Quantitative review of ruminal and total tract digestion of mixed diet organic matter and carbohydrates

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Quantitative review of ruminal and total tract digestion

of mixed diet organic matter and carbohydrates

Harry Archimède, Daniel Sauvant, Philippe Schmidely

To cite this version:

Harry Archimède, Daniel Sauvant, Philippe Schmidely. Quantitative review of ruminal and total tract

digestion of mixed diet organic matter and carbohydrates. Reproduction Nutrition Development, EDP

Sciences, 1997, 37 (2), pp.173-189. �hal-00899949�

Original

article

Quantitative

review of ruminal and total

tract

_digestion

of

mixed diet

_organic

matter

and

_{carbohydrates}

H

Archimède

D

Sauvant.

P

_Schmidely

Station de nutrition et d’alimentation, Inra, Ina-PG, 16, rue _{Claude-Bernard,}

75231 Paris cedex 05, France

(Received 19 March 1996;

accepted

20 November ₁₉₉₆₎

Summary ―

The mean _response and main factors of variation

(level

of concentrate, nature of

car-bohydrate

in the concentrate and level of _intake) for

_organic

matter, cell wall material, starch

diges-tion and microbial

_synthesis

in the

_{gastrointestinal}

tract of ruminants were

_{quantitatively}

reviewed

_using

a data base

_involving

_{157 papers. The ruminal}

_digestion

(mean ± SE%) of

_organic

matter, cell wall

material, and starch were 45.2 ± 11.2 (n = _{553), 47.7} ± 17.7 (n = 348), and 74.1 ± 16.2 (n = _140),

respectively

and the

_proportion

of each _component

_digested

in the rumen in relation to total tract

digestibility

was 64.7 ± 12.3, 78.8 ± 18.5 and 80.5 ± 16.3,

respectively.

The

_efficiency

of microbial

synthesis (g

of microbial

_protein

/

_kg

of

_organic

matter

_truly

fermented in the rumen) and the

_proportion

of microbial

nitrogen

in the total amount of

_{nitrogen leaving}

the stomachs (%) were, 23.6 ± 9.3 (n =

320) and 55.1 ± 16.5 (n =

_{289), respectively.}

_{The ruminal}

digestion

of

_organic

matter increased

by

2

_points

_{for every 10 percent} increase in concentrate

_{incorporation.}

The ruminal

digestion

of cell wall material was maximal when the concentrate

_{incorporation}

in the diet was 30%. When the

rumi-nal

_digestion

of cell wall _decreased, the substitution of ruminal

_{digestion by}

intestinal

_digestion

was

partial

(10%). The

_efficiency

of microbial

synthesis

was

_optimal

when the level of concentrate

incor-poration

was 40%. The nature of the

_{carbohydrates}

in the concentrates had a

_significant

effect on the

efficiency

of the microbial

_synthesis,

which was

_higher

_{(+ 6.6 g of}

_nitrogen/kg

of fermentable

_organic

matter in the _rumen) with

_{slowly degradable}

starch (SS) or

_digestible

fiber _(DF) than with

_rapidly

degradable

starch

_(RS).

Moreover, the mean

_{depression of cellulolysis}

in the rumen was

_higher

with

RS (-13

points) comparatively

to SS (-7

points)

or DF (-5

points).

ruminal

digestion

/ total tract

_digestion

/ microbial

synthesis

*

Correspondence

and

_reprints.

Present address: Inra, station de recherches

_{zootechniques,}

Prise d’Eau, 91170

_Petit-Bourg,

Guadeloupe,

French West Indies.

Résumé ―

_{Étude bibliographique quantitative}

de la

_digestion

ruminale et totale de la matière

organique

et des

carbohydrates.

Une étude

_quantitative

de la

_{bibliographie (157 publications)}

a

été réalisée afin d’étudier la loi de

_réponse

et les

_principaux

facteurs de variations

(niveau

et nature des

glucides

des concentrés, niveau

d’ingestion),

de la

_digestion

de la matière

_organique,

des

_parois,

de l’amidon et de la

_synthèse

microbienne dans le tube

_digestif

des ruminants. La

_digestion

ruminale

(moyenne

± écart _type %) de la matière

_organique,

des

_parois,

et de l’amidon sont de 45,2 ± _11,2 (n = _{553), 47,0 + 17,7 (n} = 348), et 74,1 ± _{16,2 (n} = ₁₄₀₎

_{respectivement.}

_{Selon le même ordre, les}

rap-ports entre la

_digestion

ruminale et la

_digestion

totale sont de 64,7 ± 12,3, 78,8 ± 18,5 et 80,5 ± _16,3

respectivement.

L’efficacité de la

synthèse

microbienne

_(g

d’azote

_microbien/kg

de _matière

orga-nique

réellement fermentée dans le rumen) est de _{23,6 ± 9,3 (n} = _{320). La}

_quantité

_{d’azote microbien}

rapporté

à l’azote total

_atteignant

le duodénum _(%) est de _55,1 ± _{16,6 (n} = _{289). La}

_digestion

_ruminale

de la matière

_organique

_augmente de deux

_points

_{pour dix}

_points

de concentré

_{supplémentaire}

dans la ration. La

_digestion

ruminale des

_parois

est maximale avec environ 30 % de concentré dans la ration. La substitution de la

_digestion

ruminale des

_parois

_par une

_digestion

_{intestinale n’est que}

_partielle

(environ 10 %). L’efficacité de la

_synthèse

microbienne est

_optimale

avec environ 40 %

_{d’incorporation}

de concentré dans la ration. La nature des

_glucides

des concentrés a un effet

_significatif

sur

l’effica-cité de la

_synthèse

microbienne

_qui

est

_plus

élevée ₍₊ 6,6 g d’azote microbien /

kg

de matière

_organique

réellement fermentée dans le rumen) avec l’amidon lentement

_dégradable

et les

_{parois dégradables}

com-parativement

à l’amidon

_{rapidement dégradable.}

Par _ailleurs, la diminution _moyenne de

cellulolyse

rumi-nale, induite par le concentré amidon à

dégradation rapide

(-13

points)

est

_supérieure

à celle du concentré amidon à

_dégradation

lente _(-7

_points)

et à celle du concentré

_paroi

(-5

points).

digestion

/ rumen / tube

_digestif

/

_synthèse

microbienne

INTRODUCTION

Improving

the

_efficiency

of ruminant diets is

_partly

conditioned

_by

a better

under-standing

of the

_digestion

sites for the various

components

of feed

_organic

matter

_(OM).

The

_partition

of

_digestion

of the

non-struc-tural and strucnon-struc-tural

_{carbohydrates}

between the rumen and the intestine

_depends

on the

nature of the

_roughage

or concentrate and their

_{constituents,}

on the

_{forage/concentrate}

ratio

and,

sometimes on the

_digestive

inter-actions between the different diet

ingredi-ents or fractions. There are nutritional

con-sequences due to variations in the

_digestive

partitioning

of the OM fractions. The starch that

_by-passes

the rumen, is

partly

digested

in the small intestine and can be absorbed as

glucose,

which could lead to a better utili-sation of

_digestible

nutrients

_(Nocek

and

Tamminga,

1991

The

animal’s _energy

sup-ply

(volatil

fatty acids)

and microbial

_protein

is

_{largely dependent}

on the OM fermented in the rumen.

The concentrate level and

_carbohydrate

involved in both the

_partitioning

of the

car-bohydrate digestion

in the ruminant

_gut

and in the nature and the

_quantities

of the

prod-ucts of

_digestion.

The main

_objectives

of this review were to

_point

out the main effects

relating

to the level and nature of concentrate

on the

_digestion

of the

_organic

matter and

ingested carbohydrate,

and the

_efficiency

of the microbial

_synthesis.

MATERIAL AND METHODS

Data base elaboration and

_coding

The relevant literature was reviewed to form a

data base. The

_major

criterion for

_selecting

a

study

was the simultaneous measurements of the total tract (TTD) and rumen

_{digestibility}

_(RD)

of

_organic

matter

_(OM),

or

_{carbohydrates.}

When

the neutral

_detergent

fiber (NDF), acid

_detergent

fiber (ADF) or cellulose _(ADF-ADL) duodenal flow exceeded the faecal flow

_by

5% or _more,

probably

because of

_inaccuracy

in the

measure-ments,

experiments

were excluded.

_Experiments

with a

_significant

effect of any feed additive (buffer, fat, antibiotics, etc) were not taken into

account. The animals were divided into cattle and small ruminants

_(sheep

and

_goat).

Little

stud-ies dealt with _goats. The

_roughage

was divided

into one of the

_following

four groups: cereal

silage (mainly

maize

_silage),

_grass

_{silage, hay}

or fresh grass, straw and raw

_by-products

_(hulls,

etc). The nature of concentrates was taken into

account, whenever

_possible,

with the

_following

classification:

_{rapidly degradable}

starch (RS,

bar-ley,

wheat and oat),

slowly degradable

starch (SS, corn and

_sorghum)

and

_digestible

fiber (DF). When the concentrate was a blend of several

ingredients,

it was included in the group with the

_{ingredient having}

the

_highest

level of

incor-poration,

or the most fermentable

_carbohydrate

if the two

_proportions

were

_equal.

_Moreover, for

starchy

concentrates, treatments were

_grouped

into whole

_grain

or

_ground,

_{flaked, rolled,}

cracked, and

_{pelleted grains.}

NDF, ADF, and

cellulose _(ADF-ADL)

_{digestibility}

data were

pooled

to

_give

the same _variable, cell wall mate-rial, (CW). However,

they

were also

_grouped

according

to the _type of chemical

_analysis

(NDF, ADF, cellulose). The duodenal flow estimation methods were classified in order to take into

account

_{methodological}

effects. Estimations

resulting

from re-entrant canulae were

differen-tiated form

_T-piece

_type canulae. In this last

group, the methods of

labelling

were divided into

rare _earths, chrome oxide, Cr mordanced on

neu-tral

_detergent

fiber,

indigestible lignin

as

single-or double-marker method,

indigestible

cell wall

material, fluid

_phase

marker as

_{single-marker}

methods, and acid insoluble ash as

_single-

or

double-marker method. Other groups were

formed based on the ruminal balance of

indi-gestible lignin

and

digesta

fluxes.

The methods of

_measuring

microbial flows

were classified on the basis of the markers used:

ribonucleic acid,

diaminopimelic

acid, D-Ala-nine,

15

N,

35S and an indirect estimation

_using

the

_{undegradable nitrogen}

of the diet

_{(nylon bag}

method). The choice of the microbial

_sample,

source of variation

(Yang,

1991; Broderick and

Merchen, 1991 ) were not taken into account. The

efficiency

of microbial

_growth

was

_expressed

as

g of

nitrogen

/

_kg

of

_organic

matter

_truly

fer-mented in the rumen. When the

_published

values

were _{per unit of}

_{apparently digested}

_OM,

_they

were transformed

_assuming

a

_nitrogen

content

of microbial OM

_equal

to 8% _(mean literature value). When the results on

_dry

matter were

pub-lished alone,

they

were transformed into

_organic

matter

_{by using}

the

_{following assumptions}

(Archimède, 1992) and mean values from the

literature:

-

organic

matter intake = 0.88*

_dry

matter

intake;

- _rumen

organic

matter

_{digestibility}

= 1.15**

rumen

_dry

matter

_{digestibility;}

- total tract

organic

matter

_{digestibility}

=

1.05 *

total tract

_dry

matter

_{digestibility.}

Energy digestibility

was calculated on the same basis as the

_dry

matter

_{digestibility.}

Cattle and small ruminants

_(goats

and

_sheep)

were differentiated in the data base.

Statistical

_analysis

Analysis

of variance was

_performed

on each

dependent

variable with the

_general

linear model

procedure

of SAS (1988)

using

two models:

where

_Y,

is the

_{digestibility}

_{data; oci is the} exper-imental

effect;

_{a!, a2,} b are the coefficients of

regression;

C is the concentrate level

(0

5 C < 1),

1 (g/kg

LW

w

s)

is the level of

_ingestion

of

organic

matter and

_e

_j

is the residual standard

deviation used as error term.

where

_Y

_iit-no

_p

is the measured

_{digestibility}

coef-ficient,

u

is

the

overall mean, F. is the

_roughage

class ₍₄

_degrees

of freedom,

d6,

_C

_k

is the

con-centrate class _{(2 df),}

_T,

is the

_{technological}

treat-ment of concentrate _{(4 df),}

_R

_m

is the animal class (1

df),

_MF!

is the method used to estimate duo-denal flows (12 df),

_MM

_.

is the microbial marker

(5 df), and

el!!mop

is the residual effect used as an

estimation of the error term.

Analyses applied

to total tract

_digestion

pre-diction did not include the methods of duodenal flow estimation, and the microbial markers were

taken into account

_only

for the microbial studies. The aim of model 1 was to test, within the

experiments,

the level of concentrate and OM intakes. Model 1 was used for

_experiments

with

at least two levels of concentrate. The aim of model 2 was to test other diet effects such as

roughage

and concentrate nature, concentrate treatment.

RESULTS

General observations

One hundred and

_fifty-seven

references

(experimental

effect)

were used to

_produce

the data base.

_{Approximately}

one half of

these dealt with cattle

studies,

the others

were from small

_ruminants,

_{mainly sheep.}

Thirty-six

references

_presented

studies with

at least two levels of the same concentrate.

The

_partition

of the observations between

organic

matter, cell

wall-material,

starch and diet

_{digestibility}

is

_presented

in table I.

The data

_partition

between the

_types

of for-ages or concentrates was unbalanced. The

combinations between

_roughages

and con-centrates did not

_generate

a normal

distri-bution. Statistical

analyses

were

_performed

with all the values in the data base.

Dis-crepancies

between the number of observa-tions in table II

_compared

to the number indicated in tables III to V result from a lack of information for all the tested effects.

Total tract and ruminal

_organic

matter

.

digestion

The mean values of rumen and the total tract

OM

_{digestibility}

_{(RDom, TTDom)}

are

pre-sented in table II.

_Figure

1 a shows the

TTDom data from studies

_using

at least two

levels of concentrate. The TTDom increases with the concentrate level as illustrated

_by

equation

1

_{a (table}

_III).

The mean effect of C on TTDom is

_quadratic.

Moreover,

for the

same level of concentrate, there are

_large

variations in the TTDom from one trial to

another as shown

_{by figure}

la. The TTDom decreases

_by

1

_point

for each 10

_{g increase}

in

_OMI/kg

LW

O.75

.

The residual standard error of model 2

(table IV)

is

_{high, indicating}

that some

important

causative factors other than nature

of

_roughage

and concentrate, type of

ani-mal,

level of

intake,

have not been taken into account in this model.

The trials based on _grass

_silage

_give

the

highest

TTDom

_values,

when

_compared

with the

_hay

or straw-based rations. The concentrates

RS,

SS and DF increase the

TTDom

_by

6.6,

8.8 and 10.8

_points

respec-tively.

The RDom increases with the level of

concentrate

_(fig

lb,

equ

lb).

For the same

level of concentrate in the

_diet,

the effect of the concentrate on the RDom

_represents

57% of the TTDom. The standard deviation of

_{<Xi indicates,}

when

_compared

to TTDom

(7.71

vs

5.51),

a

_higher

variation between

trials. Model 2

_(table

_V)

shows that the method of duodenal flow determination is an

important

source of

_experimental

variation

(24%).

The interactions between the nature

the

_roughage

and the nature of the

concen-trate

_respectively

_explain

29%,

7% and 2%

of the

_experimental

source of variation. The

level intake effect value is 30%

_higher

in the rumen

_compared

to the total

_digestive

tract.

Figure

lc indicates that the intestinal OM

digestion

varies

_largely

from one trial to

another. There is a

_quadratic

increase in the

intestinal

_digestion

of

_organic

matter with the level of concentrate in the diet as indi-cated with the

_equation

1

_{c (table III).}

There

are

_large

variations in the

_proportion

of

digestible organic

matter

_digested

in the

rumen

_(fig

_{1 d,}

_equ 2e and

_2f).

Total tract and rumen

_digestion

of cell

wall material

This

_part

of the data base contains 348

results from 113 trials. The mean values of the rumen

_(RDcw)

and total tract

digestibil-ity

of cell wall material

_(TTDcw)

are

pre-sented in table II.

_Large

variations in TTDcw

were observed for the same concentrate level

(fig

2a).

The standard deviation of

_(Xi

(± 11.39)

of model 1 confirmed the

_large

variations in TTDcw between trials.

Equa-tion 2a

_(table

_III)

illustrates a

_quadratic

effect

of concentrate level on TTDcw. The

maxi-mum value of TTDcw occurred with a level of about 40% concentrate in the diet. Model 2 showed that these variations can be

_largely

explained

(50%

of the total

_variance)

_by

the

analytical

cell wall criteria. The nature of the

_roughage

or concentrate,

explained

about 10% of the total variation of TTDcw.

Diges-tion was lower with straw and raw material.

The concentrate decreases the TTDcw

_by

about

3,

8 and 11

_points

for DF, SS and

RS,

respectively.

The

_adjustments

of the RDcw data

(fig

2b)

with model 1

_{produced equation}

2b

(table

III).

As for

_TTDcw,

there is a

quadratic effect

of concentrate level on

RDcw. The

_marginal

influence of an

increase in C is

_higher

for RDcw than for

TTDcw,

however,

it decreases at a

_higher

rate as shown

_by

the value of the

_quadratic

coefficients of C

_(20.9

vs

15.6).

Conse-quently,

the maximal RDcw was achieved

with a lower

_{incorporation}

of concentrate

when

_compared

to TTDcw

_(28%

vs

_40%).

The effect of the method used to measure

the duodenal flow and the

_dietary

factors

(the

nature of the concentrate and the

roughage,

level of

_intake)

_explain

32% and 40% of the

_experimental

variations

respec-tively

(table

V).

The effect of the

analyti-cal nature of CW on its

_{digestibility}

was

equivalent

for TTDcw and RDcw. The

pres-ence of

_RS,

SS and DF in the diet decreased

the RDcw. The intake level effect value is 20%

_higher

in the rumen

_compared

to the total

_digestive

tract. The _presence

of RS,

SS and DF in the diet decreased the RDcw

_by

12,

9 and 3

_points

_{respectively.}

The

intesti-nal

_{digestibility}

of CW

(IDcw)

is

_generally

low

_(11.2

±

_{8.1 %).}

For most of the trials with at least two levels of concentrate

(fig

2b),

there was

_initially

an increase in

intestinal CW

_{digestibility}

with an

increas-ing incorporation

of concentrate. _The

equa-tion 2c

_{(table III)}

shows a

_quadratic

increase

of one

_point

in IDcw for a 10% increase in

concentrate level.

_Figure

2d and

_equation

2d illustrate the effect of the level of

con-centrate on ruminal

_digestion

of

_digestible

CW. CW material is

_{mainly digested}

in the

rumen

_(80%).

Intestinal

_digestion

increases with the level of concentrate but remains inferior to 20% of the total

_digestible

CW

(when

C =

1)

whereas RDcw decreases

by

30%. The RDcw is 6%

_higher

with cattle

compared

to small ruminant whereas no

dif-ference was observed between animal

species

for TTDw.

Total tract and ruminal

_digestion

of starch

The mean values for starch

_digestion

are

presented

in table II. The mean value for total tract

_digestion

_(TTDst)

is 93.3

(± 7.5%).

No variation or

_only

small

varia-tions in RD for starch

_(RDst)

were observed

with

_increasing

levels of RS concentrate

contrary to what is noticed with SS

(fig

3a).The

amount of ruminal

_digestion

of starch increases with the level of

concen-trate

_(equ

3a,

table

_III).

The effect of the level of concentrate is the same when

_only

SS concentrate is taken into account _in

equa-tion 3b. The

_proportion

of starch

_digested

in the rumen increases with the level of

con-centrate

_(fig

3b,

equ

3d,

table

III).

As a

con-sequence, the intestinal

digestion

of starch decreased with the level of concentrate

_(fig

3c,

equ

3c,

table

III). Nevertheless,

when

SS concentrate

_only

is taken into account

in the

_equation,

the effect of the concentrate

level is not

_{significant. Figure}

3d indicates that the intestinal

_digestion

of starch increases when the ruminal

_digestion

decreases.

Table V shows that the RDst of SS is 12 2

points

lower than RS for which the mean

RDst is 95.5

_(±

_0.9%).

Nevertheless,

tech-nological

treatment was the first

_dietary

fac-tor in RDst variation

_(64%

of total

varia-tion).

The method of duodenal flow

determination

_explains

29% of the

experi-mental variation.

_Compared

to whole or

rolled

_grain,

treatment of

_cracking,

grind-ing,

flaking

increased the RDst

_by

7,

9 and

12

_points.

The ruminal

_digestion

of

digestible

starch remained very

high (90.9

±

_0.8%)

with RS while the contribution for SS was lower

_(77.2

±

_17%).

Efficiency

of microbial

synthesis

The

_efficiency

of microbial

_synthesis

_(EMS)

is 23.5

(± 9.3)

g/kg

OM

_truly

fermented in

the rumen

_(OMF).

_Figure

_4,

based on trials

with more than one level of concentrate,

reveals that the EMS

_initially

increases,

then decreases with

_higher

levels of concentrate.

Statistical

_analysis

of these data confirms

these tendencies as indicated in

_equation

4a

(table III).

Maximum EMS occurs for a

con-centrate level of about 40% of the diet. The SD value of the

_experimental

effect

_(a!),

7.88 _g, shows that for the same level of con-centrate there are

_large

variations in EMS. The level of concentrate was not

_significant

with model 2 in contrast to model 1. Model 2

_{(table VI)}

indicates a

_significant

effect of

concentrate nature. EMS is lower

(-7 points)

with RS

_compared

to SS and DF.

Microbial

_nitrogen

flow

The mean value of the microbial

nitrogen

duodenal flow is 1.10

_(±)

_g/kg

LW°

v5

.

This

value is

_equivalent

to

_55.06(±

_16.65)%

of

the duodenal

_nitrogen

flow. The microbial duodenal flow first increases then decreases with the level of concentrate. The

_optimum

is around 50% of concentrate

_(equ

_4b,

table

_III).

There is a

_tendency

for the microbial duo-denal flow to be lower

_(table

_VI)

with the

concentrate RS _compare to SS and DF.

DISCUSSION AND CONCLUSIONS

From a statistical

_point

of

_view,

this data base appears to be a

_{non-orthogonal}

and an

unbalanced

_design.

This is usual in data

analysis.

The current

_objective

was to

quan-tify

the main effects of the known

_qualitative

and

_quantitative

sources of variation. Models of TTD

_analysis

and

_prediction

were

_{systematically}

more accurate than those of

RD,

thus

_revealing

a

_large

unex-plainable

variability

in the measurements

of the latter. This difference is

_likely

to be

the consequence of an

_inability

of some

methods to

_accurately

measure duodenal

flow,

as

_already

mentioned

_by

Sutton

_(1977,

1979)

and Owens and Hanson

_(1992).

In the _present

_study,

marker methods were one

of the main factors of _{variation in the}

regres-sion models

_applied

to RD. _{Because of}

pos-sible interactions between methods and other

factors,

the

_{biological analysis}

of these

Never-theless the main effect of the method was

used to correct the value of the mean

(inter-cept in tables IV to

_VI).

The different

roughage

types

constitute a

_heterogenous

class. A fresh _grass

_category

would have been

_interesting

but it was not included in

enough

papers to create such a _{group. It}

appears that the

hay

group should be divided into

_legumes

_{and grass but} a

_preliminary

statistical

_{analysis rejected}

this

_hypothesis.

Stage

of

_growth

_{of grass is} an

_important

fac-tor of

_digestion

variation that we did not

take into account. The mean contribution of the stomachs to the total tract OM

digestibilities

was consistent with

_previous

reviews

_(Armstrong

and

Beever, 1969;

Arm-strong

and

_{Smithard, 1979; Sutton, 1980).}

The current work shows however that this

relative contribution varies

_widely

_(cv

=

20%).

This result has some

_{physiological}

consequences such as

_composition

of the

end-products

of

_digestion

and

_quantity

of microbial

_{nitrogen synthesized}

in the rumen.

The

_large

variations in TTDom between

ref-erences were

_significant

_(P

<

_0.05),

and

could be

_{only partly explained by}

classic main factors such as concentrate

_level,

nature of concentrate and

_roughage,

_type

of animal and level of OM intake. All these

influences were

_confirmed,

with a lower

significance,

for the RDom values. This result is

_probably

due to our classification system which was not

_optimum.

Neverthe-less this factor reveals the existence of other

large

and

_biological

variabilities

_(retention

time in the

_digestive

tract,

etc)

in TTDom and RDom.

Cell wall forms a

_{heterogeneous}

fraction.

The cellulose

_(ADF-ADL)

which does not

include

_lignin

is much more

_digestible.

The CW data

_analysis

confirmed the

_major

con-tribution of the reticulo-rumen towards CW

digestion

which is

_{approximately}

90% for

most of the rations. This

_emphasizes

the

importance

of

_maximizing

the ruminal

digestion

of cell wall constituents.

More-over, the value of 0.9*TTDcw can be used as a

_simple

estimation of

_RDcw,

at least for diets

_containing

less than 50% concentrate.

The increase

_{of RDcw,}

between 0 and 28% of concentrate, may have several

origins.

It

could be due to a better

_potential

digestibil-ity

of CW for the concentrate. It is known

however that CW

_potential

_{digestibilities}

for some concentrates

_(barley,

oats,

etc)

are

lower than those of a

_good

_{quality forage.}

The increase in CW

_{digestibilities}

could also be obtained

_{through improving}

the microbial

_activity

when a moderate level of concentrate is used. It is

_likely

that the decrease in RDcw for the

_higher

level of

concentrate results from

_digestive

interac-tions

_{(Michalet-Doreau}

and

_{Sauvant, 1989;}

Berge

and

_Dulphy,

1985,1991).

Such

neg-ative

_digestive

_{interactions,}

are sometimes

in this

_study)

in the distal

_part

of the

_gut.

Such a substitute role of the

_hindgut

has

already

been observed

_{by Ullyatt}

et al

₍₁₉₇₅₎

for poor

quality forages.

Our results

sug-gest

that the substitute role is

_higher

with small ruminants than with cattle. The

influ-ence of the nature of concentrate is

_highly

significant

for TTDcw and RDcw values.

The

_depressive

action of RS is

_higher

than SS or DF

_probably

because of ruminal

acid-ity.

The

_negative

influence of the intake level on TTDcw and

RDcw,

as on TTDom and

_RDom,

has

_already

been mentioned

_by

Blaxter et al

_(1956),

_Demarquilly

and

Andrieu (1987).

This data base confirms the

_large

differ-ences in ruminal starch

_{digestibility}

(Sauvant

et

al 1994;

Orskov

_1986;

Waldo

_1973).

The

nature of the starch

_(RS

vs

_SS)

on one

hand,

and the

_{technological}

treatment on the other

hand,

are the main factors of variation

(Theurer, 1986;

Rooney

and

_Pflugfelder,

1986; Orskov,

1979).

Moreover,

this

_study

indicates a

_{positive relationship}

between the

level of starch in the diets and the ruminal

amylolytic

capacity.

Nevertheless,

low rumi-nal

_digestion

levels for starch are

compen-sated in the intestines.

The values of EMS had the same

varia-tion range as the one indicated

_by

Verite et

al

(1986),

Demeyer

and Van Nevel

₍₁₉₈₆₎

and Durand

(1989).

The

_quadratic

_response

of EMS

_according

to the concentrate level

was first noticed

_by

Chamberlain and

Thomas

₍₁₉₇₉₎

and Mathers and Miller

(1981).

Ramangasoavina

and Sauvant

(1993)

first

_proposed

a

_quadratic

_equation

based on 49 values and 14

_experiments

car-ried out in

_dairy

cows. An

_optimum

EMS around 40% should indicate a balance

between the

_growth

_{potential and energy}

intake level on EMS has

_already

been

indi-cated

_by

Verite et al

(1986).

This was

con-sistent with the

_{positive relationship}

observed between EMS and the

_liquid

out-flow rate observed

_by

Van Nevel and

Demeyer

(1979).

With a data base

con-cerning only dairy

cows,

_{Ramangasoavina}

and Sauvant

(1993)

observed a mean

increase in EMS of 0.97

_{gNm/kg OMF/kg}

of

_dry

matter increase.

_Taking

into account

body

size

effect,

the value that we

found,

0.09 g

Nm/kg

OMF/g OMI/kg

LW

w

S

,

is

consistent with the

_previous

results. This effect of intake was the result of its

_positive

influence on Nm flow

(table

VI)

and its

neg-ative effect on RDom

_(table

_V).

In contrast

with the observations of Verite et al

_(1986),

there was no

_significant

influence of the

nature of the

_forage

on EMS.

_Probably

the different

_forage

_groups formed were too

het-erogeneous. To our

_knowledge,

the

influ-ence of the

_type

of concentrate on EMS had

not, until

recently,

been

_supported

_by

a

lit-erature data base. Hoover and Stokes

₍₁₉₉₁₎

indicated that the rate of

_carbohydrate

diges-tion is the

_major

factor

_controlling

the

energy available for microbial

growth.

The

results of this

_study

underline that

_rapidly

degradable

carbohydrates

lead to the

low-est EMS. There is

_probably

an

_energetic

uncoupling

in microbial

_requirements

and

availabilities.

Moreover,

the nature of the diet which alters the

_composition

of the microbial biomass

(adhered

bacteria,

cellu-lolytic,

amylolytic,

etc)

may

partly explain

the variation in EMS

_(Yang,

1991;

Broder-ick and Merchen,

1992).

From this

_study,

it can be concluded that

the nature of the concentrate has a

_significant

effect on

_digestion.

_Compared

to DF and

SS,

RS

results,

in the lowest

_efficiency,

of the microbial

_synthesis

and,

in the

_highest

depression,

of

_cellulolysis

in mixed diets.

Moreover,

the

_optimum

_efficiency

of the

rumen microbial

_population

is reached with 30 to 40% of concentrate level in the diet. The

_digestion

rate for starch varies with the level of concentrate.

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