• RUMEN MICROBIOLOGY AND FERMENTATION


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Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 55
RUMEN MICROBIOLOGY
AND FERMENTATION
C References: Allison (1993) & Leek (1993) in "Dukes’ Physiology of Domestic Animals"
by Swenson & Reece, ed. (1993), "http://arbl.cvmbs.colostate.edu/," and others.
INTRODUCTION
(Herbivorous strategies or utilization of forages in General;
http://arbl.cvmbs.colostate.edu/)
" Professional Fermentors? - Two distinct strategies evolved for "professional fermentors"
A. Cranial fermentors (or ruminants) - e.g., Cattle, sheep, and deer.
1) Have a large, multi-compartmented section of the digestive tract between the
esophagus & true stomach.
2) The forestomach can house a very complex ecosystem that supports fermentation.
B. Caudal fermentors, aka cecal digestors - e.g., Horses & rabbits
1) Similar to pigs & humans through the stomach and small intestine.
2) But, their large intestine, where fermentation takes place, is complex and
exceptionally large.
C. Similarities & differences?
1) The process and outcome of fermentation are essentially identical in the rumen of a
cow or the cecum of a horse.
2) However, the position of the "fermentation vat" in relation to the small intestine has
very important implications for the animal's physiology and nutrition.
3) Summary?
Function
Ability to efficiently digest and extract energy from cellulose Yes Yes
Ability to utilize dietary hexose sources directly No Yes
Ability to utilize the protein from fermentative microbes Yes No
K Remember? The small intestine is the only site where simple sugars and amino
acids can be absorbed in all animals!?
4) Utilization of dietary starch?
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 56
a) Horses? - Starch to glucose by amylase & maltase in the SI, and glucose is
absorbed into circulation.
b) Ruminants? - Very little is absorbed as glucose, and starch & others are
fermented to VFA in the forestomach.
5) Protein?
a) The bodies of microbes can be a source of high quality protein!
b) Because the fermentation vat of a horse is behind the small intestine, all their
microbial protein is lost - ?
c) Ruminants - Microbes can flow into the stomach and small intestine, where
they are digested and absorbed as amino acids and small peptides.
MICROBIOLOGY OF THE RUMEN
1. Introduction
A. Gastrointestinal tracts of ruminant species (& also others)? - Colonized by a diversity of
microorganisms, and the use of fibrous feedstuffs by microbes depends on the metabolic
activities anaerobic microbes in the rumen and the large intestine.
B. Rumen & large intestine? - Occupied by highly concentrated populations of bacteria, and
also by protozoa and anaerobic fungi.
C. Gastrointestinal tract? - Perhaps, the most intimate environment that animals are
exposed to, and has a profound impact on the physiology and health of the host animal.
2. Forestomach Fermentation
A. In the simple stomach species? - Before reaching the acidic stomach, fermentation is
limited to the ethanolic or lactic acid type, which may have minor impacts on the
nutrition of the animal (. . . obviously, some exception though!).
B Forestomach fermentation? - Occur at nearly neutral pH, and may be separated from the
acidic region.
C. Ruminants:
1) Are the most diverse (about 155 species) and best known of the herbivores with
extensive forestomach fermentation systems.
2) But, there are also others such as Camelidae (camel, llama, alpaca, guanaco, and
vicuna), hippopotamuses, tree sloths (Cholopus and Bradypus), and leaf-eating
monkeys.
D. Reticulorumen:
1) A fermentation chamber, in which bacteria and protozoa are located.
2) Can convert plant materials to volatile fatty acids (VFAs), methane, carbon dioxide,
ammonia, and microbial cells.
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 57
E. Some advantages of fermentation in the reticulorumen?
1) Allows digestion and then absorption of fermentation products that are of value to
the host (e.g., microbial cells, VFAs, and B vitamins) before the acidic abomasum.
2 Change poor quality protein/N compounds to a "good-quality" microbial protein.
3) Selective retention of coarse particles extends fermentation time and allows for
further mechanical breakdown during rumination (cud chewing).
4) Release of fermentation gas (mostly CO2 & CH4) from the system by eructation.
5) Toxic substances in the diet may be attacked by the microbes before being
presented to the small intestine.
3. Ruminal Microbes
A. Available information? - Obtained mostly from studies of cattle and sheep.
B. Knowledge on wild ruminants is largely limited to that obtained by microscopic
observations, but predominant bacteria species in rumen contents of deer, reindeer, elk,
and moose are ones also found in cattle and sheep (based on cultural studies).
C. Important bacterial species in cattle and sheep and their fermentative properties:
1) Fermentative properties of ruminal bacteria: (Hespell, 1981)
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Species Function* Products¶
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Fibrobacter (Bacteroides) succinogenes C,A F,A,S
Ruminococcus albus C,X F,A,E,H,C
Ruminococcus flavefaciens C,X F,A,S,H
Butyrivibrio fibrisolvens C,X,PR F,A,L,B,E,H,C
Clostridium lochheadii C,PR F,A,B,E,H,C
Streptococcus bovis A,S,SS,PR L,A,F
Ruminobacter (Bacteroides) amylophilus A,P,PR F,A,S
Prevotella (Bacteroides) ruminocola A,X,P,PR F,A,P,S
Succinimonas amylolytica A,D A,S
Selenomonas ruminantium A,SS,GU,LU,PR A,L,P,H,C
Lachnospira multiparus P,PR,A F,A,E,L,H,C
Succinivibrio dextrinosolvens P,D F,A,L,S
Methanobrevibacter ruminantium M,HU M
Methanosarcina barkeri M,HU MC
Treponema bryantii P,SS F,A,L,S,E
Megasphaera elsdenii SS,LU A,P,B,V,CP,H,C
Lactobacillus sp. SS L
Anaerovibrio lipolytica L,GU A,P,S
Eubacterium ruminantium SS F,A,B,C
Oxalobacter formigenes O F,C
Wolinella succinogenes HU S,C
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* C = cellulolytic; X = xylanolytic; A = amylolytic; D = dextrinolytic; P = pectinoiytic; PR =
proteolytic; L = lipolytic; M = methanogenic; GU = glycerol-utilizing; LU = lactate-utilizing; SS =
major soluble sugar fermenter, HU = hydrogen utilizer; O = oxalate-degrading.
¶ F = formate; A = acetate; E = ethanol; P = propionate; L = lactate; B = butyrate; S =
succinate; V = valerate; CP = caproate; H = hydrogen; C = carbon dioxide; M = methane.
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 58
2) All of these bacteria are anaerobes & most are carbohydrate fermenters - Including
gram-negative and gram-positive cells, sporeformers and non-sporeformers, and
motile and nonmotile cells.
3) Obligatory anaerobic mycoplasmas (. . . cells enclosed by membranes rather than by
rigid walls):
a) Some interest because detected only in rumen & can ferment starch and other
carbohydrates.
b) But, minor in terms of proportions relative to total population components,
and heir contributions would be small.
D. Numbers and relative volumes of bacteria and protozoa:
1) Approximate average volumes and numbers of microbial groups in the rumen of
sheep: (Warner, 1962)
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Organism Avg. cell volume Number/mL % of total*
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Ciliate protozoa
Isotricha, Epidinium, Diplodinium sp. 1,000,000 1.1 x 104 33.55
Dasytricha, Diplodinium sp. 100,000 2.9 x 104 8.78
Entodinium sp. 10,000 2.9 x 105 8.79
Polyflagellated fungal zoospores 500 9.4 x 103 0.01
Oscillospiras and fungal zoospores 250 3.8 x 105 0.26
Selenomonads 30 1.0 x 108 0.09
Small bacteria 1 1.6 x 1010 48.52
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*Total microbial volume was about 0.036 mi per milliliter of rumen fluid.
2) Protozoa are far less numerous than bacteria, but they are so much larger than the
bacteria that they may occupy a volume nearly equal to that occupied by the
bacteria.:
a) Most important ones are anaerobic ciliates that are differentiated on the basis
of morphology. Most of them belong to two, "holotrichous &
entodiniomorphid" protozoa.
b) Numbers and kinds of protozoa are markedly affected by diet, and the
variability among protozoa populations tends to be greater than the bacterial
population.
4. Rumen Ecology
A. Rumen - An open ecosystem, and it is a dynamic system because conditions are
continually changing (http://arbl.cvmbs.colostate.edu/).
1) Each milliliter of rumen content contains roughly:
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 59
a) 10 to 50 billion bacteria,
b) 1 million protozoa, and
c) Variable numbers of yeasts and fungi.
2) The environment of the rumen:
a) Anaerobic, and as expected, almost all these microbes are anaerobes or
facultative anaerobes.
b) Fermentative microbes interact & support one another in a complex food web,
with waste products of some species serving as nutrients for other species.
3) Bacteria? - Although many bacteria utilize multiple substrates, some of the major
groups, each of which contain multiple genera and species, include:
Cellulolytic - Digest cellulose
Hemicellulolytic - Digest hemicellulose
Amylolytic - Digest starch
Proteolytic - Digest proteins
Sugar utilizing - Utilize monosaccharides and disaccharides
Acid utilizing - Utilize lactic, succinic, malic acids, etc.
Ammonia producers
Vitamin synthesizers
Methane producers
4) Protozoa?
a) Predominantly ciliates & seems to contribute substantially to the fermentation
process.
b) Several studies have shown that lambs and calves deprived of their ruminal
protozoa had depressed growth rates.
c) In general:
(1) Protozoa utilize the same set of substrates as bacteria.
(2) Different populations of protozoa show distinctive substrate preferences.
(3) Many utilize simple sugars and some store ingested carbohydrates as
glycogen.
K Some protozoa - Cannot regulate glycogen synthesis, and when
soluble carbohydrates are in abundance, they continue to store
glycogen until they burst.
(4) An additional feature of protozoa is that many species consume bacteria.
Perhaps play a role in limiting bacterial overgrowth?
5) Microbial populations?
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 60
a) Can vary with diet!
(1) Perhaps, reflecting substrate availability?
(2) e.g., Populations of cellulolytic bugs are depressed in animals fed diets
rich in grain.
b) Environmental conditions in the "fermentation vat" can have profound effects:
(1) Rumen fluid normally has pH between 6 and 7.
(2) But, may fall if large amounts of soluble carbohydrate are consumed.
(3) If pH drops to about 5.5, protozoal populations become markedly
depressed because of acid intolerance.
(4) More drastic lowering of rumen pH, as can occur with grain overload, can
destroy many species and have serious consequences to the animal!
B. Newborn animals:
1) Glooming behavior among cud chewers may facilitates microbial transfer.
2) Strictly anaerobic bacteria (including cellulose digester) have been found in animals
< 1 wk old.
3) Transmission of protozoa depends on close or direct contact, whereas normal
rumen bacteria may be isolated from aerosols.
C. Established gastrointestinal populations create conditions that tend to exclude all but the
most competent of "invaders."
D. Anaerobiosis (life in the absence of oxygen):
1) A fundamental property that limits both the kinds of microbes to colonize the
fermentative system and reactions to occur.
2) Oxygen is metabolically removed by both bacteria and protozoa.
3) Short-chain VFA are the major end products of the fermentation simply because C
skeletons cannot be completely oxidized to CO2 in the absence of oxygen. (Also,
the e-transport systems do not function, thus low ATP generation.)
5. Basic Fermentation Chemistry
A. Microbes that digest cellulose and other substrates also provide at least three other
major "services?"
1) Synthesis of high quality protein in the form of microbial bodies:
a) i.e., Bacteria & protozoa, which can be digested and absorbed by the host
animal.
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 61
b) Animals need certain amino acids, which their cells cannot synthesize,
"indispensable amino acids" - Fermentative microbes can synthesize & provide
them to their host!
2) Synthesis of protein from non-protein nitrogen sources:
a) Fermentative microbes can, for example, utilize urea to synthesize protein.
b) In some situations, ruminants are fed urea as a inexpensive dietary supplement.
c) They also secrete urea formed during protein metabolism into saliva, which
flows into the rumen and serves as another nitrogen source for the microbes.
3) Synthesis of B vitamins:
a) Mammals can synthesize only a few B vitamins and require dietary sources of
the others.
b) Fermentative microbes can synthesize all the B vitamins, and deficiency states
are rarely encountered in some animals.
B. Substrates for fermentation (compiled by S. P. Schmidt, AU) - Also, may want to see
subsequent sections on "Functions of Ruminal Bacteria & Protozoa and Manipulations
of Ruminal Microbes!"
1) Carbohydrates:
a) Carbohydrate utilization
by the ruminant (See the
Figure).
b) Most carbohydrates are
utilized by rumen
microorganisms, thus,
very little glucose can be
absorbed by ruminants.
c) VFA account for . 70%
or more of animal's
energy needs by:
(1) Oxidation of VFA via TCA cycle.
(2) Conversion of propionate to glucose, then oxidize glucose.
2) Nitrogenous substances
a) Protein/N utilization by the ruminant:
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 62
b) Sources of rumen nitrogen:
(1) Feed - Protein N (SBM, CSM, grain, forage, silage, etc.) and nonprotein
N (NPN; usually, means urea, but from 5% of N in grains to 50% of N in
silage and immature forages can be NPN).
(2) Endogenous (recycled) N - Saliva and rumen wall.
c) Ruminal protein degradation/fermentative digestion - Enzymes of microbial
origin:
(1) Proteases and peptidases of mcroorganisms (MO) cleave peptide bonds
and release AA.
(2) AA deaminated by microbes, and release NH3 and C-skeleton.
(3) MO use NH3, C-skeleton, and energy to synthesize their own AA.
(4) Formation of NH3 is very rapid, and very little AA left in the rumen.
d) Limitations of microbial protein synthesis:
(1) Two most likely limitations - Available energy and NH3. These need to be
synchronized.
(2) For diets containing urea, may also need: sulfur (for S-containing AA) and
branched-chain C-skeletons (MO cannot make branched-chain C-chains).
These are normally not a problem.
e) Protein leaving rumen:
(1) Microbial protein.
(2) Escape protein (also called "bypass" protein).
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 63
(3) Proteins enter abomasum & small intestine, and digested by proteolytic
enzymes similar to nonruminants.
(4) Escape vs. bypass protein? - Technically not "bypass."
(5) Reticular groove? - Important for young ruminants.
6. Functions of Ruminal Bacteria
A. Fermentation of carbohydrate by diverse bacterial species (Allison, 1993):
1) "G" = Final product, and "__" = extracellular intermediate.
2) H = an electron plus a proton or electrons from reduced-pyridine nucleotides, A =
carbohydrate fermenting species, B = methanogenic species, and C =
lactate-fermenting species which often also ferment carbohydrates.
3) Catabolism by rumen microbes?
Hexose - The Embden Meyerhof
glycolytic pathway; Pentose - the
pentose phosphate cycle coupled
with glycolysis with some by
phosphoketolase pathway; Pyruvate -
a variety of mechanisms to from
acetate, butyrate, H, CO2, and
propionate.
B. Transformation of nitrogenous substances
in the rumen (Allison, 1993):
1) Proteins are hydrolyzed by bacteria,
protozoa, and anaerobic fungi.
Bacteria are most important!
2) Protozoa? - A main function being
metabolism of bacterial protein
rather than exogenous protein.
3) Ammonia is produced during
microbial metabolism, and is a
major source of N used for
biosynthesis of microbial cells.
4) Many ruminal bacteria can grow
with ammonia as the main source of
N, but some require amino acids.
5) Considerable interest in inhibition of
microbial proteases so that more dietary protein would "bypass" the rumen.
7. Functions of Ruminal Protozoa
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 64
A. Ruminal ciliate protozoa are metabolically versatile & capable of using all major plant
constituents:
1) Entodiniomorphid protozoa - Engulf particulate matter and have enzymes that
attack cellulose, hemicellulose, etc.
2) Holotrichs - Depend on nonstructural polysaccharide, especially, starch and soluble
sugars.
3) End products? - Various organic acids, CO2, and hydrogen.
B. Although bacterial predation is not important for protozoa, amino acids from ingested
bacteria are used for synthesis of protozoal protein.
C. Protozoa may not be essential for ruminant digestion, but:
1) They do have a major influence on the overall microbial process!
2) Protozoa may account for as much as one-third of ruminal cellulolysis, and their
presence may enhance the cellulolytic activity of bacteria.
8. Manipulations of Ruminal Microbes
A. Ruminal microbial protein:
1) May be adequate for maintenance and during periods of slow growth or early
pregnancy.
2) When protein demand is high, animal productivity can be enhanced by increasing
the amount of "rumen-escaped" protein.
B. Some attempts have been made to find ways to manipulate the microbial population to
minimize the degradation of feed protein, e.g.:
1) Searches for chemicals that would inhibit the activity of microbial proteases or
deaminases.
2) Treatment of feedstuffs that would inhibit ruminal proteolysis such as the use of
various drying procedures, heat, or treatment with chemicals. An example of the
effort? - The increased efficiency of growth with formaldehyde-treated feeds!
C. The use of some proteins to coat and protect fats from microbial attack to enhance
yields of milk and to increase amounts of unsaturated fatty acids in milk or animal fat.
D. The use of various chemicals to inhibit methanogenesis. About 10 percent of dietary
energy may be lost as methane.
E. The use of some compounds to increase the ratio of ruminal propionate to acetate.
K The best example of successful manipulation via dietary inclusion? - Ionophore,
monensin, which inhibits microbial methane production, proteolysis, and amino acid
degradation and causes an increase in the ruminal propionate/acetate ratio.
9. Modification and Production of Toxic Substances in the Rumen
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 65
A. Some poisonous plants are less toxic to ruminants because microbes can attack toxic
compounds before being exposed to gastric digestion and absorption.
B. Substances detoxified in the rumen:
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Substance Source Reactions Organisms
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Nitrite Nitrate Reduced to ammonia Various bacteria and protozoa
Oxalate Oxalis and halogeton Decarboxylated to formate Oxalobacter formigenes
Ochratoxin A Moldy feeds Hydrolysis Unidentified microbes
3-Nitropropanol & Miserotoxin in many Nitro-group reduction Coprococcus, Megasphaera,
3-nitropropionic acid Astragalus sp. to amine Selenomonas
Phytoestrogens Subterranean clover Degraded to p-ethylphenol Unknown
and red clover
Gossypol Cottonseed meal Bound to soluble protein Unknown
Pyrrolizidine Heliotropium Reductive fission Peptococcus heliotrinreducans
alkaloids heliotrine
3-Hydroxy-4(1 H)- Mimosine from Unknown Synergi.stes jonesii,
pyridone leucaena Clostridium sp.
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C. Toxic substances produced in the rumen:
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Substance Source Organisms involved
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3-Methylindole (skatole) Tryptophan in feeds Lactobacillus sp.
Nitrite Reduction of nitrates in feed Selenomonas ruminantium,
Veillonella alcalescens
Lactic acid Rapidly degraded carbohydrates Streptococcus spp.,
(in high-concentrate diets) Lactobacillus spp.
3-Hydroxy-4(1 H)-pyridone Degradation product of mimosine Unidentified gram negative rod
Cyanide Hydrolysis of cyanogenic Gram-negative rods and
glycosides gram-positive diplococci
Dimethyl disulfide Degradation product of S-methyl- Lactobacillus spp., Veillonella
cysteine sulfoxide (Brassica alcalescens, Anaerovibrio
anemia factor) lipolytica; Megasphaera elsdenii
Equol Demethylation and reduction of Unknown
formononetin (a phytoestrogen)
Thiaminase Microbial enzymes Clostridium sporogenes; Bacillus
spp. and various anaerobes
3-Nitropropanoic acid Hydrolysis of miserotoxins Unknown
3-nitropropanol
Goitrin Hydrolysis of glucosinolates Unknown
found in rapeseed meal and other
crucifers
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10. Small- & Large-Intestine Microbes
A. Small intestine - Concentrations of viable bacteria in the small intestine content (104 to
106/g) are much lower vs. the rumen & large intestine, and most are transients and the
impact on digestion might be minimal.
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 66
B. Large intestine:
1) Concentrations of anaerobic bacteria (1010 to 1011/g) are comparable with the
rumen.
2) The diversity in microbes among animal species and diets.
3) Microbial fermentation - A major component of digestion in the LI, and main
products include CO2, acetate, propionate, and butyrate (also, methane & H).
4) Genera of bacteria? - Bacteroides, Fusobacterium, Streptococcus, Eubacterium,
Ruminococcus, and Lactobacillus. Also, Treponema & Eschericia coli.
5) Protozoa? - Similar (not identical though!) to rumen ciliates inhabit the LI of
horses, rhinoceroses, tapirs, and elephants . . . but their roles - ???
11. Common, Well-Known Disorders in Ruminants
A. Bloat
1) Eructation-inhibition reflex is initiated when proprioreceptors in mucosa around the
cardia are in contact with fluid or foam, thus gases cannot escape!
2) The buildup of pressure can be sufficient to interfere with movement of the
diaphragm and also cause circulatory impairment, which can lead to death?
3) Causes? Rapid release of soluble proteins from the degradation of the forage
(especially, legumes?) and rapid production of gas by the microbes:
4) Susceptibility differs, thus some research to control bloat include selection of plants
that do not cause bloat, bloat-resistant animals, and the use of antifoaming agents.
B. Ruminal acidosis
1) Lactic acid accumulation in the rumen (& in the blood) if animals are overfed with,
or are abruptly switched to grain or other readily fermented carbohydrate - Can be
lethal!
2) A drastic shift in microbial populations from gram-negative predominance to
gram-positive lactic acid producers (Streptococcus bovis and Lactobacillus sp.) in
the rumen, cecum, and colon of overfed animals.
3) Ruminal pH may drop from more than 6 to 5 or less - Normal rumen microbes may
not compete well with the pH less than about 5.5, and the resulting population is
dominated by the more acid-tolerant lactobacilli.
4) Ruminal lactic acid concentrations may exceed 100 mM, which can increase the
osmolarity of the rumen, thus water is drawn into the gastrointestinal tract from the
systemic circulation, thus severe dehydration and circulatory collapse in l to 2 days?
5) Other toxic factors such as histamine or endotoxin, which is produced by lysis of
the gram-negative anaerobes at low ruminal pH, may also be involved.
6) "Gradual adaptation" to high concentrate rations may occur without the above
population shift, and several studies indicate that certain antibiotics, particularly
those selective against S. bovis, may provide protection against lactic acidosis.
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 67
C. Acute pulmonary edema
1) Also called "foggage" or "fog fever" occurs naturally 2 to 10 days after mature
cattle are switched suddenly from poor to lush pastures.
2) Animals suffer obvious respiratory distress. No effective treatment!
3) Avidly consumed lush pasture lead to abnormal ruminal fermentation - The
significant feature being the conversion of Trp to 3-methylindole:
a) Absorbed & reaches the lung where it is oxidized to an active compound.
b) The compound binds to and kills alveolar & certain bronchial cells.
c) The cells slough off, allowing the airways to fill with frothy edematous fluid.
The lungs would be enlarged, heavy, and rubbery.
D. Ketosis
1) Ketosis - A generic term for any condition in which ketone bodies (acetone &
acetoacetate) are readily detectable in the body fluids and the expired breath.
2) Formed in liver mitochondria when acetyl-coenzyme A is being formed at a greater
rate than it can be metabolized in the citric acid cycle.
3) Cause? - The inadequate provision of oxaloacetate to prime the cycle, and occurs
when either oxaloacetate precursors, predominantly propionate, are deficient or
available oxaloacetate is preferentially channeled for gluconeogenesis.
4) "Pregnancy toxemia" or twin-lamb disease, which occurs just before parturition in
ewes that are carrying more than one fetus. Often fetal?:
a) The high metabolic demands of the twin-lamb pregnancy, which can be
exacerbated by the cold and/or by a reduced availability of glucogenic
fermentation products.
b) If there is an added nutritional or metabolic stress (. . . feed restriction, colder
weather, etc.), likely to occur.
5) Acetonemia, which occurs at peak lactation in high-yielding dairy cows:
a) The demands for glucose as the precursor of lactose at peak lactation cause the
available oxaloacetate to be used for gluconeogenesis at the expense of the
needs of the citric acid cycle.
b) Exacerbated when the cow loses its appetite for concentrates, which would
provide a relatively greater proportion of propionate.
c) As the disease progresses, the milk yield falls rapidly, so that the ketosis and its
cause therefore decline.
d) The disease is usually not fatal, but the slowness of the self-cure and the
consequential loss of peak milk production make therapy worthwhile.
6) Therapies? Includes dosing with propionate, intravenous administration of glucose,
and the injection of the gluconeogenic corticosteroid hormones.
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 68
E. Ammonia toxicity
1) Arises most commonly when excessive amounts of urea are fed, especially at the
time of low amylolytic fermentation. Ruminal urease rapidly convert urea to
ammonia..
2) With cellulolytic fermentations, the VFA production rate is much lower, thus, less
substrate for protein synthesis, and also microbial growth/division is much slower,
thus less microbial protein synthesis.
3) The toxicity with ammonia arises after its absorption - Due in small part to a
systemic metabolic alkalosis and in large part to central nervous system
intoxication.
4) Toxicity is countered by oral administration of VFA and by feeding of grain.
5) The reduced ruminal acidity slows ammonia absorption, and the VFA provide
carbon skeletons for microbial protein synthesis.
RUMINAL FERMENTATION IN GENERAL
1. Functional Anatomy of the Ruminant Stomach
A. Ruminants [so named because they ruminate (chew the cud)] have a stomach consists of
a non-secretory forestomach and a secretory stomach (abomasum).
B Forestomach - Consists of three compartments, the reticulum, the rumen, and the
omasum, and serves as a microbial fermentation vat of the ingesta mainly by hydrolysis
and anaerobic oxidation.
C. Abomasum - Like the stomach of nonruminant species, largely concerned with the
hydrolysis of protein by pepsin in an acid medium.
D. A schematic diagram (Kellems and Church, 1998):
1) Reticulum - Spherical, and the esophagus enters
dorsomedially at the cardia. [The reticular
groove (not shown) runs ventrally from the
cardia to the reticuloomasal orifice.]
2) Rumen - Divided into dorsal and vetral sacs
(not shown). (The ruminoreticulum occupies
the entire left side of the addomen, and depending on the degree of filling, also
extends ventrally on the right side.)
3) Omasum - A kidney-shaped structure and consists of many leaves (laminae; bear
small papillae), which enhance the internal surface area/volume ratio of the
omasum.
4) Abomasum - Consists of fundic, body, and pyloric regions.
E. The ruminant stomach is highly vascularized, and innervated by vagal and splanchnic
nerves, both of which provide sensory (afferent) and motor (efferent) pathways.
2. Benefits and Costs of Ruminant Digestion
Copyright © 2009 by Lee I. Chiba
Animal Nutrition Handbook Section 3: Rumen Microbiology & Fermentation Page 69
A. Benefits?
1) Because of a pre-gastric fermentation, can use feeds too fibrous for nonruminants.
2) Can use cellulose, the most abundant carbohydrate present, as a major nutrient.
3) Can synthesize high-quality microbial protein from low-quality protein, nonprotein
N, and recycled nitrogenous end products.
4) Can provide all components of vitamin B complex, provided the pres


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