Ruminant carbohydrate digestion
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Ruminant Carbohydrate Digestion. References Church 145-171; 260-297 Van Soest 95-117; 118-128; 160-165, 171-177 Sejrsen 139-143 Journal of Dairy Science 84:1294-1309 Journal of Animal Science 80:1112- Carbohydrates in common feedstuffs Carbohydrate, %DM Alfalfa Grass Corn DDGS

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Ruminant Carbohydrate Digestion

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Ruminant carbohydrate digestion

Ruminant Carbohydrate Digestion

  • References

    • Church 145-171; 260-297

    • Van Soest 95-117; 118-128; 160-165, 171-177

    • Sejrsen 139-143

    • Journal of Dairy Science 84:1294-1309

    • Journal of Animal Science 80:1112-

  • Carbohydrates in common feedstuffs

    Carbohydrate, %DMAlfalfaGrassCornDDGS

    Soluble sugars 5 4 2 1-5

    Cellulose 25 30 - 16-18

    Hemicellulose 22 26 6 26-34

    Pectin 6 4 - -

    Starch 2 1 72 15-19

    Lignin 12 9 - -


Ruminant carbohydrate digestion

  • Fibrous carbohydrates

    • Cellulose

      • A chain of glucose units bound by beta-1,4-linkages

      • Intramolecular hydrogen bonding

        • Poor flexibility

        • Good tensile strengh

        • Low solubility in water or dilute acid

Starch-Groups are axial

Cellulose-Groups are equatorial


Ruminant carbohydrate digestion

  • Intermolecular hydrogen bonding

    • Allows the development of a crystalline lattice

      • In cellulose digestion, intermolecular bonds must first be broken converting crystalline to amorphous cellulose

      • More intermolecular bonds in pure cellulose than native cellulose

From Van Soest (1994)


Ruminant carbohydrate digestion

  • Hemicellulose

    • Heterogeneous mixture of pentose, hexose and uronic acids bound to a beta-1,4-linked core composed primarily of xylose

      Monomer, % HemicelluloseAlfalfaBromegrassLocation

      Arabinose 10.4 12.0 Branch point

      Xylose 58.5 59.2Chain

      Glucose 6.9 20.9Chain

      Galactose 6.9 7.8Chain

      Rhamnose 3.9 - Chain

      Glucuronic acid 13.5 - Branch point

    • Monomers of xylose chain are twisted at 60o

Van Soest (1994)


Ruminant carbohydrate digestion

  • Arabinose and uronic acid branch points

    • Arabinose binds by Beta-1,3-linkages

    • Uronic acids bind by Beta-1,2-, Beta-1,3-, or Beta-1,4- glycosal or ester linkages

    • Significance of branch points

      • Increased branch points > Greater digestibility

        > Greater solubility

  • Hemicellulose is more closely bound to lignin than cellulose


Ruminant carbohydrate digestion

  • Pectin

    • Polymers of galacturonic acid bound by alpha-1,4-linkages

      • Chains are coiled

      • Very digestible by microorganisms

    • Rhamnose units are substituted in the chains

      • Chains twist

    • Arabinose and galactose side chains bind by alpha-1,4-linkages

    • Adjacent chains of rhamnogalactans may be cross-linked through Ca+ ions bridged across galactouronyl residues

Van Soest (1994)


Ruminant carbohydrate digestion

  • Locations of fiber carbohydrates


Ruminant carbohydrate digestion

  • Lignin

    • A poorly defined polymer of phenylpropane units


Ruminant carbohydrate digestion

  • Lignin in plants is composed of a highly condensed ‘core’ lignin and a ‘non-core’ lignin composed of low molecular weight phenolics, primarily ferulic and p-coumaric acids.

    • Ratios very with plant species

    • Binding is random

  • Relation to cell wall carbohydrates

    • Only binds to hemicellulose

      • Forms a matrix around cellulose

Van Soest (1994)


Ruminant carbohydrate digestion

  • Linkages between carbohydrates and lignin vary with plant species

    • Ester linkages

      • Between carbohydrates and ferullic and hydroxycinnamic acid

      • Found in grasses

      • Saponifiable with alkali

    • Ether linkages

      • Directly between carbohydrates and core lignin

      • Found in dicotyledenous plants

      • Difficult to hydrolyze

  • Biological function

    • Strength against compression forces

    • Disease resistance

  • Factors affecting lignin content

    • Maturity

    • Ambient temperature

      • Increasing temperature increases lignin synthesis and reduces photosynthesis


Ruminant carbohydrate digestion

80

Grasses

CF dig, %

Legumes

40

10 20

Lignin/ADF, %

  • Effects of lignification

    • Lignin is the major factor limiting digestion of forage cell walls

      • Protects up to 1.4 – 2.0 x its weight in CHO and up to 8 CHO units from the lignin bond

      • Mechanisms of lignin’s effects on digestion

        • Physically encrusting the fiber

        • Altering the stereochemistry of the polysaccharides

        • Toxicity to cellulolytic bacteria


Ruminant carbohydrate digestion

  • Delignification treatments

    • Alkali treatments

      • Treatments

        • 4% NaOH

        • 3% NH3

      • Saponifies ester linkages

      • Only effective on grasses

      • Increase digestibility and intake 10-20%

    • Alkaline hydrogen peroxide lignin

      • Increases digestibility by 60%

      • Effective on all forages

    • Biological delignification

      • White rot fungi


Ruminant carbohydrate digestion

  • Other factors affecting cell wall digestion

    • Arabinose:Xylose ratio

      • Decreases with maturity, decreasing digestibility

    • Tannins

      • May reduce digestibility by:

        • Cellulase inhibition

        • Protein binding

    • Cutin

      • Waxy coating, decreasing digestibility

    • Silica

      • High in forages from arid environments, decreasing cellulose digestibility

    • Oils

      • Toxic to cellulolytic bacteria

    • Bacterial nutrition

      • N, S, and isoacids increase fiber digestion

    • Grain in diet

      • Increasing grain>Decreased pH and starch>Reduce cellulose digestibility

    • Increased rate of passage


Ruminant carbohydrate digestion

  • Cellulose digestion

    • In reticulorumen

      • Approximately 90% of cellulose digestion

    • Requires two steps

      • Microbial attachment

      • Hydrolysis

Miron et al. JDS 84:1294


Ruminant carbohydrate digestion

  • Attachment of cellulolytic bacteria on fiber

    • Results in a lag period in digestion

    • Phases

      • Transport of bacteria to fiber

        • Slow

        • Dependent on number of bacteria

      • Nonspecific adhesion of bacteria to sites on substrate

        • Binds with Glycocalyx

          Mixture of polysaccharide, glycoprotein and protein on outside of cell membrane of gram- bacteria

          Peptidoglycan of gram+ bacteria

        • Occurs mainly at cut or macerated sites of the plant

      • Specific adhesions of bacteria with digestible cellulose

        • Structures

          Cellulosome:

          Large, multienzyme complexes specialized for

          adhesion and hydrolysis of cellulose

          Fimbriae or Pili:

          Small (5-7 nm in width and 100-200 nm in length)

          structures in both gram + and – bacteria

      • Proliferation and colonization of bacteria


Ruminant carbohydrate digestion

  • Structure of the cellulosome


Ruminant carbohydrate digestion

  • Cellulose hydrolysis

    • Cellulases are extracellular

    • Enzymes

      • Endo-B-1,4-glucanase > Cleaves cellulose chains

      • Exo-B-1,4-glucanase > Cleaves cellobiose units

      • Cellobiase > Cleaves cellobiose

  • Hemicellulose digestion

    • Hemicellulose > Lignin-hemicellulose > Monosaccharides

      complexes

    • Enzymes found in cell-free rumen fluid and within cells

      • Endoxylanase > Hydrolyzes xylose linkages

      • Xylosidase > Hydrolyzes xylose linkages

      • Arabinofuranosidase > Hydrolyzes arabinoxylans

      • Glucuronidase > Hydrolyze Glucuronxylan

  • Pectin digestion

    • Rapid

      Pectic lyase & Pectin methylesterase Polygalacturonase

      Pectin > Polygalacturonic acid > Galacturonic acid


Ruminant carbohydrate digestion

  • Lower GIT tract digestion of fiber carbohydrates

    • Abomasum and small intestine

      • Little digestion

    • Large intestine

      • Fermentation of both cellulose and hemicellulose

        • Greater % of hemicellulose digestion than cellulose digestion occurs in LI

      • % of fiber carbohydrate digested in the LI increases with factors that reduce ruminal digestion


Ruminant carbohydrate digestion

CH2OH

OH

O

O

OH

CH2OH

CH2OH

CH2OH

CH2OH

CH2

CH2OH

O

O

O

O

O

O

OH

OH

OH

OH

OH

OH

O

O

O

O

O

O

O

O

OH

OH

OH

OH

OH

OH

  • Starch

    • Chief storage polysaccharide in plants

    • Two components

      • Amylose (Glucose units bound by alpha-1,4-linkages)

      • Amylopectin (Glucose units by alpha-1,4-linkages with alpha-1,6-branch points)


Ruminant carbohydrate digestion

  • Composition varies between:

    • Variety

      AmyloseAmylopectin

      Normal 30 70

      Waxy100 0

    • Maturity

      • Maturity increases amylose

  • Components are arranged in concentric spheres in granules

    • Held together by hydrogen bonds

    • Bonds limit ability to swell in water and allow access of enzymes to material in center of granules

      • Digestion proceeds from outside to center of granule

    • Bolds broken by heating, particularly in water, destroying granule structure

      • Gelatinization

      • Basis for processes like

        Steam-flaking

        Popping

      • Processes also affect seedcoat and protein matrix

      • Increases digestibility 10-20%


Ruminant carbohydrate digestion

  • Starch digestion

    • Rumen

      • 47-95% digested in rumen

      • Digested by alpha-amylase to oligosaccharides

        • Found in cell-free rumen fluid, but 70% associated with particulate-bound microorganisms

        • Activity increases in high grain diets

        • Microorganisms

          • Prevotella amylophilus

          • Streptococcus bovis

      • Oligosaccharides degraded to glucose by maltases near cells

      • Protozoa uptake

        • Primarily holotrichs

        • Stabilizes fermentation

        • Do not readily pass from rumen

      • Bacterial uptake

        • Storage polysaccharide

        • May accounts for as much as 50% of carbohydrate leaving rumen


Ruminant carbohydrate digestion

  • Small intestine

    • Mechanisms similar to nonruminants

      Pancreatic IntestinaTranl

      amylase maltase

      Starch > Oligosaccharides > Glucose

    • Glucose absorption

      • Active transport by a secondary active glucose and galactose tranporter (SGLT1) at the apical membrane

        • Activity greater in pre-ruminants than ruminants

        • Activity greater in concentrate selecting species than roughage selectors

        • Increases with glucose infusions

      • Transport at the basolateral membrane of epithelium is by facilitated diffusion using a GLUT2 transporter

    • Limitations of small intestinal starch digestion

      • 45-90% digested in the small intestine

      • Limitations

        • Inadequate amylase activity

        • Inadequate maltase

        • Intestinal pH

        • Rate of passage


Ruminant carbohydrate digestion

  • Large intestine

    • Only significant when high levels of starch escape ruminal digestion

    • Fermentation similar to rumen

    • VFAs are absorbed

    • Microbial protein is produced and excreted


Ruminant carbohydrate digestion

  • Importance of location of starch digestion

    • Since small intestinal digestion is limited, digestion in the rumen is most valuable

    • Ruminal escape starch may be associated with hemorrhagic bowel syndrome

      • Hemorrhaging in the jejunum occurs in the first 100 days of lactation

      • Symptoms

        • Abdominal distention

        • Bloody feces

        • Dehydration

        • Shock

        • Death

      • Possible causes

        • Ruminal escape starch causes growth of Clostridium perfringens type A

        • Moldy feed


Ruminant carbohydrate digestion

Results of rates of passage and digestion.

Physical form of forage plays a role

75

Ruminal

Starch Dig, %

60

52

% corn in diet

  • Factors affecting starch digestion

    • DM intake

      • Increased dry matter intake decreases starch digestion

      • Percentage of grain in diet


Ruminant carbohydrate digestion

  • Type of starch

    • Barley > Corn > Sorghum

    • Waxy > Normal

  • Processing

    • Cracking or grinding increases digestibility 2 – 5%

    • Steam-flaking, popping etc improves starch digestion by:

      • 6-10% in corn

      • 15-20% in sorghum


Ruminant carbohydrate digestion

  • VFA production

    • Importance of VFA

      Endproduct% of digested energy

      VFA 49-58

      Heat 6-12

      Gas 4-8

      Microbial mass26-32


Ruminant carbohydrate digestion

  • VFA production

    • VFA produced

      from pyruvate

    • Net production

      • Glycolysis

        (/ glucose)

        2 ATP

        2 NADH2

      • Pentose PO4

        pathway

        (/pentose)

        1.67 ATP

        2 NADPH2

        1 NADH2

        1 pentose

ATP ATP

ATP

ATP

ATP


Ruminant carbohydrate digestion

  • Pyruvate is immediately converted to VFAs


Ruminant carbohydrate digestion

  • Acetate production

    • Pyruvate oxidoreductase (Most common)

      Fd FDH2

      Pyruvate Acetyl CoA Acetate

      Coenzyme A CO2 ADP ATP

    • Pyruvate-formate lyase

      Coenzyme A ADP ATP

      Pyruvate Acetyl CoA Acetate

      Formate

      CH4 + H2O

      6H+


Ruminant carbohydrate digestion

  • Butyrate (60% Butyrate from acetate)

    • Condensation

      ATP ADP Acetyl CoA CoA

      Pyruvate Acetyl CoA Acetoacetyl CoA

      ATP CO2 NADH2

      CoA ADP CoA NAD

      Malonyl CoA B-Hydroxybutyryl CoA

      Crotonyl CoA

      NADH2

      NAD

      Butyryl CoA

      Acetyl CoA

      Acetate

      Butyryl P

      ADP

      ATP

      Butyrate


Ruminant carbohydrate digestion

  • Propionate

    • Succinate or dicarboxylic acid pathway

      • 60-90% of propionic acid production

        CO2 ATP ADP NADH2 NAD

        Pyruvate Oxaloacetate Malate

        H2O

        CO2 Fumarate

        Propionyl CoA ADP NADH2

        ATP NAD

        Succinate

        Propionate

        Methylmalonyl CoA Succinyl CoA


Ruminant carbohydrate digestion

  • Acrylate pathways

    • Important on high grain diets

      • Accounts of 40% of propionate production

    • Associated with Megasphaera elsdenii

      NADH2 NAD

      Pyruvate Lactate Acrylyl CoA

      NADH2

      Propionate

      NAD

      Propionyl CoA


Ruminant carbohydrate digestion

  • Fermentation of intermediates

    • Lactate

      • In forage-fed animals; Lactate > Butyrate

      • In grain-fed animals; Lactate > Propionate

    • Succinate

      • Supplies at least 1/3 of the propionate

    • Formate

      • Rapidly converted to H2 + CO2

    • H2

      • 4H2 + CO2 > CH4 + 2H2O

    • Ethanol

      • Rapidly converted to acetate


Ruminant carbohydrate digestion

  • Factors controlling fermentation endproducts

    • Maximum ATP yields for the microorganisms

    • Maintenance of Reduction-Oxidation balance

      • In glycolysis, 2 NADH2 are produced per glucose.

        • Must be oxidized to maintain Redox balance

        • Electron acceptors

          • Aerobic organisms

            O2 > H2O

          • Anerobic organisms

            CO2 > CH4

            Pyruvate > Propionate

            Acetate > Butyrate

            NO3 > NO2 > NH3

            SO4 > S


Ruminant carbohydrate digestion

  • Thermodynamic order of preference for electron acceptors

    • NO3 > NO2

    • NO2 > NH4

    • Crotonyl CoA>Butyryl CoA

    • Fumarate > Succinate

    • Acrylyl CoA > Propionyl CoA

    • SO4 > HS

    • Acetoacetyl CoA > B-OH-Butyryl CoA

    • CO2 > CH4

    • Pyruvate > Lactate

    • CO2 > Acetate

  • Why does CH4 supercede Propionate or Butyrate production

    • Greater ATP produciton

    • Greater affinity for H at low concentrations

    • Low amounts of other acceptors


Ruminant carbohydrate digestion

  • Redox balance in the rumen

    • 2H (Reducing equivalents) produced:

      • Glucose > 2 Pyruvate + 4H (as 2 NADH2)

      • Pyruvate + H2O > Acetate + CO2 + 2H (as 1 FADH2)

    • 2H accepted:

      • CO2 + 4H2 > CH4 + 2H2O

      • Pyruvate + 4H (as 2 NADH2) > Propionate + H2O

      • 2 Acetate + 4H (as 2 NADH2) > Butyrate + 2H2O

  • Fermentation balance

    • High forage diets

      • 5 Glucose > 6Acetate + Butyrate + 2Propionate + 5CO2

        + 3CH4 + 6H2O

      • Acetate:Propionate = 3

      • CH4:Glucose = .60

    • High grain diets

      • 3 Glucose > 2Acetate + Butyrate + 2Propionate + 3CO2

        + CH4 + 2H2O

      • Acetate:Propionate = 1

      • CH4:Glucose = .33


Ruminant carbohydrate digestion

  • VFA production

    • Usually peaks 4 hours after feeding

    • Concentration does not equal production

    • Factors that increase propionate, decrease acetate and methane

    • Factors affecting VFA produced

      • Diet forage:concentrate ratio

        • Decreased forage and increased concentrate

          • Decreased acetate and methane, increased propionate

        • Dietary buffers

          • Increased acetate and methane, decreased propionate

        • Decreased physical form of diet (Grinding, pelleting etc)

          • Decreased acetate and methane, increased propionate

        • Ionophores

          • Decreased acetate and methane, increased propionate

        • Unsaturated fatty acids

          • Decreased methane, increased propionate


Ruminant carbohydrate digestion

  • Examples of diet effects on VFA production

    • Forage:Concentrate

      Forage:Concentrate

      VFA, Molar%60:4040:6020:80

      Acetate 66.9 62.9 56.7

      Propionate 21.1 24.9 30.9

      Butyrate 12.0 12.2 12.4

      Methane, Mcal/d 3.1 2.6 1.8

    • Physical form of forage

      Alfalfa hay

      Grind

      VFA, Molar%LongCoarseFinePelleted

      Acetate 62.5 56.8 47.5 18.2

      Propionate 23.8 27.1 28.5 45.7

      Butyrate 10.8 13.6 23.9 32.8


Ruminant carbohydrate digestion

  • Methane inhibitors

    • Nitrates, sulfates, and alkaloids will inhibit CH4, but decreases propionate and butyrate as well

    • Chloral hydrate (CCl4)

      • Reduces CH4 and increases propionate

      • H2 accumulates and microbial growth is reduced

    • Myristic acid (Brit. J. Nutr. 90:529-540)

      • A 14-carbon saturated fatty acid

      • Reduced CH4 production by 58% while increasing propionate concentration (mmol/l) by 86%

      • Did not affect DM intake

      • Tended to decrease NDF digestion

    • Acetogenesis

      • 2CO2 + 2H2 > CH3COOH

      • Thermodynamically unfavorable to methane production

      • Doesn’t usually occur in the rumen

      • Does occur in the large intestine of various species and in termites

      • Why doesn’t it occur in the rumen?


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