79
(KIELGROUPSEMINAR,UPPSALA 1 3 - 1 4 JUNE19
Relationbetweendifferentcarbohydratesandmicrobial synthesis
ofprotein
S.Tamminga
Reportno.130
Institute forLivestockFeedingandNutrition
Lelystad,Netherlands
BIBLIOTHEEK
LAMDBOU'WH«*'.:.-• wn"!,
WAGEAl^KN
Uil
CENTRALE LANDBOUWCATALOGUS
0000 0162 9605
-11.lDt£2^B2ti2D
Ruminantanimalproductiongivesanimportantcontributiontotheworldshuman foodsupply.Thisismainlybecauseoftheircapability ofconverting carbohydratesin
thefibrouspartsofplantsintohighqualityhuman foods.
Thisconversionispossibleduetothesymbioticrelationshipbetweenruminantanimalsandmicro-organismsinhabitingtheirforestomachs.Thesemicro-organisms candegrade
thefibrousorstructuralcarbohydrates ingestedbythe
hostanimal,which areundigestibletothedigestive system
ofmonogastrics (includingmen),toendproductswhichcan
bedigestedandutilizedbyruminantsandconvertedtomilk,
meatandwool.
Notonly fibrouscarbohydratesaredegradedbythe
micro-organisms,soareothercarbohydrates,proteins^and
toalimitedextentlipids.Asaresultofthisdegradation
themicro-organismsextractenergyand "buildingstones"
fromthe foodingestedbythehostanimal,whichenables
themtogrow.Thismicrobialgrowthmeansasynthesisof
biomass containingprotein,nucleicacids,lipidsandcarbohydratesofwhichprotein formsthemajor component.This
microbialproteincansubsequentlybeusedasaprotein
sourcebythehostanimalandthuscancompensate theloss
ofdietaryproteinintheforestomachsresultingfrom
microbial degradation.Itbecomesevidentthattheintermediarymetabolismofruminantsreceivesprotein fromtwo
sourcesviz.dietaryproteinwhichescapesmicrobialdegredationintheforestomachs andmicrobialprotein synthesized
therein.Bothproteinsourcesaredigestedinandabsorbed
fromthesmallintestine.
Inthelastdecademuchattentionhasbeenpaidtoways
ofincreasing theintestinalproteinsupplyinruminants.So
farmostattentionwas focussedonwaysofdecreasingthe
degradationofdietaryprotein (Clark,1975;Chalupa,1978;
Tamminga,1979)andlessattentionwasgiventomeansof
increasingthesynthesisofmicrobialprotein.
Itisbeyond doubtthatthefirstlimiting factorfor
thesynthesisofmicrobialproteinisenergy inadenosine-
-2tri-phosphate (ATP),resulting fromthedegradationoffood
(ingestedbythehostanimal.Carbohydrates formamajor (
partofthissubstrateandinthe followingsections
variousaspectsoftherelationbetween dietarycarbohydratesandmicrobialproteinsynthesiswillbediscussed.
2.Microbial_growth_in_the_forestomachs_and_its_reguirements
Beforediscussingvariousaspectsoftherelationbetweencarbohydratesandmicrobialgrowthitseemsappropriatetodiscuss theprinciplesofmicrobialgrowthunder
anaerobicconditionsasitoccurs.inthe forestomachsof
ruminants.Itwasmentionedalready thatmicrobialgrowth
impliestheformationofbiomassofwhichmaincomponents
areproteins,nucleicacids,carbohydrates andlipids.The
nutritionalrequirementsofthemicroflora intheforestomachsofruminantsareverycomplex (Hungater 1966)and
includevolatile fattyacids (VFA), carbondioxide (C0„),
ammonia (NH^),aminoacids,mineralsandvitamins.Asa
generalruleitmaybestatedhoweverthatmicrobialgrowth
ismainlydeterminedbytheavailability ofenoughofa
suitablesubstrate fromwhichenergy (ATP)andprecursors
forthesynthesisofthevariousmacromoleculescanbeextracted.
Inenergetictermsthedegradationofthevarious
dietary components,mainly carbohydrates,proteinsand
lipids,ingestedby thehostanimal,meansthereleaseof
acertainamountofenergy,partofwhich istemporarily
storedintheterminalpyrophosphatebondsofATP.The
energy storedinATPcanbeusedbythemicro-organisms for
-variouspurposes suchasmotility,transport,thesynthesis
ofmacromolecules,etc.Generally thiscanbedividedinto
twoprocessesviz.maintenance andgrowth.Theratioin
whichenergy isusedforthetworequirementsmainly depends
onthegrowthrate (StouthamerandBettenhaussen, 1973).The
ATPrequired formaintenanceperunitofmicrobialdrymatter (e.g.per 100g)isconstantperunitoftime.Onthe
otherhand,theATPrequired forgrowth isexpected tobe
fairly constantperunitofdrymattersynthesized.Ata
-3lowrateofATPgenerationmostoralloftheATPwillbe
required formaintenance.AtahigherrateofATPgenerationonlypartofitwillbeneeded formaintenanceand
thesurplus isavailable forgrowth.Withanincreasing
rateofATPgenerationthesurplusofATPavailablefor
growthwillincrease,resultinginahighergrowthrate.
NotonlywillthesurplusofATPincreaseinanabsolute
sense,butalsoalargerproportionofthetotalATPgeneratedwillbeavailable forgrowth,makingthegrowthmore
efficient.
Theenergeticefficiency isoftenexpressed asthe
amountofmicrobialdrymattersynthesizedpermoleof
ATP.ForthisthetermY A m Pwasintroduced (Bauchopand
Eisden, 1960).ThisY A T P ismuch lowerthanthemaximumpossibleyieldatmaximumgrowthrate.Forthelattertheterm
M ay
Yl
5and
mT,wasintroduced andforthisvaluesofbetween 2
ATP
30gofmicrobialdrymatterwereestimated (Stouthamer,
1973;HespellandBryant,1979)incontinuouscultures.Estimates forY
underpracticalconditionsintherumenin
vivousuallyvarybetween 10and 15 (HoganandWeston,1970).
EstimatingY
valuesintherumeninvivounderpracticalconditions isverydifficult (Tamminga,1978).Besides,usuallymoreinterestexistsinmicrobialprotein
synthesis thaninmicrobialgrowth.Ithasthereforebecome
commonpractice toexpresstheefficiency ofmicrobialproteinproduction intherumenasgofmicrobialNformedper
100goforganicmatter (OM)fermented,ratherthanpermole
ofATP.Reportedvaluesusuallyrangebetween2,5 and4,0g
ofNper 100gOMfermented (Armstrong,1976;Kaufmann,1977)
BecauseofanaërobiosistheamountofATPwhich canbe
extracted fromthedegradationoforganicmatterintheforestomachsislimitedandrestricted tosome 10to15%ofwhat
canbeextracted underaerobicconditions.ThemainATPyieldingsubstrate formicrobialgrowthinthe forestomachs
arecarbohydrates.Thisisnotonlybecause carbohydrates
formthemajorcomponentoftheingested feed,butalsobecauseonbiochemicalgrounds fermentation of100gofcarbohydrates isexpected toyieldmoreATPthanfermentationof
100gofproteinsorlipids (Tamminga,1979).
-4Thenecessary precursors forthesynthesis ofbiomass
areeitherintermediates orendproductsofthedegradation
ofdietary carbohydrates,proteinsandlipids (VanEsand
Tamminga,1978). Quantitatively proteinsynthesis isthe
mostimportantanabolicprocess.Theaminoacidswhichare
required asprecursors forthisprocessmayoriginatefrom
de novosynthesis,fromhydrolyzed dietaryproteinor
fromhydrolyzedmicrobialprotein.Denovosynthesisis
possible fromintermediates andendproductsofthemicrobialdegradationofdietary carbohydrates andproteins.
BothasuitableN-source,fromwhich ammoniacanbereleasedandpreformed aminoacidsseemnecessary.Veryoften
sufficientdietaryproteinisdegraded togeneratetherequiredaminoacidsand (afterdeaminationofaminoacids)
ammonia.If,undercertainconditionsadditionalammoniais
needed,anon-protein-N sourcefromwhich ammoniacanbe
liberated,suchasurea,issufficient.
3.Availability_of_çarbohydrates_as_a_substrate_for_miçrobial
growth_and_protein_synthesis
3.1.R§te_of_degradation
Microbialgrowthandmicrobialproteinsynthesisare
continuousprocessesofwhich theratemayvary.Theenergeticefficiency ofmicrobialgrowthisinfluencedbyits
growthrateandthusfactorsinfluencing thelatterareof
interest.OneoftheseistheratewithwhichATPandprecursorsbecome available fromthedegradationofsubstrate.
Therateofthisdegradationdiffersbetweendifferentcarbohydrates (Johnson,1976)-anddependsoncertainphysicaland
•toalesserextentchemicalproperties.Thedietarycarbohydrates canbedivided invarious classeswhich differin
physicalratherthaninchemicalproperties.Suchadivision
isshownintable1.
-5-
Table 1. Distribution_of_dietarY_çarbohYdrates
fibre
polysaccharides
structural
carbchydrates
[
matrix
polysaccharides
celluloses
hemicelluloses
[
pecticsubstances
carbchydrates
starches
[
storage
polysaccharides , ,. .
non-structural
carbchydrates
tr
^
J
* fructcsans
saccharose
metabolic
intermediates
freesugars
Inthe degradation of carbohydrates twoparts canbe
distinguished. Firstly hydrolytic cleavage of the glycoside
linkages in thepolysaccharide chain occurs.This is followedby a further degradation of the resulting monosaccharides. The rate limiting partof the degradation of carbohydrates seems tobe thehydrolytic cleavage.
Before the degradation of structural carbohydrates
starts a lagperiod of severalhours is usually observed
(Mertens, 1978). This period mightbe required for thedevelopment of a slime layer,necessary formicrobes to attach
to dietary fibre (Chenget al., 1978). Part of the structural
carbohydrates is resistant against degradation aswas demonstrated inkinetic studies by Waldo et al., (1972). This is
causedby encrustration of structural carbohydrates with
lignin and silicawhich act asphysical barriers to the
rumen micro-organisms. The remaining degradable part could
be divided in two fractions ofwhich onewas degraded rather
rapidly and the other more slowly (Waldoet al., 1972).Although functionally part of the structural carbohydrates,
pectic substances are degraded much more rapidly than the
other structural carbohydrates celluloses and hemicelluloses. This may be the result of a difference in chemical
configuration. Inpectic substances a-configurations predom-
-6inatewhereas incellulosesandhemicelluloses 3-configurationsaremoreimportant.
Unlikestructuralcarbohydrates degradationofnonstructural carbohydrates doesnotrequirealagperiod.Degradationofstoragecarbohydrates isusually fast,butthe
ratemaydiffer fromonesourcetoanother.Degradationof
storagecarbohydratesmaybecomeretardedbyatemporary
storageinsidethecellsofbothbacteriaandprotozoa
(McAllanandSmith,1974;Czerkawski,1976).
Thedegradationoffreesugars,whicharemainlypresent
asmetabolicintermediateshardlyinvolvesthecleavageof
glycosidicbonds,therate limitingstepinthedegradation
ofcarbohydrates.Itisthereforenotsurprisingthatfree
sugarsaredegradedalmostinstantaneously.
3.2. Extent_of„degradation
Onlyafewdetailedstudiesareavailableontheextent
ofdegradationintheforestomachsofruminants forthe
variousclassesofcarbohydrates.Moststudiesonthepartitioningofdigestionofdietary componentsarerestricted
toorganicmatter,energyand (crude)protein.Fromthese
studiesitappearsthatusually 60-70%ofthetotalapparently
digestibleorganicmatterisdigestedintheforestomachs.Differencesintheextentofdigestionofthedifferentcarbohydrateswerehoweverreported.Thesefindingscanbesummarizedasfollows:
Withinthestructural carbohydrates totalapparentdigestionofcellulose andhemicellulosevaries,butofboth
digestionismainly restricted totheforestomachs,particularlyindairy cows (Van'tKloosterandGaillard,1976).
Hemicellulose appearstobeslightlymoreresistantagainst
degradation intheforestomachs thancellulose.Insheepthe
proportionoftotaldigestionofcelluloseandhemicellulose
takingplaceinthe (fore)stomachstendstodecreasewitha
decreasing totalapparantdigestion (Ulyattetal., 1975).
Thedigestionofpecticsubstancesintheforestomachsin
sheepappeared tobevirtually complete (Eganetal., 1975).
Withinthenon-structural carbohydrates,storagepolysac-
-7charidesusually show ahighapparentdigestion.Withinthese,
starches showdifferences dependingontheirorigin.Digestion
ofbarley starchoccursalmostcompletely intheforestomachs
butofcornstarchandsorghumstarch amountsofover20%may
escapedegradationintheforestomachs (Waldo,1973).Ofthe
watersolublepolysaccharides (fructosans,saccharose)significantamountsalsoappeartoescape degradationinthe
forestomachs.Thismaybebecausewatersolublepolysaccharidesofmicrobialoriginandmucusenterthesmallintestine
resultinginarelatively lowapparentdigestion (65-75%of
thetotaldigestion)intheforestomachs (Van 'tKlooster
andGaillard,1976).Freesugarsarealmostcompletelydigestedintheforestomachs.
Someresultsonsiteofdigestionofdifferentcarbohydrates,mainlyobtainedindairy cowsareshownintable2.
Acrudeestimateoftheextentofdegradationofstructuralandnon-structural carbohydrates canalsobeobtained
by comparing thesiteofdigestionofcrude fibre (representingpartofthestructuralcarbohydrates)andN-freeextracts (representingmostofthenon-structural carbohydrates)
respectively.Inourresearchwithdairy cows,fittedwith
Carbohydrate
source
cellulose
hemicellulose
pecticsubstances 1)
2)
starch
fructosans
saccharose
freesugars
Total apparent
digestion
(% of intake)
Digestion i n
forestomachs
(% of t o t a l
app.digestion)
Reference (s)
62-86
95-103
(19)
53-81
97-102
(19)
95-100
95-100
90-100
75-100
(11)
(19,37)
65- 90
65- 75
(19)
90-100
80- 95
(19)
Table 2.Apparentdigestionofcarbohydrates inruminantsand
theproportionofthisdigestionoccurringinthe
(fore-)stomachs.
1): Insheep
, 2):Sheepdataincluded
-8re-entrant cannulae inthesmallintestine suchstudieswere
performed.Arangeofdietsconsistingoflongroughagewith
groundandpelletedmixed concentrates invariousproportions
werefedatvariouslevelsofintake.Meantotalapparent
digestionwas foundtobe0.70 (sd=0.083)and0.84 (sd=0.40)
forcrude fibreand N-freeextractsrespectivelyof
whichproportionsof0.89 (sd=0.097)and0.77 (sd=0.050)were
digestedinthestomachs.Theresultswere analyzedbymultipleregressionanalysis.Althoughnotstatisticallysignificant,atendencywasfoundthattotalapparentdigestionof
both crudefibreandN-freeextractsslightly decreasedwith
increasingdrymatterintake,andthatthedigestionofcrude
fibretendedtoshiftfromtheforestomachstowardstheintestineswithanincreasingdrymatterintake.
Comparingsheepanddairycowssomedifferenceswith
respecttositeofdigestionofcarbohydrates seemtoexist,
particularly forstructuralcarbohydrates.Indairycowsthe
contributionofthelargeintestineseemsofminorimportance,
whereasinsheepfedchopped foragesproportionsofupto30%
ofthedigestionofstructuralcarbohydrates takingplacein
thelarge intestinewerereported (Ulyattetal., 1975).A
possibleexplanationofthisapparentdifference isthatcomparedwithsheep,dairy cowsaregenerally fedmoredigestiblediets.
Inthefermentationofcarbohydrates anumberofstages
canbe(ratherarbitrarily)distinguished,suchas:
- degradationofpolysaccharides tomonosaccharides
- conversionofmonosaccharides topyruvate
- conversionofpyruvatetovolatile fattyacids
- reductionofcarbondioxide tomethane.
Degradationofpolysaccharides tomonosaccharidesmeans
hydrolytic cleavageofglycosidicbondsinwhichnoATP
generationoccurs.However,asafinalstepofdegradationof
polysaccharides tomonosaccharides,phosphorylytic cleavage
oftheglycosidic linkagesindisaccharides seemsalsoto
occurintherumen.Cellobiosephosphorylase,sucrosephosphorylase andmaltosephosphorylase wereshowntobepresent
-9inrumenmicro-organisms (Prins,1977).Theseenzymescatalyzethegeneralreaction:
disaccharide+P.
>monosaccharide+monosaccharide-phosphate
Theimportanceofthesereactionsinthedegradationofpolysaccharidesintherumenishoweveruncertain (Prins,1977).
Variousmetabolicpathways forthedegradationofmonosaccharidesareknowntooccurintherumen,buttheEmbdenMeyerhofpathway (EMP)predominates (Prins,1977). Thispathway leadstotheformationofpyruvate.Activationofthe
monosaccharides isnecessary,requiring2molesofATPper
moleofhexose.Ifphosphorylasesareinvolvedinthedegradationofdisaccharides tomonosaccharides,theresulting
hexosephosphate requiresonly 1moleofATP foractivation.
Afteractivationofhexoseinthedegradativepathwayofhexose-topyruvate,4molesofATParegeneratedpermoleof
hexoseconverted,resultinginanetyieldof2molesofATP
(2.5molesifphosphorylases areoperative)permoleofhexose
converted.
Oneoftheotherpathwaysofdegradationofmonosaccharides,thepentosephosphatecycle,maybecomeofsignificance
ifthedietary carbohydrates containsubstantialamountsof
pentoses.Thepentosesareconverted totheglycolyticintermediateshexose-phosphate andtriose-phosphate,thenultimately
topyruvate.Theconversionof6molesofpentosewillyield
4molesofhexose-phosphate and2molesoftriose-phosphate,
resultinginanetyieldof 10molesofATP.So,perhexoseequivalent (anon-hydrolyzedmonomerinpolysaccharideswith
amolecularweightof162andequivalentto1.20 moleof
pentose),pentosebasedcarbohydratesyieldthesameamount
ofATPthanhexosebasedcarbohydrates.
Themainendproductsoftheconversionofpyruvateare
acetate,propionate,butyrateandcarbondioxide.
Pyruvate canbeconvertedtoeitheracetylCoA,CO ? and
H 2 (1)ortoacetylCoAandformate (2).Inbothcasesacetate
canbe liberated fromacetylCoA,oftenafteraconversionto
acetyl-phosphate.
Propionatecanbeformedbytwopathways.Oneinvolves
succinate asanintermediate andisknown as'the "dicarboxylic
-10acidpathway".Theotherinvolves lactylCoAasanintermediateandiscalledthe "directreductivepathway"orthe
"acrylatepathway".Undernormalconditions thedicarboxylic
pathwaypredominates,butifhigh graindietsarefedthe
acrylatepathwaymaybecomeimportant.
The formationofbutyrateoccursusuallybyareversal
ofß-oxidationfromtwomolesofacstylCoA.Fromtheresulting
butyrylCoA,butyratecanbegenerated,oftenafterconversion
tobutyryl-P.
Thedegradativepathwaysinvolvedintheconversionof
pyruvate toVFAcomprisesofanumberofsteps,someofwhich
areassociatedwithafreeenergy change largeenoughto
enablethegenerationofATP.ThegenerationofATPfromADP
andP.requiresafreeenergychangeunderstandardconditions (AG 1 )
ofatleast7.6kcal/mole (Thaueretal., 1977). Infigure1
asurvey isgivenoftheAG^ (derived fromThaueretal.,
1977)ofthevarious stepsinthepathways.ofconversionof
pyruvatetoVFA.
ComparingtheA G Q required forthegenerationofATP
with theseinfigure 1forthevarioussteps,suggeststhe
possibility forthegenerationofATPatanumberofsites.
However,theactualconditionsintherumenorwithina
bacterial cellmay differmarkedly fromthestandardconditionsunderwhichA G Q wasobserved,whichmaypreventATP
Hexose
+23.4
I
•* oxalo-acetate
+5.6
- X-9 A 2-
pyruvate
+0.6
malate
+ 0.9
fumarate 2-20.6
acetate
aceto-acetate <
-3.0 |
+11.4
3 -OH-butyrate
-2.0 A
_
crotonate
-8.5 I
butyrate
i
succinate
i
-8.4
acetate
-4.1
propionate
F i g u r e 1. The c h a n g e i n s t a n d a r d f r e e e n e r g y (AG0 i n
kcal)
associated with the various steps in the conversion
o f p y r u v a t e t o VFA i n t h e r u m e n .
2-
2-
-11-
generation atcertainsteps.
Itisbeyonddoubtthattheformationofacetateand
butyrate fromacetylCoAandbutyrylCoAviaacetyl-Pandbutyryl-Prespectively,yields 1moleofATP.Itshouldbenoted
thatbecauseoftheformationofacetoacetylCoA from2moles
ofacetylCoA 1potentialsiteofATP-generation islost.
Thereishoweversomeevidence thatthereductionofcrotonylCoAtobutyrylCoAmayyieldadditionalATPinsomestrains
ofrumenbacteria,butnotinothers (Prins,1977).
Withrespecttotheformationofpropionateviathedicarboxylicpathway thereductionoffumaratetosuccinate
causesa^ G 0 largeenough topermitATP-generation,and
thisseemstobethecaseinanumberofstrainsofrumen
bacteria (Prins,1977;ReddyandPeck,1978). Ithaseven
beenassumedthatthisreductionyields 2molesofATPincertainstrains (Stouthamer,1976).
Intheotherpropionategeneratingpathway thereduction
ofacrylate topropionatemayyieldATP,butintheconversion
oflactatetoacrylate,lactylCoAisactivated tophospholactate,whichinturnisconvertedtoaerylyCöA.Thisactivation
isattheexpenseofacetyl-P (whichwouldnormallyresultin
theformationofATP), thusnullifying theATPyielddueto
thereductionofacrylate.
Theconversionofcarbohydrates topyruvateandfurther
toVFAresultsinasurplusofreduction-equivalents,either
asNADHorasH 2 .Formattersofconveniencethetermreduction-equivalents isusedforboth.This "pool"ofreduction
equivalents isveryimportantintheregulationofthefermentationpattern.
Intheconversionofcarbohydrates topyruvatethereductionequivalentsbecomepresentasNADH.Inordertokeep
the fermentation going,anoxidationofthiscompoundback
-f.
toNAD isrequired.Thisispossibleby thegenerationofH„:
NADH+H +
»NAD++H
-12Th
eequilibriumofthisreaction liesverymuchtotheleft
andthereactionwillonlyproceed ifthepartialpressure
ofH 2 (PH„)isextremely low.ThislowPH~ canbemaintained
intherumenbecausetheH iscapturedbymethanogenicbacteriaandusedforthereductionofC0„toCH..Althoughthe
exactmechanismofCK.-formationisstillratherobscure,it
isgenerallybelieved thatCH-formationyields 1moleofATP
permoleofCH.generated (Demeijer,1976;Thaueretal.,
1977).
Thedegradationofpyruvate toacetylCoAalsogives
yieldtothe formationofH„,withoutNAD beingrequired.
ThisH-formationproceedsevenathighconcentrationsofH»
(Thauer,etal., 1977). Ifthemethanogenicbacteriacankeep
thePHp lowenough the fermentationofcarbohydrateswill
proceed,mainly toacetate.IfhoweverthePH~becomestoo
high,forinstancebecausethefermentation rateandthereforetherateofH„-formationfromtheconversionofpyruvate
toacetylCoAistoohigh,theliberationofH„fromNADHwill
nolongerproceed.TopreventaccumulationofNADH,other
electronacceptorshavetobefoundandthefermentationis
diverted fromacetatetowardsbutyrateand/orpropionate.
Theformationofbutyrate fromacetylCoAmeanstheconsumptionofreduction-equivalents ofNADH.Theformationofpropionatealsomeanstheuptakeofreductionequivalents.By
divertingthepattern,fermentationcanproceed.
Divertingthefermentationpatternaway fromthegenerationofCH.,meansthelossofthegenerationof1moleof
ATP foreach 8reductionequivalents,which isusuallynot
compensated inthediverted fermentationpattern.Ittherefore frequentlymeansanenergetically less favourable
systemanddiversion isusually restricted towhatisabsolutelynecessary.Theresultmaybeamixed fermentationwith
forinstancetheproductionofacetateandenoughbutyrate
toconsumethereductionequivalents ofNADHgeneratedin
theconversionofhexose topyruvate (Thaueretal., 1977).
Figure2showsasimplifiedbiochemicalpathwayofthe
degradationofpolysaccharides andthesiteswhereATPis
believed tobegenerated,orwhereATP (orotherenergyrich
compounds)isneeded foractivation.
•13-
polysaccharide
*
hexose
disaccharide
„•• -,i
=» hexose-P
ATP
•+
"|ATP|
hexose-di-P
i
triose-di-P
% oxaloaoetate
acetylCcA
I
acetoacetylCoA
i
succinylCoA
3-OH-butyrlCoA
l
crotonylCoA
f >|ATP?
butyrylCoA
acetyl-P
ATP
ATP ?
i
methylnalonylCoA
«——*propiony
ICoA
4
butyryl-P ,
.
I »(AH
acetate
propionate
butyrate
Fig. 2.Simplified pathway for the degradation of carbohydrates in the rumen and sites of ATP generation.
In table 2 a summary is given of theATP yields per
hexose-equivalent if acetate,butyrate,propionate or lactate are theonly end product of fermentation respectively.
It appears that differences inATP yield dooccur depending
onwhether phosphorylation is involved in the cleavage of
glycoside bonds of disaccharides ornot,depending onwhich
pathway of propionate formation is used and depending on
the surplus of reduction equivalents which canbe used for
the formation of CH 4 , yielding 1mole ofATP per surplus
of 8reduction equivalents.
-14direct
ATP- yield
acid
acetate
butyrate
propionate I J )
propionate I I 4 )
lactate
4 1 ' -4.5 2 >
3 -3.5
3 -6.5
2 -2.5
2 -2.5
W
total
ATP-yield
+8
+4
-4
-4
0
5 - 5.5
4.0- 5.0
3.0- 6.0
1.5 -2.0
2-2.5
Table 2. ATP-yield per hexose equivalent converted to the
various volatile fatty acids.
1) no disaccharide-phosphorylases involved
2) disaccharide-phosphorylases involved
3) dicarboxylic acid pathway
I
4) direct reductive pathway.
Under certain conditions, particularly when concentrate
rich diets are fed, lactic acid may become the end product
of rumen fermentation in significant quantities. From the
schematic pathway in figure 2 it can easily be seen that the
ATP yield per hexose equivalent converted to lactate is low
compared with the conversion in volatile fatty acids, the
conversion to propionate via the aerylate pathway excepted.
The degradation of carbohydrates to volatile fatty
acids and fermentation gases can be represented stoichiometrically a s :
-0.5aC 6 H 12 0 +aH 2 0
0.5bC^H 10 0,+2b [H]
l
6 126
•*
c C,H 10 0,
6 126
A -u.i/ TTTT•2a-b+2c™
4a-2b+4c [HJ+
j
>aC 2 H 4 0 2+aC0 2 +4a[H]
*•bC-IL.0«+bH„0
362
2
•cC..Ho0„+2cCO.+4c [H1
UJ
4o2
2
2a-b+2c
,
_
.
,
2a-b+2c
„
CO^* T CH.+ ~A H~0
S*±?£ C 6 H 1 2 0 6 - * a „Ac + b HP + c HE J f S H20 + ^
^
0» 2 + ^ ^ C H ,
-15ATP/mol of hexose
HAc :
HP :
HB
Max.
Min.
0.50
0.35
0.15
5.2
4.2
0.60
0.25
0.15
5.2
3.8
0.70
0.15
0.15
5.3
3.5
0.60
0.15
0.25
5.0
4.0
mole o f h e x o s e c o n v e r t e d t o v o l a t i l e
f a t t y a c i d s i n various molar p r o p o r t i o n s in the
Table 3. ATP-yield per
rumen of d a i r y c o w s .
Basedonthesestoichiometricrelationshipofdegradationof
hexoseunitstovolatile fattyacidsintheproportions a,
b andcforacetate,propionate andbutyraterespectively,
ATPyieldspermoleofhexose fermentedcanbecalculated
forvarious fermentationpatterns.Theresultsofsuchcalculations foranumberofcharacteristicVFAratiosinrumen
fermentationofdairycowsareshownintable 3.Fromthe
resultsitappearsthatrelatively smalldifferences inATP
yieldperhexoseequivalent fermentedoccurifthefermentationpatternshiftsfromanacetatebasedfermentationto
apropionateorbutyratebased fermentation.Themaindifferencesappearasaresultoftheinvolvementofphosphorylasesinthecleavageofglycosidicbondsandalsodepend
onwhichofthetwopropionate generatingpathwaysisused.
Ifdietscontainingonlyroughages andthereforeconsiderableamountsofstructuralcarbohydrates arefed,the
proportionofacetateintherumenisusuallyhighandoften
exceeds0.65.Propionateandbutyratearefoundinmuch
smallerproportions ofbetween0.15 and0.20 orevenlower
(seeChurch,1970).
Ifontheotherhanddietscontaininglargequantities
ofnon-structural carbohydrates arefedtheproportionof
propionatemayriseconsiderably.Particularly extrudedor
steam-flaked grainsinthedietmay causeveryhighproportionsofpropionate (upto0.40 orevenhigher).Duringthe
changeoverofadietrichinstructural carbohydrates toa
dietrich innon-structural carbohydrates temporarilyhigh
proportions (upto0.25) ofbutyratemayoccur (VanVuuren,
-16personalcommunication).Alsohigh levelsoflacticacidare
possibleanditisbelieved thatpropionate formationvia
theacrylatepathwaybecomes important.Thissuggeststhat
feedingexcessive amountsofconcentrates todairy cowsmay
reduce theATP-yieldperhexoseequivalent fermented inthe
rumen.
Dietscontaining largeamountsoffreesugarsarenot
often fed.Exceptionsmaybedietscontaining largequantitiesofsugar-beets,sugar-beetpulp,sugar-cane,molasses
orfreshyounggrassinspring.Ifsuchdietsarefedhigh
proportionsofbutyrateandpropionatemaybetheresultas
wasfoundifdietscontaininglargeamountsofsugar-beets
(KaufmannandRohr,1966),sugar-beetpulp (BathandRook,
1965),molasses (MartyandPreston,1970)andtoalesserextentperennialryegrassandItalianryegrasswhilegrazing
(BathandRook,1965)werefed.
Itshouldberealised thathighratesoffermentation
resultinhighacidproduction rates,causingadecreasein
ruminaipHwhichmay alsoaffectmicrobialgrowth.
5.Eff
içiency_of
_utilisation_of
_çarbohydrates_as_an_enerçfY
tion
Indiscussingtheprinciplesofmicrobialgrowthitwas
statedthattheenergeticefficiency ofmicrobialgrowthdependsonthegrowthrate.Oneofthefactorsdetermining
growthrateistherateofATP-generation inthedegradation
ofcarbohydrates.Thisseemstoplacenon-structuralcarbohydrates inafavourablepositionbecauseoftheir fastdegradation rateintheforestomachs (Johnson,1976).
Fromthefermentationpatternsdiscussedearlieritbecameevidentthatexcessiveamountsofnon-structuralcarbohydratesare lessefficientingeneratingATP,thusnullifying
toacertainextenttheadvantageofthehigh rateofdegradation.
Besides thesupplyofsubstrate inruminant feedingis
oftendiscontinuous.Forinstance,duringtheindoorperiod
thesupplyofsubstrate isusually restricted totwicedaily,
particularly fornon-structural carbohydrates.Asaconsequence therateofmicrobialgrowth isoftendiscontinuous.
-17Shortlyaftertheintakeoflargeamountsofnon-structural
carbohydratesby thehostanimalthemicrobialpopulation
intherumenwillstartgrowingataveryhigh rate.This
phaseof (exponential)growth,sometimesprecededbyashort
lagphase,isfollowedbyaperiodofmaximumstationary
growth,butpassessoonintoaso-called "deathphase".
Duringthislatterperiod,inbetween themaximumgrowth
rateandthesupplyofnewsubstrate,energy (ATP)becomes
limiting,possiblyeven formaintenance ofthemicrobialpopulation.Themicrobialpopulation starts "consumingitself",
micro-organisms die,disintegrate andaredegraded.Theresulting (smallamountsof)ATPcanbeusedtomeetthe
maintenance requirementofthesurvivingpartofthepopulation.Thefinalsizeofthelatterdependsonthelength
ofthedeathphase i.e.thetimebetweenmaximumgrowthand
thenextfeeding.Theresultisadecrease inthenetproductionofbiomassandalowerenergeticefficiency.
Ifthesupplyofnon-structural carbohydrates isabundant,significantquantitiesofmonomersaretakenupbythe
micro-organisms andstoredaspolysaccharides.Inboth
bacteriaandprotozoavaluesofupto35%ofpolysaccharides
inthedrymatterwere reported (McAllanandSmith,1974;
Czerkawski,1976). Iftheenergeticefficiencyofmicrobial
proteinproduction isconsidered,asisoftendoneinruminantnutrition,anyvariationinthesynthesisofcompounds
otherthanproteinwillaffectitsapparentenergetic
efficiency.Thisisillustrated intable5wherethetheoreticenergeticefficiencyofmicrobialproteinsynthesisis
compared formicrobialmasswithdifferentchemicalcomposition.
Fromtable 4itbecomesevidentthatafterfeeding
dietsrichinnon-structural carbohydrates themicrobial
proteinproductionmayseemlessefficientbecausepartof
theenergyisrequired forthesynthesisofcompoundsother
thanproteins.
Changing thefeedingpatternbymore frequentfeeding,
particularly ofthenon-structural carbohydrates containing
partofthediet,willrestricttheamountofsubstrate
available atonetimeandwilltherefore restrictthemaximum
-18sizeofthemicrobialpopulation.Itwillalsorestrictthe
deathperiodandthusdecrease therecyclingofmicrobial
mass.Thiswouldenable amoreefficientmicrobialgrowth
ATP/100 g 1 '
macromolecule
% i n IM
ATP
%i n
EM
ATP
N x 6.25
4.5
35
1.58
65
2.93
carbohydrates
1.2
35
0.42
5
residue
0.1
30
0.03
30
0.06
0.03
t o t a l ATP
2.03
3.02
. t o t a l ATP/100g ;N x 6. 25
5.80(125)
4.65 (100)
1) E s t i m a t e d from H e s p e l l a n d B r y a n t ,
1979.
Table 4. T o t a l
ATP r e q u i r e d f o r t h e s y n t h e s i s of
p r o t e i n i n m i c r o b i a l mass o f d i f f e r i n g
100 g c r u d e
chemical
composition.
andmicrobialproteinproduction.Insheepthiscouldbeconfirmedinvivowhentheanimalswere fedadietconsistingof
grassnutsandmixturesofbarley andsucrose.Feedingthese
dietsat2hourly intervals increased theenergeticefficiency
ofmicrobialproteinproduction from 3.0 gmicrobialprotein/
MJofenergytrulydigestedintheforestomachs in4.6a/MJ
(AlAttaretal., 1976).
Inarecentexperiment inourinstitute 3dairycows
were fedtheconcentrate partofthedietin6portionsper
dayratherthantwicedaily.Theanimalswere feddietsconsistingoflonghay (25%ofthedrymatter)and concentrates
(75%ofthedrymatter)largely (animalno.3)orentirely
(animalsno.1and2)consistingofflakedmaize.Crudeprotein
contentofthedietwas 10.8%inthedrymatter fortheanimals 1and2and 13.5%inthedrymatter foranimal3.
Frequent feedingalsoincreased the flowofNinthesmall
intestine asisshownintable5.Thedifference inN-flow
was thoughttobetheresultofadifference inmicrobial
proteinproduction.
Feeding dietsmainlybasedonlongroughageswillresult
inamore continuous degradation ofthecarbohydrates,be-
•19causestructural carbohydrates formamajorpart.Comparing
theenergeticefficiency ofmicrobialproteinproductionin
1
2
3
92.2
95.8
230.9
Treatment
A
Intake
(gN/day)
Duodenal N
(% i n t a k e )
B
Table 5.
133
133
Intake
(gN/day)
96.0
113
89.1
Mean
126
229.1
Duodenal N
(% of i n t a k e )
157
179
148
161
Duodenal N
(Ratio B/A)
1.18
1.35
1.31
1.28
The e f f e c t
centrate
(tr^atm^nt-
on
part
V.)
duodenal N-flow
of
the
diet
in
of f e e d i n g t h e con6 t i m e s p e r day
n n m n a r f >d t o 2 t i IDAS n e i r d a v
fl-TPaf-
mentA)indairy cows.
animalsfedroughagedietswithdietsmainlybasedonconcentratesshowedamuchbetterefficiencywith roughagediets
(McMenimanetal.,1976).Feeding theroughagedietsresulted
+
m 3.3-0.1gofmicrobialNsynthesizedper 100gOMtruly
fermented intherumen;afterfeedingconcentrates thisfigure
wasonly2.2-0.2g.Sincetheconcentratepartofthelatter
diets consistedofmaizeorbarley thepossibility ofNbeing
limiting formaximummicrobialgrowth canhowevernotentirely
beexcluded.
Onlyalimitednumberofexperiments inwhich theeffect
ofthedietary carbohydrate sourceuponmicrobialprotein
productionwasstudied,were reported inliterature.
Inacontinuous culturesystemwhichwas supplemented
with 3substrates differing innon-structuralcarbohydratecontenttheeffectonmicrobialproteinproductionwasstudied
(Sternet al., 1978). Increasing therationon-structuralcarbohydrates tostructuralcarbohydrates increased theamountof
microbialNproducedper 100gofdrymatter fermented (table6)
•20Non-structural
carbohydrates
(%/DM)
M i c r o b i a l
N
(g/100gDM
as%ofthe
fermented)
highestvalue
49
3.12
100
33
2.80
90
20
2.40
77
Table6.Effectofcarbohydratesonruminaimicrobialprotein
synthesis (Sternetal., 1978).
Inanotherexperimentsheepwere fedat2-hourlyintervalsaproteinrichbasaldiet (soybeenmeal+driedgrass)
whichwassupplementedwithwheatstarch,paperoramixture
ofboth (Offeretal., 1978).Theproductionofmicrobial
aminoacidswashighestifthebasaldietwas supplemented
with themixture (table7).
Maindietary
carbohydrate
source
M i c r o b i a l
N
(g/MJ)
as%ofthe
fermented
highestvalue
none
7.6
53
starch
6.3
44
paper
5.5
42
14.4
100
starch+paper
Table 7.Effectofcarbohydrates onruminaimicrobialprotein
synthesis (Offeretal., 1978).
Only fewdataareavailable ontheeffectofthepresence
offreesugars inthedietontheefficiency ofmicrobialproteinproduction.When thebarley (315g/day) intheexperiments ofAlAttaretal., (1976)wasentirely replacedby
sucrose (228g/day), theefficiency ofmicrobialprotein
•21production was severely reduced,bothwith feeding once daily
andwith feeding every 2hours. Ifonly part of the barley
was replaced no such effectwas found (table8 ) .
Dietary supplement
Barley
Sucrose
M i c r o b i a l N
(g/MJ
as %of the
fermented)
highest value
315
210
0
76
4.60
100
4.25
92
105
152
4.55
99
0
228
1.90
41
Table 8.Effect of free sugars on ruminai microbial protein
synthesis (AlAttar et al., 1976).
Diets inwhich themain carbohydrate sourcewas ground
barley straw, ground barley ormolasses were fed once daily
to sheep (Oldham et al., 1977). The highest ratio between
duodenal N-flow andN intake was found after feeding the
barley straw containing diet and particularly after feeding
the molasses containing diet this ratiowas severely reduced
(table 9).
Main dietary
carbohydrate
source
Duodenal-N
N-ingested
as %of
the
highest value
straw
0. 92
100
barley
0 82
0. 69
89
molasses
75
Table 9.Effect of carbohydrates on the duodenal N flow in
sheep (Oldham et al., 1977).
If the carbohydrate source has an influence on the
efficiency ofmicrobial protein production,the ratio between
structural and non-structural carbohydrates fermented in the
forestomachsbecomes of interest.For this the ratio between
crude fibre andN free extracts may be used. In our research
-22withdairy cowsequippedwith re-entrant cannulae inthesmall
intestine arangeofdietswithdifferentratiosbetweenlong
roughage andgroundandpelleted concentrateswere fed.This
resulted inaratiobetween crude fibreandNfEfermentedin
the forestomachs ranging from0.15 to0.70.Theefficiency
ofmicrobialprotein synthesis (expressedasmicrobialNper
100gofCF+NfEfermented intheforestomachs andbasedon
DAPA asamicrobialmarker)wasrelatedtothisratioandthe
resultsareshowninfigure3.
CF,S^DNfE,S
Figure 3.Relationbetween theefficiency ofmicrobialN
synthesis (gmicr.N/100 gcarbohydrates fermented
inthestomachs)andtheratiobetweenstructural
andnon-structural carbohydrates fermented inthe
stomachs ( D ^s / D N f E >g ).
-23Theefficiencyofmicrobialproteinproduction showsa
widevariation,which isnotsurprising taking intoaccount
allpossible factorswhichmayaffectthisefficiency.Ifthe
resultsarelookedatcarefully thereseemstobeanoptimum
ratiobetweencrudefibreandNfEfermented intheforestomachs
ofapproximately 0.3whichenablesmaximumefficiencyof
microbialproteinproduction.Athigher ratiostherateof
degradation andhencetherateofenergy (ATP)supplymaybecome limiting forthemicrobialgrowthrate;atlowerratios
thepHintherumenortheproductionofpropionateviathe
acrylatepathwayortheproductionoflactatemay reducethe
rateofATP-generation andtherefore therateofgrowth.The
storageofpolysaccharides insidethemicrobialcellsmayalso
have aninfluence.
Asaresultofdifferent fermentationpatterns theATP
yieldofthedegradationofdifferentcarbohydratesmaydiffer.
Excessive amountsofnon-structural carbohydrates orfree
sugarsareexpected toreducetheATP-yield.
TheefficiencyofutilisationofATPformicrobialproteinproductionmaydifferwithdifferent carbohydrate
sources.Thismaybecausedbydifferencesinthecomposition
ofmicrobialmasssynthesized,ingrowth rateand intheextent
ofrecyclingofmicrobialmass.
Forachievingamaximummicrobialproteinproductionan
optimumratiobetweendietary structuralandnon-structural
carbohydrates seemstoexist,butthevalueofthisratiois
notyetclearlyestablished.
-24-
Sumraary
Proteinsupplyinruminants dependsondietaryprotein
escapingmicrobial degradation intheforestomachsandon
microbialproteinsynthesized therein.Thislatterresults
frommicrobialgrowthwhich requiresasubstrate fromwhich
energy (ATP)andprecursors canbeextracted.
Energy formicrobialgrowthmainly results fromthe
conversionofcarbohydrates intovolatile fattyacids (VFA),
fermentation gasesandwater.Thisconversiondiffersbetween
structural andnon-structural carbohydrates.Totaldigestion
ofstructuralcarbohydrates (cellulose,hemicellulose)is
usually lowerthanofnon-structural carbohydrates (starch,
fructosans),butalargerproportion isdigested intheforestomachs.Therateofdegradationofnon-structuralcarbohydrates usuallyexceeds thatofstructuralcarbohydrates.
ThisresultsinahigherrateofATPgenerationandofthe
formationof "reduction-equivalents".Topreventaccumulation
ofreductionequivalents thefermentationpatternisdiverted
frommainly acetatetomorepropionate and/orlactate.This
shiftoften results inadecrease inpHandadecreasein
ATPgeneratedper 100gofcarbohydrates fermented.
Theenergeticefficiencyofmicrobialproteinproduction
isinfluencedbymicrobialgrov/thrate,theextentofrecyclingandthecompositionofmicrobialgrowth.
Thegrowth rate largely dependsontherateofATP
generation fromthedegradationofcarbohydrates.Non-structuralcarbohydrateshaveanadvantagehere.Theirfastrate
ofdegradationmayhowever initially resultinanexcessive
growthratebutlateronindyingoffofpartofthepopulationbecause ofashortageofATP,resulting inadecrease
ofnetgrowth.Thisrecyclinglowerstheenergeticefficiency
ofmicrobialproteinproduction.Thisdisadvantage canbeovercomebysupplyingnon-structural carbohydrates insmaller
quantitieswith shorterintervals.
Structural carbohydrates givealowerrateofATPgeneration,resulting inalowerefficiency ofgrowth,because
a largerproportionoftheenergy isrequired formaintenance.
Thisslowbutcontinuous releasenotonlypreventsexcessive
•25-
growth,butalso thedyingoffofpartofthemicrobialpopulation.The finalefficiency ofmicrobialgrowthisthe.reforeoftenashighorevenhigherasafterthesupplyof
non-structuralcarbohydrates.
Thecompositionofmicrobialgrowthvariesparticularly
withrespecttoproteinandcarbohydrate content.Substantial
amountsofcarbohydratesmaybe (temporarily) storedinside
microbial cells,particularly ifthesupplyofnon-structural
carbohydrates ishigh.Theenergy required forthisstorage
isnotavailable forproteinproduction andtheenergetic
efficiency ofthe latterapparentlydecreases.
Fromexperimentswithanimalsitappearsthatfeeding
concentratesmore frequently insmallerquantitieshasa
positive effectontheamountofmicrobialproteinsynthesizedper 100gorganicmatter fermented inthe forestomachs.
Large,amountsoffreesugarsappeartohaveanegativeinfluenceonthemicrobialprotein synthesis.Between structural
andnon-structural carbohydrates anoptimalratioseemsto
existatwhich themicrobialproteinproduction intheforestomachs reachesamaximumefficiencybutthisoptimum
couldnotyetaccuratelybedetermined.
§§•!?§DY §£ting
Herkauwers zijnvoorhuneiwitvoorziening afhankelijk
vantweeeiwitbronnent.w.voereiwitdatniet-afgebrokende
voormagen passeertenmicrobieeleiwitdatindiezelfdevoormagenwordtgesynthetiseerd.Ditlaatste isafhankelijkvan
microbiëlegroeiindevoormagen,waarvanhetwezen inhet
onderhavige artikelwordtuiteengezetmetspecialeaandacht
voorderelatiemetdeverschillendekoolhydraten inhet
rantsoen.
Microbiële groeihangtvooralsamenmetdebeschikbaarheidvaneensubstraatwaaruitenergie (ATP)enbouwstenen
kunnenwordenverkregen.Energievoormicrobiële groeikomt
voornamelijk beschikbaar uitdeomzettingvankoolhydraten
invluchtigevetzuren (V.FA),fermentatiegassen enwater.De
hoeveelheid diewordtafgebroken isafhankelijk vandeaard
•26-
van dekoolhydraten (structurele en niet-structurele koolhydraten).De totale vertering van structurele koolhydraten
(cellulose,hemicellulose) is doorgaans lager dan die van
niet-structurele (zetmeel,fructosanen, vrije suikers),
maar een groter deel van de vertering vindt plaats in de
voormagen.
De afbraak van niet-structurele koolhydraten in de
voormagen verloopt doorgaans sneller dan die van structurele
koolhydraten. Deze snelle afbraak resulteert niet alleen in
een snelbeschikbaar komen van veelATP,maar ook van veel
"reductie-equivalenten". Hetwegwerken van de laatsten verschuift de verhouding waarin vluchtige vetzuren v/ordengevormd van veelazijnzuurnaar meer propionzuur en/of melkzuur. Deze verschuiving gaatmeestal enerzijds gepaard met
een verlaging van depH en anderzijds meteen verlaging van
dehoeveelheid ATPwelke per 100 g afgebroken koolhydraten
wordt gevormd.
De energetische efficiëntie1van microbiële eiwitproductie hangt samen met de groeisnelheid, demate van recycling en de samenstelling van de microbiële groei.
De groeisnelheid hangt sterk afvan de snelheidv/aarmee ATP uit de afbraak van koolhydraten kanv/ordenvrijgemaakt. In dit opzicht zijn niet-structurele koolhydraten in
het voordeel. De afbraak ervan verloopt echter vaak zo snel,
dat naeen aanvankelijke stormachtige groei er een tekort
optreedt aan substraat en daarmee aanATP. De beschikbaar
komende ATP is geheel nodig voor onderhoud en resulteert
nietmeer ingroei.Vaak vindt zelfs afsterling van een deel
der micro-organismen plaats,waardoor reeds geproduceerd eiwitweerwordt vernietigd en de netto eiwitproduktie afneemt.
Deze "recycling" verlaagt de energetische efficiëntie van
microbiële eiwitproductie. Hetmetkorte tussenpozen verstrekken vankleine hoeveelheden niet-structurele koolhydraten lijkt deze nadelen grotendeels tekunnen opheffen.
Wanneer structurele koolhydraten als substre.atbeschikbaar zijn is de toevoer van ATP langzamer, resulterend in
een langzamer microbiële groei met een groter aandeel van de
energie nodig voor onderhoud. De geleidelijkheid van het
vrijkomen vanATP voorkomt echter een explosieve groei
-27-
en.daarmeeookhetdaarnaafstervenvaneendeel.vande
micro-organismen,waardoor deuiteindelijkegroeieneiwitproductieminstens zoefficient zijnalsbijnietstructurele
koolhydraten.
Desamenstellingvandemicrobiëlegroeikanvariëren,
vooralwathetgehalteaaneiwitenkoolhydratenbetreft.
Koolhydratenkunnen (tijdelijk)inaanzienlijke hoeveelheden
inmicrobiële cellenwordenopgeslagen,vooralnahetverstrekkenvanveelniet-structurelekoolhydraten.Dehiervoor
nodigeenergie (ATP)isnietmeerbeschikbaarvoormicrobiële
eiwitsynthese endeenergetischeefficiëntievandelaatste
neemtschijnbaaraf.
Uitdierproevenblijktdatvakervoerenvanvooral
krachtvoereengunstigeffectheeftopdehoeveelheidmicrobieeleiwitdieper 100gindevoormagenafgebrokenorganische stofwordtgeproduceerd.Deaanwezigheid vanveel
vrije suikersdaarentegenheefthieropeennegatieve invloed.
Er lijkteenoptimaleverhouding tussenstructureleennietstructurelekoolhydraten tebestaan,waarbijdeefficiëntie
waarmeeindevoormagenmicrobiëeleiwitwordtgevormd,maximaalis,maareen.nauwkeurige vaststellingvanditoptimumis
totnutoenietmogelijkgebleken.
•28-
Literature
1.AlAttar,A., R.A. Evans and R.F.E.Axford, 1976
The effect of frequency of feeding and of dietary energy
source upon microbial protein synthesis in the rumen of
sheep
Proc. Nutr. Soc. 35:109A
2.Armstrong, D.G., 1976
Proteinverdauung und -absorption beiMonogastriden und
Wiederkäuern.
Ubers. Tierernährng. 4:1-24
3.Bath,I.H.and J.A.F. Rook, 1965
The evaluation of cattle foods and diets in terms of the
ruminai concentrations of volatile fatty acids
J. Agric. Sei. (Camb.), 64:67-75
4.Bauchop,T. and S.R. Eisden, 1960
The growth ofmicro-organisms in relation to their energy
supply
J. Gen.Microbiol. 23:457-469
5.Chalupa,W., 1978
Digestion and absorption of nitrogenous compounds in
ruminants
Page 211-230 in Proc. 3rdWrld Conf. onAn.Feeding,Madrid
6. Clark,J.H., 1975
Nitrogen metabolism in ruminants:protein solubility and
rumen bypass ofprotein and amino acids
Page 261 inM.Friedman (ed.), Protein nutritional quality
of foods and feeds,vol.1,part II
Marcel Dekker,New York
7.Cheng,K.J., D.E.Akin and J.W. Costerton, 1978
Rumen bacteria: interaction with particulate dietary components and response to dietary variation
Fed. Proc. 36:193-197
8.Church,D.C., 1970
Digestive Physiology and Nutrition of Ruminants
O.S.U. Book Stores,Corvallis,Oregon
9. Czerkawski,J.W., 1976
Chemical composition ofmicrobial mass in the rumen
J. Sei. Fd.A g r i c , 27:621-632
10. D e m e i j e r , D . I . , 1 9 7 6
Een kwantitatieve studie vanhet metabolisme van pensmaaginhoud
Thesis State University Ghent,Belgium
11. Egan,A.R., D.J. Walker,C.J. Nader and G. Storer, 1975
Comparative aspects of digestion of four roughages by
sheep
Aust. J. Agric. Res.,26:909-922
-2912. Es,A.J.H,van and S. Tamminga, 197S
Utilisation of nutrients for net protein synthesis
Page 171-132 in Proc. 3rdWrld Conf.onAn.Feeding,Madrid
13. Hespell,R.B. andM.P.Bryant, 1979
Efficiency of rumen microbial growth; Influence of some
theoretical andexperimental factors on ï._ p
J. An. Sei. (inpress)
14. Hogan,J.P. and R.H. Weston, 1970
Quantitative aspects ofmicrobial protein synthesis in
the rumen
Page 474 inA.T.Phillipson (ed.), Physiology of digestion
and metabolism in the ruminant
Oriel Press,Newcastle upon Tyne
15. Huhgate, R.E., 1966
The rumen and its microbes
Academic Press,New York
16. Johnson,R.R., 1976
Influence of carbohydrate solubility on NPN utilisation
in the ruminant
J. An. Sei., 43:184-191
17. Kaufmann,W., 1977
Economic and other considerations governing decisions on
the advisability of incorporating additional and new
sources ofprotein and NPN into the diet ofbeef cattle
Page 101 in,Protein and non protein nitrogen for Ruminants
Pergamon Press,Oxford
18. Kaufmann,W. andK. Rohr, 1966
Ergebnisse gaschromatografischerBestimmung der flüchtigen
Fettsäuren im Pansen bei unterschiedlicher Fütterung
Z.Tierernärng,Tierphysiol. u.Futtermittelkde. 22tilg.Klooster,A.Th. van't and B.D.E. Gaillard, 1976
Digestion of carbohydrates in the intestines of dairy
cows
Misc. Papers Landbouwhogeschool 12:79-94
20. Marty, R.J. and T.R. Preston, 1970
Molar proportions of the short chain fatty acids (VFA)
produced in the rumen of cattle given high molasses diets
Rev. cubana Cienc. agric. 4:183-186
21. McAllan,A.B.and R.H. Smith, 1974
Carbohydrate metabolism in the ruminant.Bacterial carbohydrates formed in the rumen and their contribution to
digesta entering the duodenum
Br. J. Nutr. 31:77-88
22. McMeniman,N.P.,D. Ben Ghedalia and D.G. Armstrong, 1976
Nitrogen-energy interactions in rumen fermentation
Page 217 inK.N. Boorman et al., (eds.), Protein metabolism and nutrition
Butterworth, London
•30-
23. Hertens,D.R., 1977
Dietary fibre components: relationship to the rate and
extent of ruminant digestion
Fed. Proc. 36:187-192
24. Offer,N.W., R.F.E.Axford and R.A. Evans,1978
The effect of dietary energy source on nitrogen metabolism
in the rumen of sheep
Br. J. Nutr. 40:35-44
25. Oldham, J.D., P.J. Buttery,H. Swan and D. Lewis, 1977
Interactions between dietary carbohydrates and nitrogen
and digestion in sheep
J. Agric. Sei. (Camb.), 89:467-479
26. Prins,R., 1977
Biochemical activities of gut micro-organisms
Page 73 inR.T.J. Clarke and T. Bauchop (eds.), Microbial
ecology of the gut
Academic Press,London
27 Reddy,C.A. and H.D. Peck, 1978
Electron transport phosphorylation coupled to fumarate
reduction by H - andMg2+ - dependant adenosine triphosphatase activity inextracts of the rumen anaërobe Vibrio
succinogenes
J. Bacteriol., 134:982-991
28. Stern,M.D.,H. Hoover,C.J. Sniffen, B.A. Crooker and
P.H. Knowlton, 1978
Effects ofnon-structural carbohydrates,urea and soluble
protein levels on microbial protein synthesis in continuous culture of rumen contents.
J. An. Sei., 47:944-956
29. Stouthamer,A.H., 1973
A theoretical study of the amount ofATP required for
synthesis ofmicrobial cell material
Antonie van Leeuwenhoek, 39:545-565
30. Stouthamer,A.H., 1976
Yield studies in micro-organisms
Meadowfield Press Ltd., Durham
.31. Stouthamer,A.H. and C. Bettenhaussen, 1973
Utilisation ofenergy for growth andmaintenance incontinuous andbatch cultures of micro-organisms
Biochira.Biophys. Acta 301:53-70
32. Tamminga, S., 1978
Measurement of microbial protein synthesis in the rumen
Page 5.1 inD.F. Osbourn, D.E. Beever and D.J. Thomson
(eds.), Ruminant digestion and feed evaluation
Agricultural Research Council,London
33. Tamminga,S.',1979
Protein degradation in the forestomachs of ruminants
J. An. Sei., (inpress)
—o.1.—
34. Thauer,K., K. Jungermann and K, Decker, 1977
Energy conservation in chemotrophic anaerobic bacteria
Bact.'Rev. 41:100-130
35. Ulyatt,M.J., D.W. Dellow, C.S. Reid and T. Bauchop, 1975
Structure and function of the large intestine of ruminants
Page 119 in Ï.W.McDonald and A.C.I. Warner (eds.),
Digestion and metabolism in the ruminant
36.Waldo,D.R.,L.W. Smith and E.L.Cox, 1972
Model of cellulose disappearance from the rumen
J. Dairy Sei., 55:125-129
37. Waldo,D.R., 1973
Extent and partition of cereal grain starch digestion in
ruminants
J. An. Sei., 37:1062-1074