 .025 476 37 176

percent 0 0 0 0 +25 +19 -7.5 +10
change
F = cellulose fill, g; D = potentially digestible fraction of
total cellulose; U = indigestible fraction of total cellulose;
ki = cellulose digestion rate constant, hr-'; k2 = cellulose
passage rate'constant, hr-'; CI = cellulose intake, g/day; CD =
cellulose digestibility, percent; DCI = digestible cellulose
intake, g/day.

This example shows that a 25 percent increase in the passage rate constant

results in a 19 percent increase in cellulose intake, only a 7.5 percent

decrease in digestibility and a 10 percent increase in digestible

cellulose intake. It provides an explanation, in a simplified,

quantifiable way, for these variations, although under actual in vivo

conditions these relationships are certainly more complex.

The publication of the Waldo model stirred great interest due to its

potential for the prediction of forage quality. It provided a simple,

integrated scheme, and if utilized for broader fractions of forage, and

a method found to estimate the parameters, than the difficulties in

predicting intake might be solved. Golding (1976) applied this model to

rumen organic matter disappearance, using 31 tropical grasses. In vivo

lignin intake was used to estimate the passage rate constant. The rate

constant for digestion was estimated via in vitro digestion, and potentially

digestible and indigestible pools were estimated as steady-state rumen

concentrations, with the use of equations employing the digestion rate

constant and a predicted organic matter digestibility based on 72 hour

in vitro digestion. Although steady-state conditions were assumed, these

values were entered into the time dependent equation and the time required

for organic matter fill to reach 37 percent of estimated steady-state









fill was used to predict digestible organic matter intake. The

correlation achieved was quite high (r = .96). However, several

difficulties with the interpretation of the model existed in this study:

1) the basis for the pool estimates was that the retention time of

digestible organic matter is 1/(k1+k2), but since this fraction leaves the

rumen only by digestion its retention time is 1/ki, which cannot be used

to estimate these two pools; 2) in the model, intake, rather than

digestible intake, is a function of retention time, and it is intake that

should be used to validate the model; and 3) the passage rate constant

was estimated by a value containing intake, i.e. lignin intake, and

digestibility was estimated by using a prediction equation based on in

vivo values of the same forages, and these equations were different for

different species of grass.

Mertens (1973) applied the Waldo equation to cell walls, assuming

that cell walls constitute rumen fill. He was aware that previous

researchers had shown a lag phase in measurements of rates of digestion

and attempted to account for lag by various alterations in the digestion

rate component of the model (although lag may be an artifact of the

methods used to measure rates of digestion). These included the intro-

duction of a discrete lag phase, a sequential two-compartment digestion

model, analogous to that proposed by Blaxter et al. (1956) for passage, an

incremented lag phase and a model containing two potentially digestible

pools. A model with a discrete lag phase was adequate in fitting the

data, although he recognized that the rejection of other models may have

been due to an inadequate number of sampling times. To test the ability

of the model to predict cell wall and dry matter intake, he estimated the

potentially digestible and indigestible pools, digestion rate constant








and lag time with in vitro rate studies, and estimated the rate constant for

passage by an equation based on cell wall intake. A total of 187 temperate

and tropical grasses and legumes were used. He found a correlation of .81

with dry matter intake. As with Golding's estimate of the passage constant,

Merten's was also derived from intake.

Both authors.recognized certain trends and concepts in their research.

First, that the independent estimation of the passage constant was neces-

sary if the model was ever to be used in a predictive manner. Golding

suggested the use of grinding energy. Second, the importance of this passage

constant was seen by both, recognizing that a given percent change in this

value resulted in a much greater change in forage quality than did an

equivalent change in the digestion rate constant. Third, Mertens felt that

the description of particle size degradation and passage by only one rate

constant was too simplified, and that the kinetics involved were more complex.

An attempt to rectify this neglect of particle size has recently been

presented (Mertens and Ely, 1979). The new model of fiber kinetics employs

three rumen particle size pools, two potentially digestible pools (recognizing

the microanatomical work of Akin), and two lower tract digestion pools. Its

basic assumptions are that particle size is reduced at differential rates

that are dependent on the particle size pool, that particles in the large

size pool are incapable of passing out of the rumen, and that particles from

the small and medium size pools pass at different rates through the omasum.

Due to the complexity of the model, evaluation was necessary by simulation

techniques, and the results agreed fairly well with those in the literature.

The models thus far discussed might be classified as structural models

in that they attempt to understand forage quality in terms of the structural









components of the diet: cell walls and associated microanatomical

structures. A group of researchers in California have for several years

been actively synthesizing a chemical model of the determinants of feed

quality (Ulyatt et al., 1976; Reichl and Baldwin, 1976; Vera et al., 1977;

Baldwin et al., 1977). Their objectives were threefold: 1) development

of a model based on currently accepted and defensible concepts; 2)

identification of specific aspects of ruminant digestion where current

concepts are inadequate; 3) development of a model which could be used to

test hypotheses regarding factors that affect feed quality.

The model presently consists of a central subunit to account of

microbial activity and growth, a summary computation subunit, and 12

chemical subunits consisting of soluble carbohydrate, organic acids,

starch, pectin, hemicellulose, cellulose, lipids, soluble proteins,

insoluble proteins, nonprotein nitrogen, lignin and ash. Particle size

is the only physical attribute of the forage contained in the model, and

only as a binary factor (it is either greater than or less than the maximum

size capable of passage). The effects of chewing are associated with forage

input (leafiness and tensile strength), and rumination is in the model but

only as an externally altered factor. It does not respond to other

factors in the model.

The model is described by over 800 equations, and thus by necessity

it was evaluated by simulation. Their conclusion was that the model

performed generally well but was weak in three major areas: 1) existing

analytical methods are inadequate in describing pectin and organic acids

in legumes; 2) the effects of rumination and chewing are inadequately

accounted for; 3) rates of passage of soluble and insoluble material from low






;1

quality feeds are poorly described.

The author of this dissertation felt that the Waldo model presented

a compromise between empirical methods of predicting forage quality and

more complex models that may better describe the ruminant digestive

process but may be useless for prediction due to our inability to

measure all pool sizes and rate constants. Furthermore, it was felt that

this model had not been adequately validated. Due to the many abbrevia-

tions and symbols used in this text, a table of definitions is provided

as an aid to the reader (table 1).









TABLE 1. ABBREVIATIONS AND SYMBOLS USED IN TEXT, TABLES AND FIGURES


Item


Animal and


Definition


chemical data


ADF
BW
BW 75
CELL
CP
DNDF


DNDFI


DM
DOM
DOMI
HEMI
LIG
NDF
NDFD
NDFI
OM
OMD
OMI
SIL
suffix '1'
suffix '75'


Model parameters
D
F


Potentially digestible pool
Fill
Digestion rate constant
Passage rate constant
Lag time
Time
Indigestible pool


Acid detergent fiber, % dry matter
Body weight, kg
Metabolic weight, kg
Cellulose, % dry matter
Crude protein, % dry matter
Digestible ash-free neutral detergent fiber, %
dry matter
Digestible ash-free neutral detergent fiber
intake, g/day
Dry matter
Digestible organic matter, % dry matter
Digestible organic matter intake, g/day
Hemicellulose, % dry matter
Permanganate lignin, % dry matter
Ash-free neutral detergent fiber, % dry matter
Neutral detergent fiber digestibility, %
Ash-free neutral detergent fiber intake, g/day
Organic matter, % dry matter
Organic matter digestibility, %
Organic matter intake, g/day
Silica, % dry matter
Per body weight
Per metabolic weight









Table 1 continued
I tem
Statistics
CV
ns


**


Definition


Coefficient of variation, %
Not significant
Correlation coefficient
Coefficient of determination
Multiple coefficient of determination
P<.05
P<.01


P<.l001














CHAPTER III
DIGESTIBILITY AND INTAKE OF CULTIVARS OF BERMUDAGRASS, DIGITGRASS AND
BAHIAGRASS FED TO SHEEP-- EXPERIMENT 1

Introduction

Bermudagrass (Cynodon dactylon), digitgrass (Digitaria decumbens)

and bahiagrass (Paspalum notatum) are major pasture grasses in the south-

eastern portion of the United States. Plant breeders have endeavored to

develop new cultivars that combine the characteristics of high yield,

disease resistance, cold tolerance, improved propagation and improved

animal performance. Several recently developed cultivars have shown

promise in one or more of these characteristics. The objective of this

study was to determine the quality of several previously released

cultivars of these grasses at different stages of maturity, in terms of

their intake and digestibility when fed to sheep, and to provide a wide

range of forage samples for the evaluation of laboratory methods used to

predict forage quality.

Materials and Methods

Grasses were grown during the summer and fall of 1975, on three

areas of the Agronomy Farm, "Green Acres," near Gainesville, Florida on

upland sandy soil. Three digitgrasses, 'Transvala', 'Slenderstem' and

X50-1 were grown in area A. Three digitgrasses, X124-4, X215-3 and

X45-2, and three bahiagrasses, 'Argentine', 'Paraguay' and 'Pensacola',

were grown in area C. Coastcross-1 bermudagrass was grown in area D.

Each area was divided into two replications and cultivars were allotted

randomly within each replication. The plots were fertilized with 560

kg/ha of 12-12-12 in May. Prior to staging(initial mowing and removal

34









of mowed material), all plots were fertilized with 74 kg/ha of ammonium

nitrate (33 percent N), and this application was repeated after the two-

week regrowth was harvested. Area D received an application of 0-10-20 at

a rate of 560 kg/ha plus micronutrients at this time. The harvesting

schedule is shown in table 2. After staging, the two-week old regrowths

from areas A and C were harvested from the entire plot, and subsequent

four, six and eight-week cuttings were second regrowths. In area D, only

the six-week cutting was a second regrowth. On September 2, at the time

of the four week harvest of area C, an infestation of striped grass loopers

(Mocis latipes) was recorded. Damage was particularly severe in area C,

and leaf blade disappearance was noted, especially among the digitgrasses.

Applications of an insecticide (Lanate) were made on September 3 and 19 in

all areas, and October 10 in area D. At each harvest the grasses were cut

with a flail harvester, artificially dried and stored in open weave bags.

Thirty-six sheep, averaging 44.5 kg liveweight, were allotted randomly

to hays in each of six periods. Hays from one field replication were fed

during the first three periods, and from the other replication during the

second three periods. Each period consisted of a fourteen-day preliminary

period during which sheep were acclimated to metabolism crates and hays,

followed by a seven-day collection period when the amount of hay offered,

refused (orts) and wasted (dropped on the floor), and the amount of feces

excreted were measured. Hays were fed ad libitum, adjusting the hay

offered to provide approximately 200 g of orts daily. Water, calcium

phosphate and trace-mineralized salt were available at all times. At the

beginning of each collection period the anticipated amount of hay needed

for the collection period was emptied, mixed, sampled and rebagged. Feces

were collected in canvas bags. The waste, orts and 20 percent of the












Item
Date of harvest

July 25

August 4-7

August 6-8

August 13

August 25

September 2-5

September 9

September 12

September 16-

September 23

September 26

September 30-(

October 10

October 21


TABLE 2. HAY HARVESTING SCHEDULE


A C D


Staged

21


Staged

21


Staged


19





October 3


62
(8)


aNumbers refer to ages (weeks)
following staging.


at harvest; subscripts refer to cutting









feces were dried at 50 C for a minimum of two days. Samples were pooled

by animal, hays and orts ground through a hammermill and all samples ground

in a Wiley mill to pass through a 4 mm screen. Approximately 200 g were

reground through a one mm screen for subsequent analysis.

Dry matter was determined at 105 C for 24 hours and organic matter at

500 C for a minimum of six hours for the determination of organic matter

intake (OMI), digestibility (OMD) and digestible organic matter intake

(DOMI). Neutral detergent fiber was determined by a modification of the

technique of Goering and Van Soest (1970), in which the decahydronapthalene

and sodium sulfite were omitted, and residues were filtered through glass

wool in porcelain gooch crucibles. Neutral detergent fiber digestibility

(NDFD), intake (NDFI) and digestible neutral detergent fiber intake (DNOFI)

were calculated.

Least squares analysis and correlations (Snedecor and Cochran, 1967),

and multiple comparisons (Duncan, 1955) were performed utilizing SAS

(SAS Institute, 1976). TYPE-3 sums of squares were used for the analysis

of variance tests of significance.

Results and Discussion

Determination of intake expressions. Prior to the final statistical

analysis, the logarithm of the intake variables in g/day was regressed upon

the logarithm of body weight (BW) plus all the discrete variables in the

model, in order to determine the power of BW appropriate for the expression

of intake. Due to the differences in areas in staging time, insect damage,

fertilization and regrowths (second or first), the following equation was

employed:

Log intake = Bo + Bix log BW + B2 x area + B3 x rep (area) + B, x

regrowth + B5 x area x regrowth + B6 x regrowth x rep (area) + B7 x









cultivar (area) + B8 x regrowth x cultivar (area) + B, x period (rep)

+ error

The coefficient Bi is the least squares estimate of the exponent of BW.

Estimates of this exponent were .92, 1.04, .93 and 1.02 for OMI, DOMI,

NDFI and DNDFI, respectively.

The proper expression of intake for roughage diets has been a matter

of some controversy, and this has been reviewed by Capote (1975). The

results reported here agree with those of Capote, in that greater variation

is removed by expressions that relate intake to BW than to BWs75. There

exists no uniform method of reporting intake in the literature; some

researchers prefer to report it as per body weight and others per metabolic

weight.

Models of forage intake (Mertens, 1973; Baldwin et al., 1977) assumes that

fill of the rumen and rates of material disappearance from the rumen limit

intake. Since rumen volume is theoretically related to weight, intake of

the same roughage by different size animals of the same class would be

expected to vary directly with the weight of the animals. The data pre-

sented here support that conclusion. Since digestibility is assumed to be

independent of animal weight within a species, DOMI should also be related

to animal weight, since DOMI is the product of OMD and OMI.

In utilizing various expressions of intake, it is important to keep

in mind the purpose to which these expressions will be put. In evaluating

various forages, using animal performance at ad libitum intakes as a

criterion, the manner in which livestock regulate intake is irrelevant. What

is essential to know is what animal performance, in terms of wool production,

milk production, work or growth, can be expected with these diets. Basal

metabolic rate is well established as a function of metabolic weight









(Kleiber, 1932; Brody and Procter, 1932). Methods of estimating net energy

values using comparative slaughter techniques use metabolic weight as a

reference base (Garrett, 1971). The National Research Council uses

metabolic weight as its reference base in computing the net energy

requirement for both maintance and gain in beef cattle (NRC, 1976).

Therefore, DOMI/B!i'75 (DOMI75) is the appropriate parameter to use when

comparing the value of several forages, under the assumption that animal

variation in the utilization of digestible energy of a given feedstuff is

best removed by metabolic weight.

Alternatively, if the purpose of an experiment is to either validate

models of ruminant intake, or to predict intake with laboratory techniques,

the correct expression for intake data is on a weight basis, assuming fill

is a limiting factor. All suggested models of rumen digest disappearance

that incorporate fill (when the diet consists of low quality feeds fed

ad libitum) are resolvable into expressions of OMI/BV! (OMIl) and OMD, or

NDFI/BW (NDFI1) and NDFD, if estimates of pools and rate constants can be

obtained. Validation of the model with animal data should be accomplished

using body weight as the reference base. If such models are proven

reasonably accurate and useful as tools to evaluate forage quality, then

these quantities should be converted to DOMI75 for comparative purposes.

Cultivar comparisons. OMD, OMI1, DOMI75, NDFD, and NDFI1 were

statistically analyzed using equation (1) with the 'log BW' term removed.

Analyses of variance are presented in the appendix (tables 15 and 16).

Significant m iin effects included area, regrowth (except for NDFI1),

cultivar with area, and period with replication for all expressions of

intake. The greater variation accounted for by the area x regrowth

interaction than by the cultivar (area) x regrowth interaction, could









be due to any of the factors involved in area differences, including

insect damage and harvest date. The period (rep) effect for intake

measurements can be attributed to to environmental changes over the

course of the digestion trial. Across all regrowths, the digitgrasses

in area C were superior in OMD (P<.01) to the bahiagrasses.

Least square means of intake and digestibility by cultivar, replica-

tion and regrowth are presented in appendix table 17. In light of the

significant regrowth x cultivar (area) effect, multiple comparisons among

cultivars were made only within each area-regrowth combination (tables 3-

6). Within area A, the four-week Transvalva was superior in OMD (68.8) and

NDFD (71.1) to the other two cultivars. No other significant differences

in digestibility were observed in this area. Intakes of Slenderstem were

the highest of the six-week regrowths, with a DOMI75 of 41.2 versus 32.3

for the Transvala, and 31.5 for X50-1. At eight weeks the ranking of these

cultivars was reversed, with X50-1 registering its highest DOMI75 over all

regrowths, and this was superior to the other two varieties. In area C,

two-week old X124-4 had a higher DOMI75 than most other varieties (50.2)

and was higher in DOMI75 at four weeks than two of the bahiagrasses. No

other significant differences in DOMI75 were observed after four weeks.

At six weeks, cultivar X124-4 had the highest OMD, and across all re-

growths in this area, this cultivar appeared to best maintain its quality.

Regrowth effects within each area are illustrated in figures 1-9. In

area C, OMD declined rapidly from the two-week regrowth to the four-week

regrowth in all cultivars. Part of this decline may have been due to

insect damage because, with the exception of X215-3, all cultivars in

area C recovered at six weeks, recording higher digestibilities than were













TABLE 3. LEAST SQUARES MEANS OF INTAKE AND DIGESTIBILITY OF TWO-WEEK REGROWTHS


Area (date Parameter (see table 1)
of harvest) Grass Cultivar OMD OMIl DOMI75 NDFD NDFI1
Area C
(August 4-7) Digit X124-4 70.4a 27.4a 50.2a 74.7a 19.4

X215-3 67.1bc 24.2ab 42.1bc 73.5ab 18.4

X46-2 72.3a 26.2ab 49.3ab 77.3a 19.1

Bahia Argentine 63.9cd 22.9b 38.8c 67.9C 18.0

Paraguay 64.6c 23.6b 39.8c 69.1bc 18.5

Pensacola 64.9c 24.2ab 41.3c 69.0bc 19.2
-----------------.------- -- ----- ------- ------ -----.....-....-..-..-
Area A
(August 6-8) Digit Slenderstem 66.7 24.8 52.7 72.0 18.4
Transvala 68.0 24.6 43.0 72.2 18.5

Area D
(August 25) Bermuda Coastcross-1 59.7 21.3 33.6 60.9 16.0

abcColumn means within area with different superscripts are different (P<.05).


















TABLE 4. LEAST SQUARES MEANS OF INTAKE AND DIGESTIBILITY OF FOUR-WEEK REGROWTH

Area (date Parameter (see table 1)


of harvest) Grass Cultivar OMD OMI1 DOMI75 NDFD NDFI1
Area C
(September 2-5) Digit X124-4 65.2a 21.4 38.7a 70.2a 17.1

X215-3 58.9bc 20.4 30.9ab 64.6b 16.8

X46-2 58.3bc 22.2 33.7ab 63.2b 18.2

Bahia Argentine 55.0c 23.1 33.7ab 57.7c 19.2
Paraguay 54.5 23.3 33.1b 57.3c 19.3

Pensacola 58.0b 20.3 309b 60.8bc 16.8

Area A
(September 12) Digit Slenderstem 61.8a 24.2 38.9 63.8a 18.2

Transvala 68.8b 24.5 44.2 71.b 18.4

X50-1 61.6a 24.4 39.1 64.6a 18.0

Area D


(September 23) Bermuda Coastcross-1 58.1


17.3 26.4 61.1 14.2


abcColumn means within area with different superscripts are different (P<.05).


---















TABLE 5. LEAST SQUARES MEANS OF INTAKE AND DIGESTIBILITY OF SIX-WEEK REGROWTHS

Area (date Parameter (see table 1)


of harvest) Grass Cultivar OMD OMI1 DOMI75 NDFD NDFI1
Area C
(September 16- Digit X124-4 68.5a 21.5 38.4 73.1a 17.3ab
19)
X215-3 58.7bc 20.1 31.0 63.5bc 16.3b

X46-2 61.8b 21.6 34.5 65.6b 17.2ab

Bahia Argentine 55.4c 23.7 34.5 58.0c 19.6a

Paraguay 57.0bc 21.4 32.3 59.9c 17.8a

Pensacola 59.5bc 22.5 35.5 62.2bc 18,7ab

Area A
(September 26) Digit Slenderstem 59.6 26.7a 41.2a 60.3 20.4a
Transvala 61.3 20.3b 32.3b 61.6 15.3b

X50-1 60.3 20.1b 31.5b 62.0 14.8b

Area D
(October 21) Bermuda Coastcross-l 59.4 20.0 30.7 61.1 16.6

abcColumn means within area with different superscripts are different (P<.05).














TABLE 6. LEAST SQUARES MEANS OF INTAKE AND DIGESTIBILITY OF EIGHT-WEEK REGROWTHS

Area (date Parameter (see table 1)
Area (date ------------ -


of harvest) Grass Cultivar OMD OMI1 DOMI75 NDFD NDFI1
Area C
(September 30- Digit X124-4 60.6a 19.7 30.9 64.3a 15.7
October 3)
X215-3 60.2a 19.7 30.9 64.3a 16.0

X46-2 59.2ab 22.1 33.7 61.2ab 17.4

Bahia Argentine 60.1a 22.2 34.4 62.9ab 18.4

Paraguay 54.1b 21.9 31.1 57.5b 18.2

Pensacola 57.1ab 20.2 29.6 61.0ab 17.0

Area A
(October 10) Digit Slenderstem 58.7 27.0a 37.0a 58.9 18.6

Transvala 57.8 21.4b 32.8a 57.6 16.0

X50-1 61.7 25.3a 41.3b 62.0 18.0

Area D
(October 21) Bermuda Coastcross-1 55.2 21.5 31.2 55.4 17.8


abColumn means within area with different superscripts are different (P<.05).

























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observed at four weeks. Most cultivars then showed a decline in OMD at

eight weeks. In area A there were no drastic shifts in OMD over regrowths,

with a gradual decline evident. OMD of the Coastcross-1 was generally

constant. OMI1 of the digitgrasses and the Pensacola declined rapidly from

the two-week regrowth to the four-week regrowth in Area C, while the OM11

of the Argentine and Paspalum remained constant. Changes in OMI1 from four

to eight weeks were more moderate. In area A, intake of cultivars

Transvala and X50-1 declined sharply from four to six weeks of regrowth and

then increased at eight weeks, while Slenderstem showed increases in OMIl

over these regrowths. OMI1 of Coastcross-1 in area D declined from two to

four weeks and then increased.

Ventura et al. (1975) observed OMD and DOMI75 in Pangola digitgrass at

two weeks regrowth to be 69.7 and 47.8, respectively. The average for the

two-week digitgrasses in this study were comparable at 68.9 and 45.5;

superior cultivars were X124-4 and X46-2 with OMD's of 70.4 and 72.3, and

DOMI75's of 50.2 and 49.3, respectively. Minson (1972) found the OMD and

DOMI75 of Pangola to be 68.9 and 33.8, respectively, for four-week old

material. At this stage of regrowth, digitgrasses in area C were inferior

in OMD and about equivalent in DOMI75 to the values Minson observed, while

the digitgrasses in area A were superior in DOMI75. Again, insect damage

was more severe in area C than in area A, and digitgrasses were damaged more

than bahiagrasses.

Moore et al. (1970) determined OHD and DOMI075 on four-week old regrowths

of Pensacola bahiagrass to be 61.6 and 28.8, respectively. The four-week

Pensacola in this study had an OMD of 58.0 and DOMI75 of 30.9. Four-week

regrowths of Coastal bermudagrass studied by Grieve and Osbourn (1965) had

dry matter digestibility of 64.8 and digestible dry matter intake per









metabolic weight of 42.4 compared to OMD of 58.1 and DOMI75 of 26.4 for the

four-week Coastcross-1 in this experiment.

Interrelationships among animal measurements. The relationship be-

tween various measures of intake and digestibility were examined with a

correlation procedure (table 7). NDFD was highly correlated with OMD

(r = .96), and DNDFI1 with DOMI1 (r = .95). NDFI1 was also highly

correlated with OMI1 (r = .94). These correlations suggest the importance

of cell wall intake and digestibility in determining forage quality. Intake

and digestibility as separate functions were not well related, the highest

being that between OMD and OMI1 (r = .47). This is in agreement with other

studies that have used several forage species. Minson (1972) demonstrated

that while within-species correlations between intake and digestibility

may often be has high as .96, correlations across several species will be

lower. In the research presented here, however, within-genus and within-

species correlations were also low. The average correlation between OMI1

and OMD, within genera was .42, and within species, .48.

TABLE 7. CORRELATION MATRIX OF INTAKE AND DIGESTIBILITY

Itema OMD DOM OMIl DOMI75 NDFD DNDF NDFII
DOM .96
OMIl .48 .49
DOMI75 .77 .75 .92
NDFD .96 .90 .41 .70
DNDF .71 .73 .24 .48 .81
NDFII .36 .38 .94 .82 .33 .35
DNDFI1 .73 .72 .88 .95 .74 .65 .88

aExplanation of abbreviations in table 1.








Sul iia a ry

Six cultivars of digitgrass (Digitaria decumbens), three cultivars of

bahiagrass (Paspalum notatum) and one cultivar of bermudagrass (Cynodon

dactylon) were harvested at different stages of regrowth (two to eight

weeks), artificially dried, and fed ad libitum to sheep in a digestibility

and intake trial. .Animal variation in organic matter intake (OMI) and

neutral detergent fiber intake (NDFI) was best removed by powers of body

weight that were closer to 1.0 than to .75. Organic matter digestibility

(OMD) varied from 57.8 to 72.3, 54.1 to 64.9, and 55.2 to 59.7 percent for

the three genera, respectively. Digitgrasses grown in the same area as the

bahiagrasses were superior in OMD (P<.01), with a mean digestibility of

63.4 versus 58.7 percent. Digestible organic matter intake per metabolic

weight (g/kg 75) for the three genera varied from 30.9 to 50.2, 29.6 to

41.3, and 26.4 to 33.6, respectively. No consistent regrowth effects,

either within genera or within species, were noted. This may have been

because there was severe insect damage to some of the grasses between the

two and four-week cuttings. The experimental digitgrass X124-4 and

'Slenderstem' digitgrass maintained higher digestible organic matter intakes

across all regrowths. The overall correlation between organic matter in-

take and digestibility of these grasses was .47, which was not improved

when considered within genera (r = .42) or within species (r =.48).

Digestibilities of organic matter and neutral detergent fiber were closely

related (r = .96) as were OMI and NDFI (r = .94).














CHAPTER IV
ESTIMATION OF CELL WALL DIGESTION RATES AND POTENTIALLY DIGESTIBLE AND
INDIGESTIBLE CELL WALL IN FORAGE AND FECES BY IN VITRO AND IN SITU
DIGESTION-- EXPERIMENT 2

Introduction

Several recent efforts to improve the prediction of forage quality

have involved the development of models that describe rumen digesta

disappearance (Waldo et a]., 1972; Mertens, 1973; Baldwin et al., 1977).

These models presuppose the existence of a potentially digestible fraction,

capable of complete digestion given sufficient fermentation time, and an

indigestible fraction which undergoes no digestion, regardless of the

duration of fermentation. The concept of these fractions originates from

the research of Blaxter et al. (1956) and Wilkins (1969), both of whom

showed a ceiling of digestion that was below 100%, the latter author

employing long term in vitro digestions. Goto and Minson (1977) had

difficulty in quantifying these fractions in vitro. Reinoculation of

forage samples after four days incubation resulted in a 10.2% increase in

dry matter digestibility and up to a 28% increase in lignin solubility.

These authors suggested that 21 days was the minimum incubation time. In

vitro derived estimates of lag times, rate constants and digestible and

indigestible pools have been employed to estimate the corresponding in vivo

values (Mertens, 1973; Golding, 1976). Such use assumes that in vitro values

are approximately equal to, or highly correlated, with in vivo values.

The purpose of this experiment was threefold: (1) to determine the

relationship of in vitro estimates of lag times, rate constants and pot-

entially digestible and indigestible fractions to estimates obtained in situ;









(2) to determine in which of these parameters two hays, of the same species

but differing in quality, differed; and (3) to determine if the indigestible

fraction is fully recoverable in the feces.

Materials and Methods

Digestion and intake trial. Two bermudagrass (Cynodon dactylon) hays,

one designated high quality (HQ) and the other low quality (LQ), were fed

to mature wethers. The wethers were subjected to a fourteen day preliminary

period during which they were acclimated to metabolism crates and hays,

followed by a seven day collection period when the amounts of hay offered,

refused (orts) and wasted, and the amount of feces excreted were measured.

The hays were fed ad libitum, adjusting the hay offered to provide approx-

imately 200 g of orts daily. Water, calcium phosphate and trace mineralized

salt were available at all times. Feces were collected in canvas bags. The

waste, orts and 20 percent of the feces were dried at 50 C for a

minimum of two days. Samples were pooled by animal, ground to pass through

a 4 mm screen and approximately 200 g ground through a 1 mm screen for

subsequent analysis.

Rate study. The HQ and LQ hays were fed to two fistulated steers in a

switchback design. Samples of hay, orts and feces from the above digestion

trial were composite by hay, across animals, using amounts proportional to

that obtained from individual animals. Samples of both hays were incubated

in vitro and in situ, two replications per cell. Orts and feces from sheep

fed the LQ hay (LQ orts and LQ feces) were incubated only when the steers

were consuming LQ hay. Similarly, orts and feces from sheep fed the HQ hay

(HQ orts and HQ feces) were incubated only when the steers were consuming

the HQ hay. Residues were recovered after 2, 4, 6, 10, 15, 21, 27, 33, 39,

45, 51, 57, 69, 84, 100 and 129 hours of incubation, both in vitro and in

situ.








Acropor (A-5000)' copolymer material, with a porosity of 5 microns,

was used in the in situ study. Bags, 12.5 cm long with a diameter of 2.5

cm, were fabricated by sealing the seam and one end with epoxy glue. One

half gram of sample was placed in each bag, and the open end sealed with

epoxy. Four of these bags, consisting of samples of both hays, and the

orts and feces representing the hay which the steer was consuming, were

placed in nylon utility net bags2, 17 x 16 cm, with a mesh size of .3 cm.

The addition of three marbles (1.5 cm diameter) to each bag gave them the

approximate density of water. Sixteen of these bags were attached to a

doubled nylon monofilament (36.3 kg test) and placed in the rumen of each

steer. To insert the bags in the rumen it was necessary to remove some

digesta, but it was replaced when the bags were in place. At each

recovery time, one net bag (with Acropore bags) was removed, washed with

cold running water, placed in ice and taken to the laboratory. Acropor

bags were removed from the net bags, thoroughly washed with cold running

water, and refrigerated until analysis, which was carried out within 40

hours of removal from the rumen.

Within one week after the last in situ sample was removed, rumen fluid

was collected for the in vitro phase of the study. The fluid was strained

through cheesecloth and glass wool, and combined with a buffer solution

(McDougall's Saliva), one part rumen fluid to four parts buffer. Samples

of hay, orts and feces, .5 g each, were innoculated in 100 ml polyethylene

centrifuge tubes with 50 ml of inoculum. The tubes were flushed with C02,

capped with rubber stoppers fitted with Bunsen valves, and incubated at

Gelman Instrument Company, Ann Arbor, MI

2Sterling Marine Products, Montclair, NJ






,,0

39 C, swirling the tubes three times daily. At each recovery, centrifuge

tubes were placed in an ice water bath and refrigerated until analysis

which was carried out within 40 hours.

Chemical analyses. Dry matter was determined at 105 C for 24 hours

and organic matter at 500 C for a minimum of six hours. Protein was

determined by Kjeldahl (AOAC, 1970). Acropor bags were opened at

both ends and the residue washed into 600 ml Berzelius beakers with 100 ml

of neutral detergent fiber solution. Centrifuge tubes were emptied into

600 ml Berzelius beakers and rinsed with 100 ml neutral detergent solution

into the beakers. Ash-free neutral detergent fiber (NDF) as a percent of

dry matter was determined by a modification of the technique of Goering

and Van Soest (1970) in which the decahydronapthalene and sodium sulfite

were omitted, and residues were filtered through glass wool in porcelain

gooch crucibles.

Statistical analysis and estimation of model parameters. Initial

analysis of ODF residue, as a function of time and the discrete variables

in the statistical model, was accomplished by least squares analysis

(Snedecor and Cochran, 1967), using the GLM procedure of SAS (SAS Institute,

1976). TYPE-1 sums of squares were used for polynomial effects and TYPE-3

sums of squares for all other effects. NDF residue was then fitted, one

run at a time,to three models, utilizing the NLIN procedure of SAS:

NDF residue = De-ki(t-L)+ U (for t>L) (1)

= D + U (for t
NDF residue = De-k"1(t-L)+ D02-k'2(t-L) + U (for t>L) (2)

= Di + D2 + U (for t
NUF residue = (-D/(k,1-kl,)(k14e-k13t -k3e-k, t) + U (3)

D, Di and D2 are potentially digestible pools, expressed as percent of NDF.








U is the indigestible pool, t is time (hr), L is lag time (hr) and e is

the base of the natural logarithm. Rate constants for digestion (hr-'),

are ki, k1j, k12, ki3, and k14. Model (1) is a lag time model, model (2)

assumes two separate potentially digestible pools ( a fast and a slow

digesting pool, Di or D2), each with its own rate of digestion (ki1 or

ki2), and model (3) is a sequential model that assumes NDF disappearance

occurs in two phases, disappearing at k1i and k14, respectively. Estimates

of model parameters were analyzed by least squares analysis using the

original statistical model, but with time omitted.

Results and Discussion

Digestion and intake trial. Results of the digestion trial are pre-

sented in table 8 and analysis of variance in appendix table 18. The HQ

hay had a higher OMD and DOMI75 than did the LQ hay (P<.001), and a higher

OMI1 (P<.01). It was also lower in NDF and higher in CP.

Comparison of hays and methods of digestion. Analysis of variance of

NDF residue as a function of time is presented in appendix table 19. A

quartic time effect was present (P<.01). Method of digestion (in vitro vs.

in situ) as a main effect was not significant, but all interactions with

method were (P<.001). These interactions are illustrated in figure 10.

With increasing time of digestion the difference in NDF residue between

the HQ and LQ hays widened, from a 6% difference at zero time to an 11%

difference at 129 hours. A larger difference between the two methods was

observed in the LQ hay than in the HQ hay. The method x time interaction

was evidenced by the in vitro method yielding less NDF residue than the

in situ method prior to 10 hours. After 10 hours, greater digestion was

observed with the in situ method, and this difference increased with in-

creasing duration of digestion. Wheeler et al. (1979) observed greater

digestion at 72 hours in situ than invitro. The higher rate of in vitro









digestion during the early hours of fermentation might be due to the more

immediate contact of sample and inoculum that occurs in the centrifuge

tube, compared to Acropor bags where the small porosity might restrict

liquid flow into the bags. It was also possible that the dilution of

the NDF solution with 50 ml inoculum during analysis of the in vitro

samples might affect NDF values, although intuitively this should increase

rather than decrease NDF. Mertens (personal communication) suggested that

TABLE 8. IN VIVO AND CHEMICAL CHARACTERISTICS OF HQ
AND LQ HAYSa

Hay OMDa OMI1 DOMI75 NDF CP
HQ 54.3 24.7 36.7 73.2 10.5

LQ 44.3 22.2 25.4 78.1 8.3

aExplanation of abbreviations in table 1.

the initial rapid rate of in vitro fermentation may be due to analytical

technique. An experiment to examine this is described at the end of this

chapter.

Differences in hays and method observed in the initial analysis of

variance are not readily resolvable into biologically meaningful comparisons.

The conversion of the data to expressions of model parameters affords a

better method of comparison and provides estimates that can be introduced

into models of rumen digesta disappearance. When NDF residue was fitted

to models (2) and (3), estimates of the rate constant for the fast

digesting pool (kH ), and the initial rate constant (k12), respectively,

nearly always gave confidence intervals that included zero. For several

runs the NLIN procedure was unable to solve the equations. Therefore,

these two models were discarded.

Estimates of the rate parameters (for model 1) are given in table 9 and

selected contrasts in table 10. For the hays only, which were in a cross-






























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TABLE 10. SELECTED CONTRASTS

Parametera
Contrast D UDTT71TF-TNDF-- U (%DM) ki L

HQ vs. LQ hays *** *** *** ns ns

In vitro vs. in situ for hays *** ** ** ns ns

HO hay vs. HQ ortsb ** *** ** ns ns

LQ hay vs. LQ orts ns ns ns ns ns

HQ hay vs. HQ feces *** ***

LQ hay and LQ orts vs. LQ feces *** *** *** *** *

aExplanation of abbreviations and symbols in table 1.

Not orthogonal.








over design with steer diet, least squares analysis of variance is provided

in table 20. All differences in hays and method occurred in the estimates

of the potentially digestible (D) and indigestible (U) cell wall pools,

both of which were different for the hays and the two methods. No dif-

ferences were detectable in the digestion rate constant or the estimates

of lag time, the latter being highly variable from run to run (CV = 19.0%).

The HQ hay had a higher potentially digestible fraction and a lower

indigestible fraction than did the LQ hay, whether expressed as a percent

of dry matter or percent of NDF. The in situ method provided higher

estimates of the potentially digestible fraction and lower estimates of the

indigestible fraction than did the in vitro method. Estimates of NDF

residue at 129 hours were always greater than actual values, indicating

that simple first order kinetics, using a single digestible pool, may not

adequately describe digestion kinetics. This effect was also observed by

Mertens (1973) in vitro. Disappearance of NDF from 84 to 129 hours was

approximately linear, rather than approaching an asymptote (figure 10).

Comparison of hays, orts and feces. Analysis of variance of NDF

residue as a function of time by steer diet is provided in appendix table

21. The analysis of variance for parameter estimates is given in appendix

tables 22 and 23. No difference between the LQ hay and LQ orts were

observed (tables 9 and 10). Pool estimates of the potentially digestible

fraction were higher for the HQ hay than for the HQ orts, even though the

HQ orts contained approximately 1% less NDF at zero time (figure 11).

Thus it would be incorrect to conclude that no animal selectivity took

place just because the cell wall content of hay and orts were approximately

equal.



































C-
4)








C'j







cr
C-
CD



































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C)
sa -a

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--'






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SC-


CC) U C) CC) CC
CD C)C) C) C)

(vu tu~ ;~rn~ed) np~s~ VO









Comparison of hay with feces showed significant differences in all

parameters (tables 9 and 10). It is expected that fecal material would be

higher in indigestible cell wall, and lower in potentially digestible cell

wall. However, if the potentially digestible pool is truly of a uniform

nature, i.e. a combination of unlignified hemicellulose and cellulose

(Minson, 1976), the digestion rate constant should be equivalent with that

of the respective hay. Forage microanatomical studies by Akin (Akin and

Burdick, 1975; Akin et al., 1974) strongly indicate that more than one

potentially digestible pool exist. However, as observed earlier, the

resolution of single value data into two rate curves is no easy matter. It

demands extensive sampling times and extremely accurate data (Gutfreund,

1972), the latter being difficult when dealing with a chemically hetero-

geneous substance such as cell walls.

Digestibility of the indigestible fraction. The use of an indigestible

fraction makes determination of rate constants tractable. If it satisfies

the requirements of an 'ideal' marker (Engelhardt, 1974; Faichney, 1975),

it might prove useful in rate of passage studies and in determining

digestibility when complete fecal collection is impractical. Berger et al.

(1979) used indigestible NDF and ADF as passage markers, determined by 96

hour in vitro digestions followed by NDF and ADF analyses. In this study,

application of the estimates of U in hay, orts and feces to the intake and

excretion data of the digestion trial resulted in positive digestibilities

of U in both hays, irrespective of method of estimation. Using in situ,

estimates of U, the digestibilities of U were 18.1 and 14.3 percent for the

HQ and LQ hays, respectively; using in vitro estimates, digestibilities

were 15.6 and 14.8 percent, respectively.


~









Effect of method of determination of the NDF content of five hays. An

experiment was conducted to determine if analytical factors were the cause

of the observed rapid rate of in vitro digestion during the first few hours.

Five hays were studied, in a 5 x 6 design, with the following treatments:

A. Hay samples placed in centrifuge tubes, the following solutions added,

centrifuge tubes immediately iced, and analyzed for NDF:

1. 50 ml normal inoculum (40 ml buffer + 10 ml rumen fluid)

2. 40 ml buffer + 10 ml sterilized rumen fluid

3. 50 ml buffer

B. Hay samples placed in Berzelius beakers, the following solutions added,

and analyzed for NDF:

4. 50 ml buffer

5. 50 ml deionized water

6. no solution added (control)

The results of this trial appear in table 11 and analysis of variance

in appendix table 24. All samples containing buffer (no.'s 1, 2, 3, 4)

showed a drop in NDF compared to the control (no. 6) of approximately 2

percent. The addition of 50 ml deionized water (no. 5) did not have this

effect and NDF values were the same as in the control. Evidently, the

buffer solution solubilizes, either by itself or in combination with NDF

solution, a portion of cell wall that is not solubilized by the NDF

solution alone. Therefore, part of the apparent digestion of NDF which

took place in the first few hours of in vitro digestion (figure 10) was an

artifact of the in vitro technique.





















TABLE 11. EFFECT OF METHOD OF DETERMINATION ON NDF CONTENT OF FIVE HAYS

Treatment
A B
Genus Cultivar Regrowth 1 2 3 4 5 6
Digit X46-2 2 63.5 63.7 63.9 64.1 64.9 65.8

Bahia Argentine 2 72.4 72.4 72.1 72.3 74.9 74.6

Bahia Paraguay 4 77.8 76.9 76.6 77.0 79.8 79.7

Ditit Slenderstem 6 68.6 69.3 69.0 67.9 69.8 70.3

Bermuda Coastcross-1 8 74.2 73.3 74.6 73.9 76.4 76.8

71.3 71.1 71.2 71.0 73.2b 73.4b
x
aA=samples initially in centrifuge tubes; B=samples initially in beakers;
1=50 ml normal inoculum; 2=40 ml buffer + 10 ml sterilized rumen fluid;
3=50 ml buffer; 4=50 ml buffer; 5=50 ml deionized water; 6=no solution
added (control).
beans in a row with different superscripts are different (P<.01).








Suimman ry

Two bermudagrass hays (Cynodon dactylon), one high quality (HQ) and one

low quality (LQ), were fed ad libitum to wethers in a digestion and intake

trial. Organic matter digestibilities (%) were 54.3 and 44.3, and organic

matter intakes per body weight (g/kg) were 25.7 and 22.2, respectively.

Samples of hays, orts and feces from this trial were digested in vitro,

and in situ (utilizing Acropor bags), in two fistulated steers fed these

hays, in a crossover design. Samples were removed at 16 times, ranging

from 2 to 129 hours and analyzed for neutral detergent fiber (NDF). NDF

residue was fitted to the set of equations: NDF residue = De-k(t-L) +U for

t>L, and NDF residue = D+U for t
wall (%), k1=digestion rate constant (hr-1), t=time (hr), L=lag (hr), and

U=indigestible cell wall (%). No differences were detected in k] or L,

the latter being highly variable between runs. All differences between

hays and method of digestion occurred in the estimates of D and U. The HQ

hay had a higher fraction of D than did the LQ hay. In situ digestion pro-

vided higher estimates of D than did in vitro digestion. Orts from sheep

fed the HQ hay had less of fraction D than did the HQ hay, although they

were approximately equal in NDF content. Application of the estimates of

U in hay, orts and feces to the intake and excretion data of the digestion

trial resulted in positive digestibilities of U in both hays, irrespective

of method of digestion, with a range of 14.3 to 18.1 percent. In a

separate experiment the addition of McDougall's saliva to 100 ml neutral

detergent reagent solubilized an additional two percent of forage NDF.

Therefore, apparent increased early digestion of NDF in vitro, compared to

in situ, was an artifact.














CHAPTER V

EVALUATION OF A MODEL OF RUMEN CELL WALL
DISAPPEARANCE-- EXPERIMENT 3

Introduction

Several models of rumen digesta disappearance have been suggested

(Baldwin et al., 1977; Mertens and Ely, 1979). Validation of these models

has been accomplished by computer simulation, whereby the effect of

alteration in the parameters of the model are compared with values found

in the literature, since direct estimates of the pool sizes and rate con-

stants have not been possible. Waldo et al. (1972) proposed a model of

rumen cellulose disappearance that is of less complexity than the above

models. It describes two cellulose fractions, a potentially digestible

pool and an indigestible pool, both of which can disappear from the rumen

by passage, but only the former is capable of being digested. Mertens

(1973) applied this model to rumen cell wall disappearance. He used in

vitro estimates of the rate constant for passage by an equation relating

cell wall intake to passage. Predicted dry matter intake was correlated

with actual dry matter intake (r = .81) in a wide variety of 187 forages.

Golding (1976) applied the Waldo model to organic matter intake, again using

in vitro techniques, and estimated the passage rate constant with an equation

relating lignin intake to passage. A correlation of .96 was found between

predicted and actual digestible organic matter intake per metabolic weight.

Both lertens (1973) and Golding (1976) used intake to both predict and

validate the Waldo model. Thus, there was an internal correlation that may

have lead to spurious conclusions about the validity of this model. The









purpose of this experiment was to evaluate the Waldo model, as applied to

cell walls, using a procedure that was independent of the methods used to

generate parameter estimates.

Materials and Methods

In vitro rate study. From the 76 hay samples used in experiment 1,

60 were selected (table 12) on the basis of the number of animal observations

(>1), coefficient of variation (CV) for organic matter intake (<19) and CV

for organic matter digestibility (<10). In each of 5 runs, hay samples were

incubated in vitro and NDF residues recovered at 96 hours and at one of the

following: 12, 24, 36, 48 or 60 hours. Thus there were five estimates of

the 96 hour residue.

Rumen fluid was collected from a fistulated crossbred steer, maintained

on bermudagrass hay. One hour prior to obtaining rumen fluid, the steer was

fed 600g soybean meal. The rumen fluid was strained through cheesecloth and

glass wool, and combined with a buffer solution (McDougall's Saliva), one

part rumen fluid to four parts buffer. Samples of hay were inoculated in

centrifuge tubes with 50 ml of inoculum. The tubes were flushed with CO2,

capped with rubber stoppers fitted with Bunsen valves, incubated at 39 C,

and swirled three times daily. At each recovery, centrifuge tubes were

placed in an ice water bath and refrigerated until analysis, which was

carried out within 40 hours.

Dry matter was determined at 105 C for 24 hours and organic matter at

500 C for a minimum of six hours. The centrifuge tubes were emptied into

600 ml Berzelius beakers, and rinsed with 100 ml neutral fiber solution

into the beakers. Ash-free neutral detergent fiber (NDF), as a percent of

initial dry matter, was determined by a modification of the technique of

Goering and Van Soest (1970), in which the decahydronapthalene and sodium









sulfite were omitted, and residues were filtered through glass wool in

porcelain gooch crucibles.

TABLE 12. IN VIVO AND CHEMICAL CHARACTERISTICS OF HAYSa

Genus (# of cultivars) OMD OMIl DOMI75 NDF CP

Cynodon (1) 54-62 17-24 26-36 68-80 5-14

Digitaria (6) 55-73 19-30 28-57 73-80 6-14

Paspalum (3) 54-67 18-26 26-45 63-76 4-13

aExplanation of abbreviations in table 1.

Estimation of digestion rate constant, and potentially digestible and

indigestible pools. Examination of the raw data indicated that the first

twelve hours of digestion were part of the lag time (L) for many of the

samples, and residues collected at this time were not included for parameter

estimation. The 96-hour residue was considered the indigestible fraction

(U), and ((hay NDF content)-U) considered the potentially digestible

fraction (D). Residues at 96-hour were subtracted from the residues at 24,

36, 48 and 60 hours, and subjected to a log transformation for regression

prediction of the digestion rate constant (kI) in hr-1 and the zero-time

intercept of the regression line (Di), where time (t) is the dependent

variable:

NDF residue = De-k't+u

(NDF residue)-U = D.e

In((NDF residue)-U)= In(Di)-kit

The sum of D. + U was almost invariably greater than the hay NDF content

due to the assumption that digestion is not initiated until the end of the

lag time. Lag time was determined as:
L = (In(Di)-In(D))/k,









Estimation of rumen NDF digestibility. The Waldo model yields two

equations, one for intake and one for digestibility. If the digestion rate

constant, the potentially digestible pool and the indigestible pool are

estimated by the in vitro procedure, and a constant rumen fill assumed,

these values can be entered into the intake equation, and the equation can

be solved for the passage rate constant (k2).

It is, of course, worthless to re-enter this estimate into the intake

equation to evaluate the model since the estimates of intake will be equal

to actual intake. However, the parameters can be entered into the equation

for rumen NDF digestibility and the resulting estimates for digestibility

should, if the model is accurate, yield better estimates for digestibility

than do any of the parameters alone.

Two methods for predicting rumen NDF digestibility, depending on how

k2 is estimated, are possible. The first assumes that lag (L) is an arti-

fact of the in vitro system and does not occur in vivo. In this case cell

wall intake (NDFI) is a function of rumen cell wall fill (F), the rate

constant for digestion (ki), the rate constant for passage (k2), the

potentially digestible cell wall as a fraction of total cell wall (D), and

the indigestible cell wall as a fraction of total wall (U):

NDFI/hr = F/[(D/(kl+k2))+(U/k2)]

Dividing both sides by body weight (BW):

(NDFI/hr)/BW = (F/BW)/[(D/(ki+k,))+(U/k2)]

Assuming F is a constant 16.03 g per kg BW (Mertens, 1973):

(NDFI/hr)/BW = 16.03/[(D/(kk+k2))+(U/k2)]

Converting intake to a daily basis per kg BW (NDFI1), by multiplying by 24:
NDFIl = 385/[(D/(k1+k2))+(U/k2)J








Collecting terms, this then takes the form of the quadratic equation:

0 = 1385/NDFIl](k2)2+[(385kl/NDFIl)-1l(k2) Ukl

This equation can be solved for k2 and entered in the digestibility equation

to yield a predicted cell wall digestibility (NDFDPI):

NDFDPI = D (kl/(k1+k2))

The second method of estimating NDFD (NDFDP2) is with the assumption

that the lag observed in vitro occurs in vivo. If we again assume a con-

stant fill per unit body weight, than daily cell wall intake per kg BW is

defined by:

NDFI1 385/[((D/k2)l-e-k2L)) + ((De-k2L)/(ki+k2)) + (U/k2)]

This cannot be solved directly for k2, but a solution can be achieved by

Newton's method for approximating the roots of equations (Thomas, 1972).

Predicted digestibility NDFDP2 will then be:

NDFDP2 = D[e-k2L((k k2L)/(k+k2))]

The two estimates of rumen NDF digestibility should be highly correlated

with in vivo NDF digestibility (NDFD), if the model is valid. Regression

and correlation (Snedecor and Cochran, 1967) were performed with the

computer statistical package, SAS (SAS Institute, 1976).
Results and Discussion

NDF residues (percent of initial dry matter) are presented in appendix

table 25 and model parameter estimates in appendix table 26. Model parameter

ranges by genera are presented in table 13. Estimates of these parameters

fell within the ranges previously found for tropical grasses (Mertens, 1973).

The regression equation for estimation of digestion parameters yielded an

r' of .98 and a CV of 7.4. Digitgrasses had generally lower quantities of

indigestible cell wall (U) than the other two genera, when expressed on a

dry matter basis. Variation in D, when expressed as a percentage of dry

















TABLE 13. MODEL PARAMETER RANGESa


Genus
Parameter Cynodon Digitaria Paspalum

D (%Di) 42-48 44-57 45-56

D (%NDF) 53-64 62-85 62-77

U (%DM) 25-36 10-28 17-30

ki .043-.069 .038-.078 .035-.062

k21b .020-.036 .015-.032 .020-.038

k22C .025-.041 .020-.039 .030-.050

L 5.2-11.2 -1.0-17.4 11.5-19.3

aExplanation of abbreviations and symbols in table 1.

k21 derived from model that assumes zero lag (L) in vivo.

Ck22 derived from model that assumes lag (L) in vivo is same as L in vitro.









matter,was less than when expressed as a percentage of NDF,suggesting that

the former is a more constant proportion of forage dry matter than U or NDF.

Correlations of predicted NDFD with in vivo NDFD were .75 for NDFDP1

and .34 for NDFDP2. Evidently, the lag contributed random variation to the

estimate. Correlation of NDFDPI was not above what has been reported

elsewhere for correlations of in vitro estimates with in vivo digestibilities

(Weller,1973; Velasquez, 1974). Since NDFDP1 and NDFDP2 are predictions of

rumen cell wall digestibility, the remaining potentially digestible fraction

was mathematically subjected to further digestion, simulating digestion and

passage in the large intestine, using the same rate constants for digestion

and passage that were estimated for the rumen. This improved the

correlation with NDFDPI to .81, but had no effect on the correlation with

NDFDP2. It should be observed, however, that as the remaining potentially

digestible fraction is subjected to increasing digestion, the limit is its

complete digestion, which in this case is the 96-hour in vitro digestibility.

This highest correlation with NDFD was with parameter D, the 96-hour

in vitro digestibility. In vitro digestibility at each recovery time was

calculated (figures 12 17). The coefficient of determination between

in vivo and in vitro NDF digestibility increased with increasing fermenta-

tion times from .16 at 12 hours to .80 at 96 hours, with a concomitant

decline in deviation from the regression lines. A considerable difference

existed between the standard 48-hour in vitro digestibility and that at 96

hours, suggesting that 96 hours might provide more accuracy in predicting

NDFD in routine forage screening procedures. A possible explanation for

this result is that actual retention times of digesta subject to digestion

are closer to 96 hours, when the large intestine is included, and that

digestion of potentially digestible cell wall approaches completion during








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this time. Correlations of ki and L with in vivo measurements were not high

(appendix table 27).

Correlations between model parameters and chemical composition (Hartadi,

unpublished data) are presented in table 14. Highest correlations were

between lignin (as a percent of dry matter or of acid detergent fiber) and

the potentially digestible and indigestible pools. This is consistent with

the concept that lignin serves as a limit to potential digestibility, rather

than an inhibitor of the rate of digestion (Minson, 1976). Lag time was

not highly correlated with any chemical component. The correlations of

the digestion rate constant with NDF and cellulose (-.71 and -.76) suggest

that with increasing amounts of fiber in forages, the character of the

potentially digestible fraction changes. This is consistent with the

concept that there is more than one potentially digestible pool (Mertens.and Ely,

1979), and that a single pool model, while conceptually more desirable

than empirical models, is inadequate in describing rumen cell wall dis-

appearance. However, as previously stated, in the absence of the ability to

measure rates of digestion on individual pools, the ability to resolve

one-component data into two-component models is dependent on strict

adherence to first order kinetics. With the degree of biological variation

that occurs in the measurement of NDF digestion over time, accurate fitting

of a two-component model is not feasible. An additional difficulty in

identifying pool sizes and rate constants would occur if rumination tends

to free some of the lignin-bound cellulose, thus increasing the potentially

digestible fraction.

















TABLE 14. CORRELATIONS BETWEEN MODEL PARAMETERS AND CHEMICAL COMPOSITIONa

Chemical component
Parameter NDF CP ADF CELL LIG(%ADF) LIG HEMI SIL

D (%DM) .08 .32 -.07 .27 -.77 -.61 .07 -.24

D (%NDF) -.56 .56 -.41 .23 -.79 -.79 -.25 .09

U (%DM) .73 -.60 .48 .37 .71 .77 .34 -.20

ki -.71 .53 -.61 -.76 -.23 -.45 -.02 <.01
b
k21 .70 -.39 .30 .36 .45 .50 .41 -.37

k22c .53 -.18 .03 .18 .03 .06 .55 -.30

L .34 -.13 -.06 .11 -.27 -.22 .47 -.22

aExplanation of abbreviations and symbols in table 1.

bk21 derived from model that assumes zero lag (L) in vivo.

ck22 derived from model that assumes lag (L) in vivo is same as L in vitro.









Summary

Sixty hay samples, representing three genera (Cynodon dactylon,

Digitaria decumbens and Paspalum notatum) and previously fed to sheep in

Experiment 1, were subjected to in vitro neutral detergent fiber (NDF)

digestion for 12, 34, 36, 48, 60 and 96 hours. NDF residue at 96 hours was

assumed to be the indigestible NDF (U), and initial NDF less U was assumed

to be the potentially digestible pool (D). The rate constant for digestion

(k,) was determined by the regression equation:
In((NDF residue)-U) = In (Di)-kit

where In (Di) is the zero-time intercept of the regression line and t is

time (hr). Lag time (L) was determined as:

L = (1n(Di)-In(D))/k,

The rumen NDF passage rate constant (k2) was estimated using these parameter

estimates, an assumed rumen NDF fill of 16.03 g per kg body weight, in vivo

NDF intake per kg of body weight (NOFI1) and one or the other of two options:

1) that digestion lag does not occur in vivo:

0 = [385/NDFIl](k2)2+[(385kl/NDFIl)-l](k2)-UkI

which is in the form of a quadratic equation; or

2) that digestion lag does occur in vivo, in which case:

NDFI1 = 385/[((D/k2)(l-e-k2L))+((De-k2L)/(kl+k))+(U/k2)]

which was solved for k2by means of an iterative method for finding the roots

of equations. Predicted rumen NDF digestibilities are then defined as:

1) NDFDP, = D(k,/(k1+k2)

2) NDFDP2 = D[e-k2L-((k2e-k2L)/(k,+k2))]

Correlations (r) of NDFDPI and NDFDP2 with in vivo NDF digestibility were







8:

.75 and .34, respectively. In vivo NDF digestibility was more closely

related to D (r = .89), which is the 96-hour in vitro NDF digestibility,

than to NDFDPi, NDFDP2, or any of the other model paratmeters. The co-

efficient of determination between in vivo and in vitro NDF digestibility

increased with increasing fermentation times, from .16 at 12 hours to .80

at 96 hours.




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