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Saliva and the Fiber Requirements of Ruminants. Church: 117-124, 229-231 Van Soest: 246-249, 153-155 Sjersen et al. 155-163 Nutrient Requirements of Beef Cattle:Seventh Revised Edition:Update 2000. pp. 129-130. Available at:

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saliva and the fiber requirements of ruminants

Saliva and the Fiber Requirements of Ruminants

Church: 117-124, 229-231

Van Soest: 246-249, 153-155

Sjersen et al. 155-163

Nutrient Requirements of Beef Cattle:Seventh Revised Edition:Update 2000. pp. 129-130. Available at:

Nutrient Requirements of Dairy Cattle:Seventh Revised Edition, 2001. Chapter 4, pp. 34-42. Available at:

Armentano, L. and M. Pereira. 1997. Measuring the effectiveness of fiber by animal response trials. J. Dairy Sci. 1416-1425

Available at:

Mertens, D. 1997. Creating a system for meeting the fiber requirements of dairy cows. J. Dairy Sci. 80:1463-1481.

Available at:

functions of saliva in ruminants
Functions of saliva in ruminants
  • Moistens and lubricates feeds
  • Water balance
  • Bloat prevention
  • Intake control
  • Recycling of nitrogen and minerals to the rumen
  • Buffering the rumen fermentation
  • Unlike nonruminants
    • No enzymes secreted in saliva of mature ruminants
Moistening and lubricating feed

Components responsible




Protects mucus membrane of mouth and esophagus

Aids in bolus formation

Water solubilizes soluble components providing access to taste buds

Water balance

70% of the fluid entering the rumen

Bloat prevention

Mucin is a strong anti-foaming agent

Intake control (?)

Saliva infused into the abomasum increased reticular contractions and DM intake in sheep

Infused into the abomasum, ml/hr

Saliva:McDougall’s solution


DMI, % BW 1.23 3.5 5.1 1.23

Reticular contractions, 1.4 5.7

% increase over no infusion

saliva s role in recycling n and minerals
Saliva’s role in recycling N and minerals
  • Nitrogen
    • In a 24 hour period, a 700 kg cow receiving a mixed hay:grain diet with secrete:
      • 190 l saliva
      • 30 to 80 gm total N
      • 50-130 gm urea
    • N recycling
      • Will be important on low protein diets
      • An important consideration in minimizing N excretion

Dietary protein









Amounts recycled

General estimates

% dietary N recycled = 15-20%

CNCPS program

% N recycled = (121.7 – 12.02 x %CP + .3235 x %CP2)/100

% CP in diet% N recycled

6 61

8 46

10 34

12 24

14 17

16 12

18 10

Marini et al. (2003)

Holstein heifers fed a corn meal-molasses- citrus pulp diet fed at 1.8 x maintenance

% CP in diet% N recycled

9.1 30

11.8 37

15.7 25

18.6 22

Routes of N recycling


15 to 50% of total recycled N


Blood urea concentration

Saliva flow

Gut wall

Major route


Low ruminal [NH3]

Upregulates a urea transporter which increases transfer of urea from blood to epithelium or vice versa

Decreases microbial urease activity of microbes adhered to the rumen wall:

decreases conversion on urea to NH3 at rumen wall

Decreased ruminal pH

Converts NH3 to NH4+ in the rumen

Only NH3 can cross the rumen wall

Marini et al. (2003)

% CPN recycled (saliva)N recycled (Gut wall)

g/d% of totalg/d% of total

9.1 0.8 3.0 25.1 97.0

11.8 1.5 3.6 39.6 96.4

15.7 3.8 10.4 32.7 89.6

18.6 5.4 13.7 33.9 86.3


Urea Diffusion into RumenUpdate

Rumen wall

Urea transporter


urea Urea

High [NH3]



Bacterial population



700 kg cow producing 190 l saliva/day will secrete:

1100 gm NaHCO3

350 gm Na2 HPO4

100 gm NaCl

Minerals recycled in saliva




classes of salivary glands
Classes of salivary glands
  • Serous glands
    • Include:
      • Parotid glands
      • Inferior molar glands
    • Properties
      • Saliva is quite fluid
        • Parotid glands secrete ½ of all saliva
      • Saliva is isotonic with plasma
        • Saves osmotic work
      • Saliva is strongly buffered with HCO3- and HPO4-2
      • Secrete continuously, but increased with eating and ruminating
Mucus glands


Palatine glands

Buccal glands

Pharyngeal glands


Vary mucus saliva

Isotonic with plasma

Saliva is strongly buffered with HCO3- and HPO4-2

Low flow when not stimulated

Mixed glands






Very mucus saliva

Hypotonic to plasma

Poorly buffered

Variable flow

composition of saliva
Composition of saliva
  • Composition from different glands


Parotid 95 75 13 186 5

Inferior molar 134 48 10 175 9

Palatine and Buccal 109 25 25 179 4

Submaxillary 6 54 6 15 26

  • Composition control
    • Adrenal cortex
      • Aldosterone
    • Kidney
      • Renin
  • Factors affecting saliva composition
    • Sodium deprivation
      • As concentration of Na decreases, the concentration of K increases to maintain concentration of total cations
    • Rate of saliva secretion
      • As rate of secretion increases
        • [Na+] and [HCO3-] increases
        • [K+] and [HPO4-2] decreases
saliva secretion
Saliva secretion
  • Control of secretion
    • Controlled by the vagus nerve through receptors in the mouth, esophagus, reticulum, reticuloruminal fold, and reticulo-omasal orifice
    • Stimuli
      • Stretch up to 20 mm Hg
      • Rumination
Factors affecting saliva flow

Activity of animal

Activity% of saliva flow

Resting 36

Eating 27

Ruminating 37

Feed consumption

Increased DM intake increases saliva flow

Type and physical form of diet

Factors that limit rumination will limit saliva flow

Saliva secretion will be decreased as:

Grain level in the diet increases

Maturity of forage in the diet decreases

The particle size of the feedstuffs decreases

The diet moisture level increases

DietSaliva secretion (gm/gm feed consumed)

Dairy cubes .68

Fresh grass .94

Silage 1.13

Dried grass 3.25

Hay 3.65

saliva s role in buffering the rumen
Saliva’s role in buffering the rumen
  • Significance of the rumen buffering system
    • Enough organic acids are produced in the rumen to cause the pH to drop to 2.8 to 3.0 without buffering
    • Normal rumen pH range is 5.5 to 7.1
  • Components of the rumen buffering system

__pK__Buffering range

HPO4-2 (second H+) 7.1 6-7

HCO3- (first H+; saliva and 6.4 5.5-7

rumen wall)

Acetate 4.8

Propionate 4.9

Butyrate 4.8

Lactate 3.9 5-6

Glutamate 5.6

Aspartate 5.2

Alfalfa protein isoelectric point 5.5

NH3 9.3

Cation exchange capacity

VFA absorption

role of cation exchange in buffering the rumen
Role of cation exchange in buffering the rumen
  • Cation exchange capacity
    • The concentration of charged groups like proteins, lignins, and pectins that exchange cations like Ca+2, Mg+2, and K+ for H+
    • Cation exchange capacity of different forages

CEC, mEq/100 gm

ForageMechanical pulpNDF

Fescue 59 111

Timothy 68 132

Orchardgrass 72 120

Rice straw 43 57

Alfalfa 152 104

Red clover 169 139

White clover 294 249

buffering range in the rumen
Buffering range in the rumen
  • The rumen is well-buffered for acid, but poorly for alkali
  • Buffer curve








40 20 0 20 40 60 80 100 120

1N KOH added 1N HCl added

ruminant fiber requirement effects of fiber on ruminant intake digestion and metabolism
Ruminant fiber requirementEffects 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

eNDFpH% 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 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

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





Molar %


7 6 5


Cause of changes in VFAs

Primary end-products of cellulolytic bacteria (pHopt6-7)

Acetic acid

Butyric acid

Carbon dioxide


Primary end-products of amylolytic bacteria (pHopt5-6)

Acetic acid

Propionic acid

Lactic acid



VFAs, 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 ratio


ME intake, Mcal 36.12 36.42 34.87

Energy balance, Mcal, RE 11.94 12.63 12.16

Milk energy, Mcal, LE 13.94 13.17 10.41

LE/RE x 100 117 104 86

Tissue energy, Mcal -2.00 -.54 1.75

Milk fat, % 3.5 3.0 2.7

Acetate/Propionate 3.32 2.57 2.00





Milk or body weight

Synthesis, kcal /

100 Kcal ME

above maintenance

Body tissue

30 40 50 60 70

Acetic acid, % of total VFA

Cause for difference in energy partitioning

Old theory

Decreasing [Acetate] and increasing [Propionate] reduces milk fat synthesis and increases body tissue synthesis


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

Microbial yield

Inadequate dietary fiber

Decreased salivary buffers

Decreased pH Decreased osmotic pressure

Decreased liquid turnover

Decreased efficiency of microbial growth

eNDFTheoretical maximum microbial synthesis, g/g CHO fermented

24 .4

20 .4

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:


Particle size reduction


40 kg milk

20 kg milk





Physical limitation



20 30 40 50


At low fiber levels, feed intake is under physiological control



Increased [Acetate] in the rumen decreases feed intake

Increased [Propionate] in the portal vein decreases feed intake





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


Long-term health problems


Liver abscess


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

the fiber requirements of ruminant animals
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
        • 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



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



Feed volume .78 .62 .71

NDF is more highly related to chewing time than CF or ADF



Chewing time .86 .73 .76

Using a static fiber percentage prevents the opportunity to meet the fiber requirement and come close to meeting the energy requirements of high producing dairy cows

Feed intake, lb/day

Milk production, lb/day

Body weight, lb

0 10 20 30 40

Week of lactation

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


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

High concentrate diets to maximize 5 – 8

Gain/Feed, good bunk management

& ionophore

Mixed diet, variable bunk management or 20

no ionophore

High concentrate diet to maximize 20

non-fiber carbohydrate (NFC) use

& microbial yield

Lactating dairy cows


Total mixed ration fed

Adequate particle size of the forage

Grain is corn

Recommendations (Adjusted for minimum forage NDF in diet DM)


Minimum NDF, %DMMinimum NDF, %DMMaximum NFC, % DM

19 25 44

18 27 42

17 29 40

16 31 38

15 33 36


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 DM

Nonstructural carbohydrates 30-40

Non-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


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


First 3 weeks 28

Peak lactation 25

use of buffers in ruminant diets
Use of buffers in ruminant diets
  • Functions of buffers
    • Increase ruminal pH
    • Maintain DM intake
    • Prevent acidosis
    • Increase liquid turnover
  • Buffers commonly used

BufferAdditional effectsPreventative level

Sodium bicarbonate - 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


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:


Urinary calculi