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Cellulose Digestion

Cellulose Digestion. Cellulose. Endoglucanase. Cellodextrins. Cellobiose Cellotriose Glucose. Exoglucanase. Glucose chains. β -Glucosidase (cellobiase). Starch Digestion. Starch Amylopectin Amylose  Amylase. Dextrins  Amylase.  Amylase. Maltase.

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Cellulose Digestion

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  1. Cellulose Digestion Cellulose Endoglucanase Cellodextrins Cellobiose Cellotriose Glucose Exoglucanase Glucose chains β-Glucosidase (cellobiase)

  2. Starch Digestion • Starch • Amylopectin • Amylose •  Amylase • Dextrins •  Amylase  Amylase Maltase Maltose Glucose

  3. Carbohydrate Digestion in the Rumen • Starch dextrins maltose glucose • Enzymes are extracellular • -amylase yields mixture of maltose and glucose • Hydrolyze: -1,4 bonds and -1,6 bonds • -amylase yields maltose • Hydrolyze: -1,4 bonds • Debranching enzymes • Hydrolyze: -1,6 bonds • Maltase yields glucose • Cellulose cellodextrins cellobiose glucose • Enzymes are extracellular • First an endo--1,4-glucanase cleaves cellulose chains • Then an exo- -1,4-glucanase removes cellobiose units • Hydrolyze: -1,4 bonds in cellulose

  4. Carbohydrate Digestion in the Rumen • Hemicellulose (Neutral detergent insoluble fiber) • Enzymes from rumen have been described • Specific for sugars in the polymer • Xylose linkages – Endoxylanase • Xylose linkages – Xylosidase • Arabinoxylan – Arabinofuranosidase • Glucuronoxylan – Glucuronidase • Pectins (Neutral detergent soluble fiber) • Readily digested in the rumen • Galacturonide linkages – Pectate lyase • Methylester bonds – Pectin methylesterase

  5. Rumen Digestion Cellulose Hemicellulose Pectin Starch Uronic acids Galactose Cellobiose Pentoses Pentose Dextrose pathway Maltose Glucose

  6. Digestion in the Rumen • Lag time • Delay prior to apparent digestion • Exposing substrate • Wetting of feed • Attachment of microbes • Model • Feed Rumen Outflow • Kd • Kp • Potentially Kp • Digestible pool Indigestible pool

  7. Digestion Measure loss of nutrient (protein, dry matter, NFC, fiber) from Dacron bags containing feed when incubated for different times in the rumen. (Pool is material in the bag) % Nutrient left in bag 0 24 48 72 Hours

  8. Calculations CHOH Fraction A B1 B2 C Log of % nutrient remaining Hours Calculate slope (change per hour) of each line. Slope = kd, has units of % of pool remaining that is lost per hour.

  9. Calculation of Digestion in the Rumen • Digestion in the rumen is determined by rate of digestion and rate of passage. • Portion of each fraction digested in the rumen = • kd/(kd+kp) • kd = Disappearance (digestion) rate, % of pool/hr • kp = Passage rate from the rumen, % of pool/hr • Determined by marking feed • 2. Amount of each fraction digested in the rumen = • Feed DMIntake x Fraction% feed DM x kd/(kd+kp)

  10. Calculation of Fractions Leaving the Rumen Rumen Escape • Portion of each fraction leaving the rumen = • kp/(kd+kp) • kd = Disappearance (digestion) rate, % of pool/hr • kp = Passage rate from the rumen, % of pool/hr • Determined by marking feed • 2. Amount of each fraction leaving the rumen = • Feed DMIntake x Fraction% feed DM x kp/(kd+kp)

  11. Digestion • Each CHOH fraction within a feed has • a characteristic rate of digestion, Kd. • A = sugars - very fast • B1 = starch and pectin - fast • B2 = available fiber - slow • C = unavailable fiber - not digested

  12. Some Kd ValuesCarbohydrates A B1 B2 Corn ---------%/h--------- Whole 75-150 5-10 3-5 Cracked 100-200 10-20 5-7 High moist 200-300 15-20 6-8 Sorghum Dry roll 100-200 5-15 4-5 Steam flake 200-300 15-20 6-8 Legume hay 200-300 25-35 3-6 Grass hay 200-300 25-35 2-4 Corn silage 200-300 10-20 3-6

  13. Feed Passage • Each feed has its own characteristic rate • of passage, kp • Feed • Particle size • Density • Hydration rate • Amount of feed consumed by the animal • Passage rate increases when fed • above maintenance level of intake • Environmental temperature • Increases in very cold temperatures

  14. Some Kp Values Level of maintenance 1X 2X 3X --------%/hr-------- Wheat mids 2.0 2.5 3.0 Whole corn 2.5 4.0 6.5 Cracked corn 3.5 4.0 5.0 Legumes Long 2.5 3.0 4.0 Chopped 4.0 5.0 6.0 Grasses Long 2.0 2.5 3.0 Chopped 3.0 3.5 4.5 Corn silage 2.0 2.5 3.0

  15. Rate and Extent of Digestion in the Rumen 1. Quantity of feed consumed Intake = Rate of passage = % Digested 2. Composition of diet Grain > Roughage Storage CHOH > Structural CHOH 3. Other factors a) Physical form of diet (particle size) Rate of passage b) Nutrition of microbes Nitrogen requirements c) pH effects

  16. Rate and Extent of Digestion in Rumen - Continued • d) Processing of feed • Availability of substrate • e) Associative effects of feed • Response to combined feeds not additive • f) Change in diets • Effects on microbial population • g) Feed additives • Ionophores • Antibiotics • Other

  17. Associative Effects % Digested 100 80 60 40 Calculated Observed 0 50100 Percentageof concentrate

  18. Carbohydrate Digestion in Intestines CHOH in intestines Plant CHOH escaping degradation in the rumen Cell walls Starch Microbial polysaccharides Bacteria Protozoa

  19. Postruminal Digestion of Starch • Pancreas secretes -amylase • Starch Maltose + -limit dextrins • Brush boarder secretes • Maltase glucoamylase Not characterized in ruminants • Sucrose isomaltase No activity in ruminants • Starch is digested to glucose in the intestine

  20. Postruminal Digestion of Starch 20 to 40% of starch consumed might escape digestion in the rumen. The quantity of starch digested in the intestine increases with quantity entering, but % digested usually decreases. 45 to 88% of starch entering is digested. Digestion of starch in small intestine of ruminants is limited.

  21. Postruminal Digestion of Starch • What limits digestion of starch in small intestine? • Infuse glucose or hydrolyzed starch into abomasum • (JAS 80:1112, 2002) • Increase volume of pancreatic secretion • Decrease concentration of -amylase • Decrease in units of -amylase/hr • Seems to be limited by enzymatic digestion • Infuse casein with starch • (JAS 80:3361, 2002) • Starch digestion in small intestine (g/d) increased • but % of dietary starch entering not changed • Positive relation of starch disappearance with • protein disappearance • Additional protein might stimulate amylase secretion

  22. Sugar Absorption • Facilitated transporters (GLUT 5) • Transports fructose down a concentration gradient • Not significant in ruminants • 2. Through intercellular spaces (solvent drag) • Glucose must be present in high concentrations in • intestinal lumen • Na+-dependent glucose transporter (SGLT 1) • High affinity for glucose • Major transporter • Regulated by glucose in the intestine • Increases with glucose infusion • Less active in forage fed ruminants

  23. Glucose Transport Systems LumenCellBlood Glucose Glucose Na+ Na+ Na+ SGLT1 K+ ATPase K+ Glucose Glucose GLUT2 Glucose Glactose Fructose GLUT5 Fructose GLUT2 Fructose Luminal membrane Basolateral membrane Metabolism

  24. Postruminal Digestion of Carbohydrates • Carbohydrates that are not digested in small • intestine pass into large intestine • Digestion in large intestine is microbial • Sugars are fermented • VFA are absorbed • Microbial mass excreted in feces • Feeding systems should be designed to limit • starch digestion in large intestine • Concern expressed about intestinal acidosis • but little is known

  25. Postruminal Digestion of Starch • Authors have theorized that digestion of starch • in the small intestine rather than the rumen • would improve energetic efficiency. • Energy balance calculations support the concept. • Difficult to demonstrate experimentally. • Cattle fed high moisture corn or steam flaked corn • are more efficient than those fed dry corn. • Starch in high moisture and steam flaked grains • more digestible in the rumen and the intestine.

  26. Fecal Output of CHOH Fractions Fraction A (Sugars): All digested Fractions B1 and B2 (Starch and potentially digested fiber) = (1 – Intestinal digestibility of fraction) x Quantity of fraction escaping the rumen Fraction C (Indigestible CHOH): All of feed fraction appears in the feces

  27. Effect of Fiber and Grain Processing on CHOH Digestion in Dairy CowsJ. Dairy Sci. 84:2203, 2001 1. Fiber Content (Alf hay and alf & barley silage) Low High F:C ratios: 35:65 55:45 2. Grain processing (Rolled barley) Coarse Flat Kernel thickness, mm 1.60 1.36 Kernel width, mm 4.68 5.74

  28. Effect of Fiber and Grain Processing on In Situ Digestion of Dry Matter in Diet Components ARate of degradation of B fraction bERD = Effective ruminal degradability, Kp = 4%/hr

  29. CHOH Digestion in Dairy Cows Duodenal Cannula – Whole Diet JDS 84:2203, 2001

  30. CHOH Digestion in Dairy Cows JDS 84:2203, 2001

  31. Effect of Corn Particle Size on Starch Digestion Lactating cows fed 48.4% roughage (Grass silage & hay) Particle size: ground 568 μm and rolled 3458 μm

  32. CHOH Digestion in Steers JAS 73:1239, 1995

  33. CHOH Digestion in Steers JAS 80:797, 2002

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