FIBER IN RUMINANT DIETS
o Fiber can be defined as carbohydrates
not digested by mammalian enzymes but
can be digested by rumen
microorganisms.
o Fiber includes cellulose, hemicellulose,
lignin, and soluble fiber (fructans,
pectans, galactans, and beta-glucans).
o Most fiber in plant material is found in
the structural components of cell walls.
o Ruminant livestock (cattle, goats, and
sheep) can use large amounts of forage
with high fiber content.
o Fiber-digesting bacteria digest structural
carbohydrates in the rumen into VFA,
which is as main energy source for
ruminant.
Rumen changes in response to decreased fiber intake
(Adapted from Trenkle, 2002)

Digestibility

Inadequate fiber
 Results in reduced fiber digestion
 Cause
 Maximum growth of cellulolytic bacteria and protozoa occurs
between pH 6 and 7
 If the effective fiber concentration of the diet is < 24.5%, rumen
pH will decrease resulting in reduced fiber digestion
Effective fiber is the NDF remaining on a 1.18 screen, as a % of
total DM
eNDF
pH
% of maximum fiber digestion
24
6.4
98
20
6.3
95
16
6.1
87
12
5.9
70
8
5.7
28
4
5.6
0
 Physiological cause for the inhibition of cellulolytic bacteria
 ATP energy production from the proton motive force across the
cell membrane is inhibited by acids entering the cells
 Inadequate quantities of HCO3- which is the active form of CO2
for anerobic bacteria
 Toxicity of the VFAs and lactate is greater because nonionized
forms more readily cross cell membranes
 Reduced ruminal turnover reduces efficiency of microbial
growth

Excess fiber
 If lignified, high levels of fiber may reduce DM digestibility
because soluble constituents are diluted or the ratio of soluble
nutrients to the insoluble ones is low

Fermentation endproducts

Volatile fatty acids
 Decreased fiber causes reduced pH which causes
 Increased production of total VFAs
 Decreased molar proportions of acetate and butyrate
 Increased molar proportions of propionate
80
Acetate
Molar %
40
Propionate
Lactate
7
6
pH
5
 Cause of changes in VFAs
 Primary end-products of cellulolytic bacteria (pHopt6-7)
 Acetic acid
 Butyric acid
 Carbon dioxide
 Hydrogen
 Primary end-products of amylolytic bacteria (pHopt5-6)
 Acetic acid
 Propionic acid
 Lactic acid
Hay:Concentrate
60:40
40:60
20:80
VFAs, molar %
Acetic acid
Propionic acid
Butyric acid
66.9
21.1
12.2
62.9
24.9
12.2
56.7
30.9
12.4
 Effects of changes in VFA concentrations on efficiency of energy use for body
tissue or milk synthesis
 Decreasing the concentration of acetate and increasing the concentration of propionate will
decrease the energetic efficiency of milk production while increasing that of body tissue
synthesis
70
Milk or body weight
Synthesis, kcal /
40
100 Kcal ME
above maintenance
10
Milk
Body tissue
30
40
50
60
70
Acetic acid, % of total VFA
Item
ME intake, Mcal
Energy balance, Mcal, RE
Milk energy, Mcal, LE
LE/RE x 100
Tissue energy, Mcal
Milk fat, %
Acetate/Propionate
Hay:grain ratio
60:40 40:60 20:80
36.12 36.42 34.87
11.94 12.63 12.16
13.94 13.17 10.41
117
104
86
-2.00
-.54
1.75
3.5
3.0
2.7
3.32
2.57
2.00
 Cause for difference in energy partitioning
 Old theory
Decreasing [Acetate] and increasing [Propionate] reduces milk fat
synthesis and increases body tissue synthesis
Basis:
Propionate is needed to synthesize glucose
Glucose needed for acetate metabolism for energy and fat
synthesis
Glucose stimulates insulin secretion
Insulin increases glucose uptake by adipose and muscle
tissue, but
not mammary tissue
Results in acetate being preferentially used by adipose and
muscle tissue
 Current theory
Reduced pH increases production of trans-10, cis-12 conjugated linoleic
acid from polyunsaturated fatty acids
Trans-10, cis-12 conjugated linoleic acid inhibits long chain fatty acid
synthesis in the mammary gland (decreases)

Microbial yield
Inadequate dietary fiber
Decreased salivary buffers
Decreased pH
Decreased osmotic pressure
Decreased liquid turnover
Decreased efficiency of microbial growth
eNDF
24
20
16
12
8
4
Theoretical maximum microbial synthesis, g/g CHO fermented
.40
.40
.36
.32
.28
.24

Feed consumption

At high fiber levels, feed intake is limited by the physical
volume occupied by fiber
40 kg milk
20 kg milk
4
DMI, % BW
3
Physical limitation
2
Physiological
control
20

30
40
NDF, % DM
Physical limitation is freed by:
 Digestion
 Particle size reduction
 Passage
50

At low fiber levels, feed intake is under physiological control
 Limitations
 VFAs
 Increased [Acetate] in the rumen decreases feed intake
 Increased [Propionate] in the portal vein decreases feed intake
 Hormones
 Insulin
 Glucagon
 Osmolality
 Increased [H+] in duodenum reduces reticuloruminal contractions to reduce
feed intake
 Acidosis a problem in feedlot cattle and dairy cows rapidly changed from
a high forage to a high grain diet
 Fiber’s role on low fiber diets
 Saliva flow
 Provides buffers
Prevents undesirable microorganisms
Dilutes VFAs
Increases liquid turnover
 Motility

Long-term health problems due to low fiber feeding



Parakeratosis
Liver abscess
Laminitis
Inadequate fiber
Decreased pH
Increased VFA and lactic acid
Decreased gram- bacteria
Release histamine and endotoxins (?)
Increased blood pressure
Dilation and damage to blood vessels

Displaced abomasum
Decreased fiber
Muscle atrophy
Subclinical acidosis
Decreased feed intake
Empty abomasum
Displaced abomasum

Previous requirements

Dairy
 Before 1989
 Minimum of 17% CF
 1989 NRC
 Minimum of 21% ADF for first 3 weeks lactation period
 Minimum of 19% ADF at peak lactation

Beef
 Before 1996 NRC
 Minimum of 10% roughage

Limitations of previous requirements
 CF and ADF do not represent all fiber fractions
 CF contains variable amounts of cellulose and lignin
 ADF contains cellulose and lignin
 NDF contains cellulose, lignin, hemicellulose and pectins
 While related to digestibility,
 CF and ADF are not as highly related to the rate of digestion as NDF
NDF
ADF
CF
r
TDN
.65
.76
.80
 Rate of digestion is important at high feed intakes
 NDF is more highly related to feed volume than CF or ADF
NDF
Feed volume
.78
ADF
CF
r
.62
.71
 NDF is more highly related to chewing time than CF or ADF
NDF
Chewing time
.86
ADF
r
.73
CF
.76
 Fiber requirements have not considered the physical form of the fiber
 Physical form affects chewing time
 Particularly a problem with high fiber byproduct feeds
 To consider physical form, the Beef NRC used effective NDF (eNDF) to
express the fiber requirement of beef cattle
 Definition - % NDF remaining on a 1.18 mm screen after dry sieving
eNDF
Feed
% NDF
% of NDF % of DM
Corn cobs
87
56
49
Cracked corn
10.8
60
6.7
Whole corn
9.0
100
9.0
Corn gluten feed
36.0
36
12.8
Corn silage
41.0
71
29
Alfalfa haylage (1/4” cut)
43.0
67
29
Alfalfa hay, late vegetative
37.0
92
34
Oat straw
63.0
98
62
Bromegrass hay, pre-bloom
55.0
98
54
 Relationship to rumen pH
Rumen pH = 5.425 + .04229 x eNDF
for eNDF < 35% DM
 Doesn’t consider cation exchange capacity

Current fiber requirements

Beef cattle
Minimum eNDF, % DM
5–8
High concentrate diets to maximize
Gain/Feed, good bunk management
& ionophore
Mixed diet, variable bunk management or
no ionophore
High concentrate diet to maximize
non-fiber carbohydrate (NFC) use
& microbial yield
20
20

Lactating dairy cows
 Assumptions
 Total mixed ration fed
 Adequate particle size of the forage
 Grain is corn
 Recommendations (Adjusted for minimum forage NDF in diet DM)
Forage
Diet
Minimum NDF, %DM
19
18
17
16
15
Minimum NDF, %DM
25
27
29
31
33
Maximum NFC, % DM
44
42
40
38
36
 Adjustments
 Starch source
 High moisture corn
27% NDF (Minimum)
 Barley
27% NDF (Minimum)
 Forage particle size
 Desire length of chop of forage at ¼”
15 to 20% of particles > 1.5”
 If mean particle size of forage decreases below 3 mm, then the minimum
dietary NDF % should be increased several percent
 Dietary buffers
 Can lower NDF requirements
 Method of feeding
 Feeding separate components will increase the NDF requirement
 Additional recommendations for dairy cattle
Nonstructural carbohydrates
Non-fiber carbohydrates
% of diet DM
30-40
32-42
 Merten’s approach to meeting the fiber requirements of dairy cattle
 Daily requirement for NDF in optimum ration is 1.2% of BW
 Assumptions
Forage supply 70 to 80% of the NDF
Forages are chopped at no less than ¼”
 Allows the percentage of fiber in the diet to vary with milk production and
feed intake
 Recommended minimums
% NDF
First 3 weeks
28
Peak lactation
25

Functions of buffers





Increase ruminal pH
Maintain DM intake
Prevent acidosis
Increase liquid turnover
Buffers commonly used
Buffer
Additional effects
Sodium bicarbonate
-
Preventative level
1.2 to 1.6% of grain
.75% of diet
Sodium sesquicarbonate
.3 to .75 lb/d
Magnesium oxide
Increase uptake
.4 to .5% of grain
of acetate by mammary gland
.1 to .2 lb/d
Potassium carbonate Provides potassium
.5 to .9 lb/d

Buffers are most effective when:





Early lactation
Switching from high forage to high grain diets
Diet is deficient in effective fiber
Concentrates and forages are fed separately
Fermented forages are the only forage source
 Particularly a problem with corn silage



Large amounts of fermentable carbohydrates are fed at infrequent intervals
Small particle size or high moisture level of the grain
Milk fat percentage of dairy cows is low
 Milk fat % is .4 units < Protein %
 Milk fat % is < 2.5% in Holsteins



Off-feed problems caused by feeding rapidly fermenting feeds
Heat stress
Limitations of buffers

Unpalatable
 2% sodium bicarbonate or 1% Magnesium oxide will reduce feed intake


Responses are short-lived
Buffers don’t cure all problems associated with low fiber diets
 Displaced abomasum

Health problems associated with buffers:
 Bloat
 Urinary calculi
 Diarrhea
Neutral detergent fiber (NDF) =
o fiber that is insoluble in neutral detergent and
includes cellulose, hemicellulose, and lignin.
o represents all plant cell wall material, is only partly
digestible by animals, and
o is negatively correlated with dry matter intake. As
NDF increases in the diet, dry matter intake
decreases.
Acid detergent fiber (ADF) =
o is the portion of fiber that is insoluble in acid
detergent (cellulose and lignin),
o is composed of highly indigestible plant material,
generally only the lignified or otherwise undigestible
portions of plant cell walls.
o is negatively correlated with digestibility. Generally, as
ADF increases, forages or feeds become less
digestible.