Carbohydrates

1
Cellulose Digestion
Cellulose
Endoglucanase
Cellodextrins
Cellobiose Cellotriose Glucose
Exoglucanase
Glucose chains
ß-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 cellodextri
ns 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 Pen
tose 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/(kdkp)
• 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/(kdkp)

10
Calculation of Fractions Leaving the Rumen Rumen
Escape

• Portion of each fraction leaving the rumen
• kp/(kdkp)
• 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/(kdkp)

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 10-2
0 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 Whol
e 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 gt Roughage Storage CHOH gt 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 50 100
Percentage of concentrate
18
Carbohydrate Digestion in Intestines
CHOH in intestines Plant CHOH escaping
degradation in the rumen Cell walls Starch Micr
obial 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 801112, 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
803361, 2002) Starch digestion in small
intestine (g/d) increased but of dietary starch
entering not changed Positive relation of starch
disappearance with protein disappearance Additiona
l 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 Lumen Cell
Blood Glucose Glucose Na
Na Na SGLT1 K
ATPase K Glucose Glucose
GLUT2 Glucose Glactose Fructose
GLUT5 Fructose GLUT2 Fructose Lum
inal 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. 842203,
2001
1. Fiber Content (Alf hay and alf barley
silage) Low High FC ratios 3565 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 842203, 2001
30
CHOH Digestion in Dairy Cows
JDS 842203, 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 731239, 1995
33
CHOH Digestion in Steers
JAS 80797, 2002