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