1. FIBER IN RUMINANT DIETS
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2. Fiber can be defined as carbohydrates not digested by mammalian enzymes but can be digested by rumen microorganisms.
Fiber includes cellulose, hemicellulose, lignin, and soluble fiber (fructans, pectans, galactans, and beta-glucans).
Most fiber in plant material is found in the structural components of cell walls.
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3. Ruminant livestock (cattle, goats, and sheep) can use large amounts of forage with high fiber content.
Fiber-digesting bacteria digest structural carbohydrates in the rumen into VFA, which is as main energy source for ruminant.
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Rumen changes in response to decreased fiber intake (Adapted from Trenkle, 2002)

5. Ruminant fiber requirement Effects of fiber on ruminant intake, digestion and metabolism
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

6. 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

7. 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 40
Acetate
Propionate
Molar %
Lactate pH

8. Hay:Concentrate 60:40 40:60 20:80 VFAs, molar %
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)
Propionic acid
Lactic acid
Hay:Concentrate
60: : :80
VFAs, molar %
Acetic acid
Propionic acid
Butyric acid

9. 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
Hay:grain ratio
Item : : :80
ME intake, Mcal
Energy balance, Mcal, RE
Milk energy, Mcal, LE
LE/RE x
Tissue energy, Mcal
Milk fat, %
Acetate/Propionate 70 40 10
Milk or body weight
Synthesis, kcal /
100 Kcal ME
above maintenance
Milk
Body tissue
Acetic acid, % of total VFA

10. 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)

11. Inadequate dietary fiber
Microbial yield
Inadequate dietary fiber
Decreased salivary buffers
Decreased pH Decreased osmotic pressure
Decreased liquid turnover
Decreased efficiency of microbial growth
eNDF Theoretical maximum microbial synthesis, g/g CHO fermented

12. Feed consumption
At high fiber levels, feed intake is limited by the physical volume occupied by fiber
Physical limitation is freed by:
Digestion
Particle size reduction
Passage
40 kg milk
20 kg milk 4 3 2
DMI, % BW
Physical limitation
Physiological
control
NDF, % DM

13. 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

14. 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

15. Displaced abomasum
Decreased fiber
Muscle atrophy Subclinical acidosis
Decreased feed intake
Empty abomasum

16. The fiber requirements of ruminant animals
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

17. 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
Rate of digestion is important at high feed intakes
NDF is more highly related to feed volume than CF or ADF
Feed volume
NDF is more highly related to chewing time than CF or ADF
Chewing time

18. 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
Cracked corn
Whole corn
Corn gluten feed
Corn silage
Alfalfa haylage (1/4” cut)
Alfalfa hay, late vegetative
Oat straw
Bromegrass hay, pre-bloom
Relationship to rumen pH
Rumen pH = x eNDF for eNDF < 35% DM
Doesn’t consider cation exchange capacity

19. Current fiber requirements
Beef cattle
Minimum eNDF, % DM
High concentrate diets to maximize – 8
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. 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 Minimum NDF, %DM Maximum NFC, % DM
Adjustments
Starch source
High moisture corn % NDF (Minimum)
Barley % 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

21. Additional recommendations for dairy cattle % of diet DM
Nonstructural carbohydrates
Non-fiber carbohydrates
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
Peak lactation

22. Use of buffers in ruminant diets
Functions of buffers
Increase ruminal pH
Maintain DM intake
Prevent acidosis
Increase liquid turnover
Buffers commonly used
Buffer Additional effects Preventative level
Sodium bicarbonate to 1.6% of grain
.75% of diet
Sodium sesquicarbonate to .75 lb/d
Magnesium oxide Increase uptake to .5% of grain
of acetate by mammary gland to .2 lb/d
Potassium carbonate Provides potassium to .9 lb/d

23. 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

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26. Neutral detergent fiber (NDF) =
fiber that is insoluble in neutral detergent and includes cellulose, hemicellulose, and lignin.
represents all plant cell wall material, is only partly digestible by animals, and
is negatively correlated with dry matter intake. As NDF increases in the diet, dry matter intake decreases.
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28. Acid detergent fiber (ADF) =
is the portion of fiber that is insoluble in acid detergent (cellulose and lignin),
is composed of highly indigestible plant material, generally only the lignified or otherwise undigestible portions of plant cell walls.
is negatively correlated with digestibility. Generally, as ADF increases, forages or feeds become less digestible.
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