Rumen digestive physiology and microbial ecology

44 Pages·1992·1.9 MB·English

by  Hoover, W. H.

Rumen
Digestive
Physiology and
Microbial Ecology
Bulletin 7Q8T J November 1992
'^"nculture and Forestrx Experiment Station
.i/J y L
:
^1, est \' i r 2 i n i a U n i \" e r s i t v
Authors
W.H.HooverisprofessorandT.K.Millerisresearchassistant,Division
ofAnimal and Veterinary Sciences, College ofAgriculture and Forestry,
WestVirginiaUniversity.
Front Cover: Dr. William H. Hoover with the continuous culture system
WVU
developedat forstudyingrumenfunction.
WestVirginiaUniversity
AgriculturalandForestryExperimentStation
CollegeofAgriculture andForestry
RobertH.Maxwell,Director
Morgantown
Rumen Digestive Physiology and
Ecology
IVIicrobial
W.H. Hoover and T.K. Miller
RUMEN CHARACTERISTICS
Rumen Conditions
Under normal circumstances, the rumen provides a suitable environ-
mentforthegrowthofnumerousstrictlyanaerobicmicrobes,aswellassome
facultative anaerobes. The temperature usually is maintained within the
rangeof38-41Cwith39usedasacommonmeantemperature.Ameemredox
potentialof-350mV,withfluctuationsbetween-250and^50mV,reflects
thestrongreducingmediumandtheabsenceofo?Q^gen.Therumenisusually
well-buffered, duetothepresenceoforganicacidsproducedfromfermen-
tation, thebuffering capacityofvarious feeds (87) and the copious flowof
saUva,inwhichtheprincipalbuffers arebicarbonatesandphosphates. The
pHrangesofthesevariousbuffersareinTable 1.
Table 1.BufferingRangesofSubstancesintheRiunen.
Buffer pHrange
Fermentationacids 3.8-5.0
Forages 5.0-6.0
Bicarbonates 5.0-7.0
Phosphates 6.0-8.0
In addition, the presence ofammonia can act to prevent decreases in
rumenpH,particularlywhentheconcentrationofNH3is20mg/dlorgreater.
SaUvaproductioncanbeveryhighinacow,reachingover1801/day.Inspite
ofthis potential to buffer rumen fermentation, rumen pH can range from
around7.0onforage dietstoaslowas4.6whenanimalsarefedhigh-grain
diets. This is not only because of the production of acidic fermentation
end-products,butalsobecausecertainfeedsreducesalivaflowbyfailingto
stimulatechewingandrumination.Comparedtodrylonghay,whichstimu-
latesthegreatestextentofchewingandrumination,highmoisturefeedssuch
as pasture or silage can reduce the amount ofsaliva produced per kg dry
matterbyhalf, andgrainsorpelletedfeedscanreduceflowto20% ofthat
onalonghaydiet(15).Theaveragepercentcompositionofthegasesinthe
rumenisshowninTable2.Carbondioxideandmethanearethemajorgases
present. Since a large proportion ofCO2 is reduced to methane, the con-
centrations of these two gases become closer to equahty with time after
feeding. Oxygen varies from <.1% to slightly over 5% depending on the
amountingestedinwaterandfeed.Theosmoticpressureofrumencontents
ranges between 260 and 340 miUi-osmoles (m OSM), with an averzige of
about 280. High osmotic pressure is detrimental to rumen function, and
rumen osmotic pressure in the range of 350-380 has been shown to stop
rumination(97).
Table2.CompositionofRumenGases.
Component Averagepercent
Hydrogen .2
Oxygen .5
Nitrogen 7.0
Methane 26.8
Carbondioxide 65.5
Osmotic pressure affects not onlywater movement across the rumen
epitheUal tissue, but movement of volatile fatty acids (VFA) as well. As
osmoticpressure increases, absorption ofVFAfrom the rumen decreases
(64). The magnitude of this effect for acetic acid is shown in Figure 1.
Propionic and butyric acids are similarly affected. Feeding high levels of
grainscontributestohighosmoticpressure,asdoesthefeedingofminerals.
Consumptionof10kgofaforage-graindietcontaining1.5% sodiumbicar-
m
bonate, .5% salt and 1.0% dical can cause a transient increase of75-80
OSMinan80Uterrumen. Forthisreason, caremustbe exercisedto avoid
theinclusionofexcessivequantitiesofmineralsinthediet.
En2ymatichydrolysisofthefeedcomponentsrequiresaliquidinterface.
The rumen has a high moisture content that remains fairlyconstant in the
range of85-90%. In addition, the moisture contentincreases fromthe top
to the bottom of the rumen due to the stratification ofthe contents, thus
providingadiversehabitatforavarietyoforganisms.
Absorption From the Rumen
Volatile fatty acids produced by the microbes are passively absorbed
throughtherumenwall.Therateofabsorptionisdependentonchainlength,
pH, concentration and, as indicated in the preceding section, osmolality.
BetweenpH4.5and6.5,ratesofVFAabsorptionarebutyrate > propionate
> acetate; above 6.5, the rates are similar. Since the undissociated acid is
morerapidlyabsorbed,aspHdecreases,VFAabsorptionincreases.Thenet
absorption,whichistheamountofVFA'sthatreachtheblood,isdependent
onbothconcentrationintherumenandthequantitymetabolizedbythecells
RATE OF ACETIC ACID ABSORPTION, %/H
30
200 250 300 350 400 450 500 550
OSMOLARITY OF RUMEN FLUID
(MILLI-OSMOLES)
Figure 1.EffectsofOsmoticPressureonMovementofAceticAcid
AcrosstheRumenEpithelium (64).
ofthe rumen wall. The rates ofmetabolism by cells ofthe rumenwall are
butyrate > propionate > acetate.Acetateisfoundinthegreatestconcentra-
tion in the rumen (ratios are usually 5:1.5:1 for acetate, propionate and
butyrate,respectively).Asaresultofthehigherconcentrationandlowerrate
ofmetabolism by the nmienwall, acetate enters the blood in the greatest
quantity, followed by propionate. Because of the smaller amount in the
rumenandextensivemetabolismbytherumenwall,veryUttlebutyrateenters
the blood. Lactate, when present, is absorbed directly through the rumen
wall. In dairycattle fed andmanagedwell, lacticaciddoesnot accumulate
intherumencontentstoanylargeextent.Itisfoundinhighestconcentration
intherumenofcattleplacedsuddenlyonahighgraindietbecausesufficient
numbersoflactateutilizingbacteriahavenotbeenculturedintherumenin
thosecircumstances.However,ifgradualintroductionofgrainispracticed,
thelactate-utilizingbacteriawilldevelopandpermitonlyatransientincrease
in lactic acid accumulation following ingestion of a diet high in readily
fermentablecarbohydrates (46).LowpHfavorsabsorptionoflacticacid,as
wellasproduction. RumenmicrobesproducebothL(+) andD(-) lactate
inessentiallyequalamountsandbotharereadilyabsorbed.Comparedtothe
L(+)form,theD(-) formisslowlymetabolizedbyruminanttissuessuchas
the liver, and is a major cause oflactic add toxicity. Glucose also can be
absorbed through the rumen wall, but is usually present in nutritionally
insignificant quantities. Ammonia is readily absorbed, with the rate being
dependent on concentration and pH. Absorption is rapid at pH 6.5 and
higher, £inddeclinestonearly atpH 4.5.Aminoadd absorptioncantake
place, but concentrations ofamino acids are maintained at lowlevels, and
<10% of total N absorption is usually as amino acids. Of the minerals,
sodium and magnesium are actively absorbed through the rumen v^all.
MagnesiumabsorptionisdecreasedbyaNa:Kratiointherumenoflessthan
5:1, andbyhigh ammoniaconcentrations (50). Potassium and chlorinecan
pass through the rumen wall in either direction, depending on concentra-
tions, other ion effects and osmolaUty. The rumen also is responsible for
about one-third of total zinc absorption. The rumen wall is apparently
impervioustocalcium,phosphorus,copper,molybdenumandsulfur.
RUMEN MICROBES
The major microbial populations in the rumen are bacteria, protozoa
andfungi.Ofthese,bacteriaarefoundinthegreatestnumbersanddiversity.
Sofar,22generaand63 specieshavebeenidentified,withtotalconcentra-
tions of10' to 10^°cells/gofrumen contents. Protozoa numbers canbe as
high as 10^ to 10^ cells/g contents, and are greatly affected by feeding
practices.Todate,sixgeneraand16specieshavebeenidentified.Protozoal
numbersincreaseinproportiontotheamountofgrainfedunlesstheincrease
inacid productioncausedbyhigh grain levelsreduces rumenpH to 5.5 or
below.InstudiesatKansasStateUniversity(82)itwasfoundthatinfeedlot
cattlefedonly8-10%foragediets, 13%weredefaunatedwhiletheremain-
derhadprotozoaconcentrationsrangingfrom 10^to 10^/ml.Theanaerobic
fungi are the most recently recognized group of rumen microbes. These
organisms were first identified in the rumen in 1975 by Orpin (63), who
reportedthat certainflagellatedcells thought tobe protozoawere, in fact,
zoosporesoffungi. Asof1987, threegeneraand four specieswere known.
Sincethen,additionalgenerahavebeendiscoveredbyBarretal.(3)in1989
andBretonetal. (8)in 1990,andother,unidentifiedfungiwerereportedin
ruminantsinAustralia(65).
Microbial Compartmentalization
Themicrobes arenotrandomlydistributedthroughouttherumen,but
tend to associatewithvarious fractions ofthe rumen contents, orwith the
rumenwallitself.Inadultruminants,somebacteriaandprotozoaarefound
associatedwiththeUquidphaseoftherumencontents,someadheredtothe
solid digesta, and some associated with the rumen wall. The quantity and
speciesofmicrobesineachofthesepools arenot static,butvarywithtime
afterfeeding.
Bacteria. The compartmentation ofthe bacteria appears to be depen-
dent largely on nutrient availability or specialized metaboUc activity. The
liquid-associated, or non-adherentbacteria, consist oforganisms found in
therumenfluidthatfeedonsolublecarbohydratesandproteins.Thesolid-
associated, or adherent bacteria, are bound to feed particles in order to
digest theinsoluble polysaccharides such as starch andfiber, aswellasthe
less soluble proteins. An exchange of microbes between the soUd- and
liquid-associatedcompartments occursimmediatelyafterfeeding,wherein
some of the soUd-associated bacteria are displaced from the particulate
matter and become part of the Uquid-associated pool. These microbes
appeartobe attractedchemotacticallybythepresence ofsolublenutrients
inthenmienfluidas aresult offeeding. Inthismanner, newfeedparticles
arelocatedandcolonized(99).
Inadditiontothebacteriaassociatedwiththenmiencontents,therealso
isapopulationfoundontheepithelialsurfaceofthenmien.Althoughmost
of these "epimural" organisms are species that also are found in rumen
contents,thispopulationappearstocontainhigherlevelsofureolytic,aero-
bicandfacultative anaerobicmicrobes than does the rumen contents. The
uniquemetaboUcfunctionsascribedtotheepimuralmicrobesbyChengand
Costerton (13) are: 1. Utilization of oxygen that diffuses from the blood
throughthestratifiedsquamousepitheUalceUsoftherumen,thushelpingto
maintainthe anaerobicnature ofthe rumen contents; 2. Conversionofthe
ureathatdiffusesthroughthenmienwall,toammonia.Thisisofsignificance
inthatfewanaerobicmicrobesproduceureases;3.Digestionofthesloughed
epitheUal cells of the rumen wall. These highly keratinized cells wouldbe
indigestiblewereitnotforthemicrobialhydrolysis.Ithasbeenalsoobserved,
particularlyonhigh concentratediets,thatsomedigestionofepitheUalcells
canoccurbeforethecellsaresloughed,resultingindamagetotheunderlying
tissue.Jensen and co-workers (39) proposed that the subsequentbacterial
invasionmaybe the source ofUver abscesses inbeefcattle.Fusohacterium
necrophomm,anetiologicalagentinUverabscess,hasbeenisolatedfromthe
epimuralpopulation.
Protozoa. The physical compartmentaUzation ofprotozoais similar to
that of bacteria, in that there are Uquid-associated, soUds-associated and
epimuralpopulations. In additionto the digestionofsoluble andinsoluble
nutrientsbyliquid-andparticulate-associatedpopulations,respectively,the
protozoa have a further motivation regarding adherence to feed particles
and to the nmien waU. Because the time required for most species of
protozoatoreproduceis longerthan theturnover time oftherumenfluid,
the protozoa must attach to large feed particles or to the walls of the
reticulum andrumeninorderto avoidwash-out. Thishas ledto theuse of
theterm"sequestered" inreference totheattachedprotozoa.Withregard
to digestion of soluble and insoluble nutrients and colonization of feed
particles, however, the protozoal responses are similar to those described
forbacteria.Uponfeeding,thesolublenutrientsleachingfromthecutends
ofthe plant materials chemotacticallyattract sequesteredprotozoa, which
rapidlylocateandcolonizethenewfeedparticles.Speciesofbothholotrich
and entodinimorph protozoa can ingest starch granules and smedl plant
fragments, which they digest internally. In addition, there is evidence that
organisms of both groups attach to large particles and hydrolyze fibrous
polysaccharidesviaenzymesthataresecretedintothesurroundingmedium
orboundtothesurfaceofthemicrobes(exoenzymes).
Fungi. The fungi appear also to be compartmentedbetween attached
andUquid-associatedforms,butinamorecomplexmannerbecause ofthe
morphologicaltypesinthelifecycleoftheseorganisms.Thevegetativestage
ofthefungiisaparticulate-associatedsporangiumwithanextensiverhizoid
networkthatinvadestheplantmaterial.Thesporangiaproduceandrelease
flagellated,motilezoosporesthatbecomefluid associateduntilattachment
to anewfeedparticle. Likebacteria andprotozoa, chemotaxis plays arole
inthelocationandcolonizationofplantmaterialbyfungi.21oosporogenesis
is triggered by the presence of heme and other components in the newly
ingested feed, and the presence of soluble nutrients serves to attract the
newlyreleasedzoosporetothefreshfeedparticles(35).
Thus,eachofthemajormicrobialcommunities—bacteria,protozoaand
fungi—maintainapatternoffeedcolonizationthat allowsthemtorespond
tothetypesoffeedandthevarietyoffeedingcyclesassociatedwithvarious
ruminants.
Establishment of Microbial Populations
Bacteriain therumen contents. Bacterialcolonizationdevelopsrapidly,
with a very diverse population, soon after birth. By three days of age,
anaerobicbacteriaarepresentatconcentrationsof10^colony-formingunits
(CFU) per g of contents. At this age, facultative anaerobes are found in
moderatenumbers, 10^to 10^CFXJ/g.Thesedeclinesteadilytoadultlevels
of about 10^ to 10^ CFU/g by 5 weeks of age. According to the work of
Andersonetal. (2),otherchangesare:
ColonyFormingUnits/grams
Organisms 3days 12weeks*
Proteolytics lO^-lO'^ lO^-loS
Amylolytics lO^-lO^ lO'-lQlO
CeUulolytics lO^-lCp >10'7
Methanogenics 10^ 10^-10^
*generallyrepresentativeofadultpopulations
Ingestion ofsoUd feed is necessary for the establishment ofthe adult
microflora,andearlyweaningwillcausemicrobestoreachadultproportions
sooner.Earlyweaningfrommilkisnotanabsolutenecessity,sincemicrobes
capableofhydrolyzingallfeedcomponents,includingcellulose,arepresent