Date post: | 26-Dec-2015 |
Category: |
Documents |
Upload: | samson-blankenship |
View: | 215 times |
Download: | 0 times |
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 digestionEffective fiber is the NDF remaining on a 1.18 screen, as a % of total DM
eNDF pH % of maximum fiber digestion
24 6.4 9820 6.3 9516 6.1 8712 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
Acetate
Propionate
Lactate
7 6 5 pH
Molar %
80
40
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 20:80VFAs, molar % Acetic acid 66.9 62.9 56.7 Propionic acid 21.1 24.9 30.9 Butyric acid 12.2 12.2 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
Hay:grain ratioItem 60:40 40:60 20:80ME intake, Mcal 36.12 36.42 34.87Energy balance, Mcal, RE 11.94 12.63 12.16Milk energy, Mcal, LE 13.94 13.17 10.41LE/RE x 100 117 104 86Tissue energy, Mcal -2.00 -.54 1.75Milk fat, % 3.5 3.0 2.7Acetate/Propionate 3.32 2.57 2.00
70
40
10
30 40 50 60 70Acetic acid, % of total VFA
Body tissue
MilkMilk or body weightSynthesis, kcal /100 Kcal ME above maintenance
Cause for difference in energy partitioning
Old theoryDecreasing [Acetate] and increasing [Propionate] reduces milk fat synthesis and increases body tissue synthesisBasis:
Propionate is needed to synthesize glucoseGlucose needed for acetate metabolism for
energy and fat synthesisGlucose stimulates insulin secretionInsulin increases glucose uptake by adipose
and muscle tissue, but not mammary tissueResults in acetate being preferentially used
by adipose and muscle tissue
Current theoryReduced pH increases production of trans-10, cis-12 conjugated linoleic acid from polyunsaturated fatty acidsTrans-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 Theoretical maximum microbial synthesis, g/g CHO fermented
24 .40 20 .40 16 .36 12 .32 8 .28 4 .24
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
20 30 40 50 NDF, % DM
4
3
2
DMI, % BWPhysical limitation
Physiologicalcontrol
20 kg milk
40 kg milk
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 microorganismsDilutes VFAsIncreases 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
NDFNDF ADF CF
rTDN .65 .76 .80 Rate of digestion is important at high feed intakes
NDF is more highly related to feed volume than CF or ADFNDF ADF CF
rFeed volume .78 .62 .71
NDF is more highly related to chewing time than CF or ADF
NDF ADF CF rChewing time .86 .73 .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
eNDFFeed % NDF % of NDF % of DM
Corn cobs 87 56 49Cracked corn 10.8 60 6.7Whole corn 9.0 100 9.0Corn gluten feed 36.0 36 12.8Corn silage 41.0 71 29Alfalfa haylage (1/4” cut) 43.0 67 29Alfalfa hay, late vegetative 37.0 92 34Oat straw 63.0 98 62Bromegrass 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, % DMHigh concentrate diets to maximize 5 – 8Gain/Feed, good bunk management & ionophoreMixed diet, variable bunk management or 20no ionophoreHigh concentrate diet to maximize 20non-fiber carbohydrate (NFC) use & microbial yield
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 DietMinimum NDF, %DM Minimum NDF, %DM Maximum NFC, %
DM 19 25 44 18 27 42 17 29 40 16 31 38 15 33 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 % of diet DMNonstructural carbohydrates 30-40Non-fiber carbohydrates 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
AssumptionsForage supply 70 to 80% of the NDFForages 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 28Peak lactation 25
Functions of buffers Increase ruminal pH Maintain DM intake Prevent acidosis Increase liquid turnover
Buffers commonly usedBuffer Additional effects
Preventative levelSodium bicarbonate - 1.2 to
1.6% of grain .75
% of dietSodium sesquicarbonate - .3 to
.75 lb/dMagnesium oxide Increase uptake .4
to .5% of grain of acetate by mammary gland .1
to .2 lb/dPotassium 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) =ofiber that is insoluble in neutral detergent and includes cellulose, hemicellulose, and lignin.
orepresents all plant cell wall material, is only partly digestible by animals, and
ois 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),
ois composed of highly indigestible plant material, generally only the lignified or otherwise undigestible portions of plant cell walls.
ois negatively correlated with digestibility. Generally, as ADF increases, forages or feeds become less digestible.