Cellulose Digestion

Cellulose

Endoglucanase

Cellobiose Cellotriose Glucose

Glucose chains

β-Glucosidase (cellobiase)

Exoglucanase

Cellodextrins

Starch Digestion

Starch Amylopectin Amylose

Amylase

Dextrins

Amylase

Maltose Glucose

Maltase

Amylase

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 bondsDebranching enzymes

Hydrolyze: -1,6 bondsMaltase yields glucose

Cellulose cellodextrins cellobiose glucose Enzymes are extracellular

First an endo--1,4-glucanase cleaves cellulose chainsThen an exo- -1,4-glucanase removes cellobiose unitsHydrolyze: -1,4 bonds in cellulose

Carbohydrate Digestion in the Rumen

Hemicellulose (Neutral detergent insoluble fiber)Enzymes from rumen have been described

Specific for sugars in the polymerXylose linkages – EndoxylanaseXylose linkages – XylosidaseArabinoxylan – ArabinofuranosidaseGlucuronoxylan – Glucuronidase

Pectins (Neutral detergent soluble fiber)Readily digested in the rumen

Galacturonide linkages – Pectate lyaseMethylester bonds – Pectin methylesterase

Rumen Digestion

Cellulose Hemicellulose Pectin Starch

Uronic acids Galactose

Cellobiose Pentoses Pentose Dextrosepathway

Maltose

Glucose

Digestion in the RumenLag time Delay prior to apparent digestion

• Exposing substrate• Wetting of feed• Attachment of microbes

ModelFeed Rumen Outflow

Kd

Kp

Potentially Kp

Digestible pool Indigestible pool

Digestion

0 24 48 72 Hours

% N

utrie

nt le

ft in

bag

Measure loss of nutrient (protein, dry matter, NFC,fiber) from Dacron bags containing feed whenincubated for different times in the rumen.

(Pool is material in the bag)

Calculations

Hours

Calculate slope (change per hour) of each line.Slope = kd, has units of % of pool remaining that is lost per hour.

Log

of %

nut

rient

rem

aini

ngCHOH FractionAB1

B2 C

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 = kd/(kd+kp)

kd = Disappearance (digestion) rate, % of pool/hrkp = 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)

Calculation of Fractions Leaving the RumenRumen Escape

1. Portion of each fraction leaving the rumen = kp/(kd+kp)

kd = Disappearance (digestion) rate, % of pool/hrkp = 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)

Digestion

Each CHOH fraction within a feed hasa characteristic rate of digestion, Kd.

A = sugars - very fastB1 = starch and pectin - fastB2 = available fiber - slowC = unavailable fiber - not digested

Some Kd ValuesCarbohydrates

A B1 B2Corn ---------%/h--------- Whole 75-150 5-10 3-5 Cracked 100-200 10-20 5-7 High moist 200-300 15-20 6-8Sorghum Dry roll 100-200 5-15 4-5 Steam flake 200-300 15-20 6-8Legume hay 200-300 25-35 3-6Grass hay 200-300 25-35 2-4Corn silage 200-300 10-20 3-6

Feed Passage

Each feed has its own characteristic rateof passage, kp

FeedParticle sizeDensityHydration rate

Amount of feed consumed by the animalPassage rate increases when fedabove maintenance level of intake

Environmental temperatureIncreases in very cold temperatures

Some Kp Values Level of maintenance1X 2X 3X

--------%/hr--------Wheat mids 2.0 2.5 3.0Whole corn 2.5 4.0 6.5Cracked corn 3.5 4.0 5.0Legumes Long 2.5 3.0 4.0 Chopped 4.0 5.0 6.0Grasses Long 2.0 2.5 3.0 Chopped 3.0 3.5 4.5Corn silage 2.0 2.5 3.0

Rate and Extent of Digestion in the Rumen

1. Quantity of feed consumedIntake = Rate of passage = % Digested

2. Composition of dietGrain > RoughageStorage 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

Rate and Extent of Digestion in Rumen - Continued

d) Processing of feed Availability of substratee) Associative effects of feed

Response to combined feeds not additivef) Change in diets

Effects on microbial populationg) Feed additives

IonophoresAntibioticsOther

Associative Effects

100

80

60

400 50 100

% Digested

Calculated

Observed

Percentage of concentrate

Carbohydrate Digestion in Intestines

CHOH in intestines

Plant CHOH escaping degradation in the rumenCell wallsStarch

Microbial polysaccharidesBacteriaProtozoa

Postruminal Digestion of Starch

Pancreas secretes -amylaseStarch Maltose + -limit dextrins

Brush boarder secretesMaltase glucoamylase Not characterized in ruminantsSucrose isomaltase No activity in ruminants

Starch is digested to glucose in the intestine

Postruminal Digestion of Starch

20 to 40% of starch consumed might escapedigestion in the rumen.

The quantity of starch digested in the intestineincreases with quantity entering, but % digestedusually decreases. 45 to 88% of starch enteringis digested.

Digestion of starch in small intestine of ruminantsis limited.

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 secretionDecrease concentration of -amylaseDecrease 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) increasedbut % of dietary starch entering not changedPositive relation of starch disappearance withprotein disappearance

Additional protein might stimulate amylase secretion

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 infusionLess active in forage fed ruminants

Glucose Transport SystemsLumen Cell Blood

Glucose GlucoseNa+ Na+ Na+

SGLT1 K+ ATPase K+

Glucose Glucose GLUT2 Glucose Glactose

Fructose GLUT5 Fructose GLUT2 Fructose

Luminal membrane Basolateral membrane

Metabolism

Postruminal Digestion of Carbohydrates

Carbohydrates that are not digested in smallintestine pass into large intestine

Digestion in large intestine is microbialSugars are fermentedVFA are absorbedMicrobial mass excreted in feces

Feeding systems should be designed to limitstarch digestion in large intestine

Concern expressed about intestinal acidosisbut little is known

Postruminal Digestion of Starch

Authors have theorized that digestion of starchin the small intestine rather than the rumenwould improve energetic efficiency.

Energy balance calculations support the concept.

Difficult to demonstrate experimentally.Cattle fed high moisture corn or steam flaked cornare more efficient than those fed dry corn.Starch in high moisture and steam flaked grainsmore digestible in the rumen and the intestine.

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

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

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, % Kd, %/hra ERDb, %

Barley

Coarse 3.6 87.1 2.71 30.6

Flat 5.8 71.3 9.20 48.9

Forage

Alfalfa silage 24.8 32.0 5.81 40.4

Barley silage 24.3 50.2 2.99 40.5

Alfalfa hay 28.2 37.8 8.11 49.6ARate of degradation of B fractionbERD = Effective ruminal degradability, Kp = 4%/hr

CHOH Digestion in Dairy CowsDuodenal Cannula – Whole Diet

Grain processing Fiber content

Coarse Flat Low High

OM intake, kg/d 17.6 18.8 18.7 17.7

Dig rumen, % 48.2 49.1 47.7 49.5

Dig post rumen, %a 30.7 34.2 33.7 31.2

Dig post rumen, %b 42.2 49.0 47.9 43.2

Starch intake, kg/d 5.49 6.08 6.80 4.77

Dig rumen, % 37.8 50.1 48.3 39.6

Dig post rumen, %a 43.9 40.1 37.0 47.0

Dig post rumen, %b 68.4 78.7 71.2 75.8 a% of intake b% of passing into duodenum

JDS 84:2203, 2001

CHOH Digestion in Dairy Cows

Grain processing Fiber content

Coarse Flat Low High

NDF intake, kg/d 6.87 7.29 6.69 7.47

Dig rumen, % 39.3 37.8 38.7 38.5

Dig post rumen, %a 6.5 8.7 9.2 6.0

Dig post rumen, %b 8.3 12.3 13.3 7.3

ADF intake, kg/d 4.11 4.37 3.63 4.84

Dig rumen, % 31.3 28.4 25.7 34.0

Dig post rumen, %a 5.8 11.3 11.3 5.8

Dig post rumen, %b 6.3 14.4 13.3 7.4 a% of intake b% passing to duodenum

JDS 84:2203, 2001

Effect of Corn Particle Size on Starch Digestion

JDS 87:1389, 2004 Dry corn

Ground Rolled

Starch intake, kg/d 4.73 4.66

Duodenal flow, kg/d 1.43 2.17

Terminal ileal flow, kg/d 0.32 0.68

Apparent digest in rumen, kg/d 3.30 2.49

Apparent digest in rumen, % intake 69.8 53.5

Apparent digest in s. intest, kg/d 1.11 1.48

Apparent digest in s. intest, % duo pass 77.8 68.3

Apparent digest in s. intest, % intake 23.4 31.9

Apparent digest in l. intest, kg/d 0.19 0.18

Apparent digest in l. intest, % ileal pass 59.2 25.3

Apparent digest in l. intest, % intake 4.1 3.8

Apparent digest in total tract, % 97.3 89.2

Lactating cows fed 48.4% roughage (Grass silage & hay)Particle size: ground 568 μm and rolled 3458 μm

CHOH Digestion in Steers Corn processing Level intake Dry roll Steam

flake Low High

OM intake, kg/d 3.96 3.93 3.22 4.67 Dig rumen, % 56.5 65.5 63.5 58.5 Dig post rumen, %a 37.3 34.8 36.2 35.9 Starch intake, kg/d 2.06 2.03 1.67 2.42 Dig rumen, % 70.6 85.4 79.9 76.2 Dig post rumen, %a 19.2 13.6 14.8 18.0 ADF intake, kg/d 0.33 0.33 0.27 0.39 Dig rumen, % 31.3 25.3 31.6 25.0 Dig post rumen, %a 12.5 16.1 14.3 14.4

a% of intake

Diet: 74.7% corn and 12% roughage in the diet

JAS 73:1239, 1995

CHOH Digestion in Steers

Form of corn grain Dry roll High

moisture Steam flake

DM intake, kg/d 12.7 14.5 12.5 Dig total tract % 83.5 86.7 87.2 OM intake, kg/d 12.1 13.8 11.9 Dig rumen, % 64.4 76.0 69.3 Dig post rumen, %a 69.1 67.1 72.5 Dig total tract, % 84.6 88.0 88.1 Starch intake, kg/d 7.8 8.4 7.5 Dig rumen, % 81.2 97.3 94.7 Dig post rumen, %a 84.4 86.5 98.3 Dig total tract, % 96.1 98.7 99.8

a% entering Diet: 81.75% corn and 10% roughage in the diet

JAS 80:797, 2002