Cellulose Digestion
Cellulose
Endoglucanase
Cellodextrins
Cellobiose Cellotriose
Glucose
Exoglucanase
Glucose chains
β-Glucosidase (cellobiase)
Starch Digestion
Starch
Amylopectin
Amylose
 Amylase
Dextrins
 Amylase
 Amylase
Maltase
Maltose
Glucose
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
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
Rumen Digestion
Cellulose
Hemicellulose
Pectin
Starch
Uronic acids Galactose
Cellobiose Pentoses
Pentose
pathway
Dextrose
Maltose
Glucose
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
Digestible pool
Kp
Indigestible pool
Digestion
% 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)
0
24
48
Hours
72
Log of % nutrient
remaining
Calculations
CHOH Fraction
A
B1
B2 C
Hours
Calculate slope (change per hour) of each line.
Slope = kd, has units of % of pool remaining that is
lost per hour.
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/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)
Calculation of Fractions Leaving the Rumen
Rumen Escape
1. 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)
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
Some Kd Values
Carbohydrates
A
Corn
Whole
Cracked
High moist
Sorghum
Dry roll
Steam flake
Legume hay
Grass hay
Corn silage
B1
---------%/h--------75-150
5-10
100-200
10-20
200-300
15-20
B2
100-200
200-300
200-300
200-300
200-300
4-5
6-8
3-6
2-4
3-6
5-15
15-20
25-35
25-35
10-20
3-5
5-7
6-8
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
Some Kp Values
2.0
2.5
3.5
Level of maintenance
2X
--------%/hr-------2.5
4.0
4.0
2.5
4.0
3.0
5.0
4.0
6.0
2.0
3.0
2.0
2.5
3.5
2.5
3.0
4.5
3.0
1X
Wheat mids
Whole corn
Cracked corn
Legumes
Long
Chopped
Grasses
Long
Chopped
Corn silage
3X
3.0
6.5
5.0
Rate and Extent of Digestion in the Rumen
1. Quantity of feed consumed
Intake =
Rate of passage =
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
% Digested
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
% Digested
Associative Effects
100
Calculated
80
60
Observed
40
0
50
Percentage of concentrate
100
Carbohydrate Digestion in
Intestines
CHOH in intestines
Plant CHOH escaping degradation in the rumen
Cell walls
Starch
Microbial polysaccharides
Bacteria
Protozoa
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
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.
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
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
Glucose Transport Systems
Lumen
Cell
Glucose
Na+
Glucose
Na+
Na+
K+
SGLT1
Glucose
Glactose
Fructose
Blood
Glucose
ATPase
GLUT2
K+
Glucose
Metabolism
GLUT5
Luminal membrane
Fructose
GLUT2
Fructose
Basolateral membrane
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
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.
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 Cows
J. 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
Kernel thickness, mm
1.60
Kernel width, mm
4.68
Flat
1.36
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
Kd, %/hra ERDb, %
A, %
B, %
Barley
Coarse
Flat
3.6
5.8
87.1
71.3
2.71
9.20
30.6
48.9
Forage
Alfalfa silage
Barley silage
24.8
24.3
32.0
50.2
5.81
2.99
40.4
40.5
28.2
37.8
8.11
49.6
Alfalfa hay
ARate
of degradation of B fraction
bERD = Effective ruminal degradability, K = 4%/hr
p
CHOH Digestion in Dairy Cows
Duodenal Cannula – Whole Diet
Grain processing
JDS 84:2203, 2001
Coarse
Flat
OM intake, kg/d
17.6
18.8
Dig rumen, %
48.2
49.1
Dig post rumen, %a 30.7
34.2
Dig post rumen, %b 42.2
49.0
Fiber content
Low
High
18.7
17.7
47.7
49.5
33.7
31.2
47.9
43.2
Starch intake, kg/d
Dig rumen, %
Dig post rumen, %a
Dig post rumen, %b
6.80
48.3
37.0
71.2
a
% of intake
b
5.49
37.8
43.9
68.4
6.08
50.1
40.1
78.7
% of passing into duodenum
4.77
39.6
47.0
75.8
CHOH Digestion in Dairy Cows
Grain processing
Flat
JDS 84:2203, 2001 Coarse
NDF intake, kg/d
6.87
7.29
Dig rumen, %
39.3
37.8
Dig post rumen, %a
6.5
8.7
Dig post rumen, %b
8.3
12.3
Fiber content
Low
High
6.69
7.47
38.7
38.5
9.2
6.0
13.3
7.3
ADF intake, kg/d
4.11
4.37
3.63
Dig rumen, %
31.3
28.4
25.7
Dig post rumen, %a
5.8
11.3
11.3
Dig post rumen, %b
6.3
14.4
13.3
a
b
% of intake
% passing to duodenum
4.84
34.0
5.8
7.4
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
JAS 73:1239, 1995
Dry roll Steam
flake
OM intake, kg/d
3.96
3.93
Dig rumen, %
56.5
65.5
Dig post rumen, %a 37.3
34.8
3.22
63.5
36.2
4.67
58.5
35.9
Starch intake, kg/d
Dig rumen, %
Dig post rumen, %a
2.06
70.6
19.2
2.03
85.4
13.6
1.67
79.9
14.8
2.42
76.2
18.0
ADF intake, kg/d
Dig rumen, %
Dig post rumen, %a
0.33
31.3
12.5
0.33
25.3
16.1
0.27
31.6
14.3
0.39
25.0
14.4
a
% of intake
Diet: 74.7% corn and 12% roughage in the diet
Level intake
Low
High
CHOH Digestion in Steers
JAS 80:797, 2002
DM intake, kg/d
Dig total tract %
Form of corn grain
Dry roll
High
Steam
moisture
flake
12.7
14.5
12.5
83.5
86.7
87.2
OM intake, kg/d
Dig rumen, %
Dig post rumen, %a
Dig total tract, %
12.1
64.4
69.1
84.6
13.8
76.0
67.1
88.0
11.9
69.3
72.5
88.1
Starch intake, kg/d
Dig rumen, %
Dig post rumen, %a
Dig total tract, %
7.8
81.2
84.4
96.1
8.4
97.3
86.5
98.7
7.5
94.7
98.3
99.8
a
% entering
Diet: 81.75% corn and 10% roughage in the diet