Ruminant Carbohydrate Digestion

1
Ruminant Carbohydrate Digestion

• References
• Church 145-171 260-297
• Chapter 8 - Ruminal Fermentation
• Chapter 14 Carbohydrate Nutrition in Ruminants
• Carbohydrates in common feedstuffs
• Carbohydrate, DM Alfalfa Grass Corn DDGS
• 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 -
  -

2

• Fibrous carbohydrates
• Cellulose
• A chain of glucose units bound by
  beta-1,4-linkages
• Intramolecular hydrogen bonding
• Poor flexibility
• Good tensile strength
• Low solubility in water or dilute acid

Starch-Groups are axial Cellulose-Groups are
equatorial
3

• Hemicellulose
• Heterogeneous mixture of pentose, hexose and
  uronic acids bound to a beta-1,4-linked core
  composed primarily of xylose
• Monomer, Hemicellulose Alfalfa Bromegrass Locati
  on
• Arabinose 10.4 12.0
  Branch point
• Xylose 58.5 59.2 Chain
• Glucose 6.9 20.9 Chain
• Galactose 6.9 7.8 Chain
• Rhamnose 3.9 -
  Chain
• Glucuronic acid 13.5
  - Branch point
• More closely linked to lignin than cellulose

4

• Pectin
• Polymers of galacturonic acid bound by
  alpha-1,4-linkages
• Chains are coiled
• Very digestible by microorganisms
• Higher concentration in legumes than grasses

5

• Locations of fiber carbohydrates

6

• Digestion of cell wall carbohydrates
• In reticulorumen
• Approximately 90 of cellulose digestion
• Cellulose digestion
• Requires two steps
• Microbial attachment
• Hydrolysis
• Results in a lag in digestion
• Hemicellulose and Pectin
• Enzymes found in rumen fluid
• Hydrolysis of cellulose and hemicellulose results
  in glucoses and pentoses that are fermented

Miron et al. JDS 841294
7

• 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

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• Factors limiting digestion of cell wall
  carbohydrates
• Lignin
• A poorly defined polymer of randomly bound
  phenylpropane units
• 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 vary with plant species
• Binding is random

9

• Relation to cell wall carbohydrates
• Only binds to hemicellulose
• Forms a matrix around cellulose

Van Soest (1994)
10

• Linkages between carbohydrates and lignin vary
  with plant species
• Ester linkages
• Between carbohydrates and ferulic 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

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• 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 lignins effects on digestion
• Physically encrusting the fiber
• Altering the stereochemistry of the
  polysaccharides
• Toxicity to cellulolytic bacteria

12

• Delignification treatments
• Alkali treatments
• Treatments
• 4 NaOH
• 3 NH3
• 4 CaO
• 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
• Silage inoculants with bacteria that secrete
  ferulic esterase
• Genetically engineered reduced-lignin crops
• Brown mid-rib varieties of corn and sorghum
• Low lignin alfalfa

13

• Other factors affecting cell wall digestion
• Grain in diet
• Increased grain gt Decreased pH gt Reduced
  cellulolytic bacteria gt Reduced cell wall
  digestion
• Fats, particularly unsaturated oils
• Toxic to cellulolytic bacteria gt Reduced cell
  wall digestion
• Limit dietary fat to 5 of DM
• Bacterial nutrition
• N, S, and isoacids increase fiber digestion
• Increased rate of passage gt Reduced cell wall
  digestion
• Factors increasing rate of passage of forages
  from the rumen
• Increased feed intake
• Grinding (Through its effect on feed intake???)
• Cold temperatures
• Pregnancy
• Rumen size

14
Laboratory analysis of fiber carbohydrates
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• 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)

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• Composition varies between
• Variety
• Amylose Amylopectin
• Normal 30 70
• Waxy 0 100
• 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

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• 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
• Oligosaccharides degraded to glucose by maltases
  near cells
• Protozoa uptake
• Stabilizes fermentation
• Do not readily pass from rumen
• Bacterial uptake
• Storage polysaccharide
• May accounts for as much as 50 of carbohydrate
  leaving rumen

18

• Small intestine
• Mechanisms similar to nonruminants
• Pancreatic
  Intestinal
• amylase
  maltase
• Starch gt Oligosaccharides
  gt Glucose
• Glucose absorption
• Active transport
• Limitations of small intestinal starch digestion
• 45-90 digested in the small intestine
• Limitations
• Inadequate amylase activity
• Inadequate maltase
• Intestinal pH
• Rate of passage
• Consequences of poor small intestinal starch
  digestion
• Limited supply of glucose for animal
• Hemorrhagic bowel syndrome in dairy cows
  

19

• 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

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• Factors affecting starch digestion
• DM intake
• Increased dry matter intake decreases starch
  digestion
• Percentage of grain in diet
• Increasing forage to high grain diet will
  generally decrease starch digestion by increasing
  passage of liquid and small particles from the
  rumen
• Dependent on forage source

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• Type of starch
• Barley gt Corn gt Sorghum
• Waxy gt 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