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

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

3. Carbohydrate Digestion in the Rumen Starchdextrinsmaltoseglucose 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 Cellulosecellodextrinscellobiose 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 CelluloseHemicellulosePectinStarch Uronic acids Galactose CellobiosePentosesPentose 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 FeedRumen Outflow K d K p Potentially K p Digestible poolIndigestible pool

7. Digestion 0 24 48 72 Hours % Nutrient left in bag 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)

8. Calculations Hours Calculate slope (change per hour) of each line. Slope = k d, has units of % of pool remaining that is lost per hour. Log of % nutrient remaining CHOH Fraction A B 1 B 2 C

9. Calculation of Digestion in the Rumen Digestion in the rumen is determined by rate of digestion and rate of passage. 1.Portion of each fraction digested in the rumen = k d /(k d +k p ) k d = Disappearance (digestion) rate, % of pool/hr k p = Passage rate from the rumen, % of pool/hr Determined by marking feed 2. Amount of each fraction digested in the rumen = Feed DM Intake x Fraction % feed DM x k d /(k d +k p )

10. Calculation of Fractions Leaving the Rumen Rumen Escape 1.Portion of each fraction leaving the rumen = k p /(k d +k p ) k d = Disappearance (digestion) rate, % of pool/hr k p = Passage rate from the rumen, % of pool/hr Determined by marking feed 2. Amount of each fraction leaving the rumen = Feed DM Intake x Fraction % feed DM x k p /(k d +k p )

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

12. Some K d Values Carbohydrates AB1B2 Corn---------%/h--------- Whole 75-150 5-103-5 Cracked100-20010-205-7 High moist200-30015-206-8 Sorghum Dry roll100-200 5-154-5 Steam flake200-30015-206-8 Legume hay200-30025-353-6 Grass hay200-30025-352-4 Corn silage200-30010-203-6

13. Feed Passage Each feed has its own characteristic rate of passage, k p 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 K p Values Level of maintenance 1X2X3X --------%/hr-------- Wheat mids2.02.53.0 Whole corn2.54.06.5 Cracked corn3.54.05.0 Legumes Long2.53.04.0 Chopped4.05.06.0 Grasses Long2.02.53.0 Chopped3.03.54.5 Corn silage2.02.53.0

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 100 80 60 40 050100 % Digested Calculated Observed Percentage of 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 StarchMaltose +  -limit dextrins Brush boarder secretes Maltase glucoamylaseNot characterized in ruminants Sucrose isomaltaseNo 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 1.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 3.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 LumenCell Blood Glucose Na + Na + Na + SGLT1K + ATPaseK + Glucose Glucose GLUT2 Glucose Glactose Fructose GLUT5 Fructose GLUT2 Fructose Luminal membraneBasolateral 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 B 1 and B 2 (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 Cows J. Dairy Sci. 84:2203, 2001 1. Fiber Content (Alf hay and alf & barley silage) Low High F:C ratios:35:6555:45 2. Grain processing (Rolled barley) CoarseFlat Kernel thickness, mm 1.601.36 Kernel width, mm 4.685.74

28. Effect of Fiber and Grain Processing on In Situ Digestion of Dry Matter in Diet Components Parameters J. Dairy Sci. 84:2203, 2001 A, %B, %K d, %/hr a ERD b, % Barley Coarse3.687.12.7130.6 Flat5.871.39.2048.9 Forage Alfalfa silage24.832.05.8140.4 Barley silage24.350.22.9940.5 Alfalfa hay28.237.88.1149.6 A Rate of degradation of B fraction b ERD = Effective ruminal degradability, K p = 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 JDS 87:1389, 2004 Dry corn GroundRolled Starch intake, kg/d 4.734.66 Duodenal flow, kg/d 1.432.17 Terminal ileal flow, kg/d 0.320.68 Apparent digest in rumen, kg/d 3.302.49 Apparent digest in rumen, % intake 69.853.5 Apparent digest in s. intest, kg/d 1.111.48 Apparent digest in s. intest, % duo pass 77.868.3 Apparent digest in s. intest, % intake 23.431.9 Apparent digest in l. intest, kg/d 0.190.18 Apparent digest in l. intest, % ileal pass 59.225.3 Apparent digest in l. intest, % intake 4.13.8 Apparent digest in total tract, % 97.389.2 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