Saliva and the Fiber Requirements of Ruminants

Nutrient Requirements of Beef Cattle:Seventh Revised Edition:Update 2000. pp. 129-130. Available at: http://search.nap.edu/books/0309069343/html/

Nutrient Requirements of Dairy Cattle:Seventh Revised Edition, 2001. Chapter 4, pp. 34-42. Available at: http:search.nap.edu/books/0309069971/html/

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

Available at: http://jds.fass.org/cgi/reprint/80/7/1416.pdf

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

Available at: http://jds.fass.org/cgi/reprint/80/7/1463.pdf

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

• Water• Mucin

– Functions• 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 0:1000 250:750 500:500 0:1000DMI, % BW 1.23 3.5 5.1 1.23Reticular contractions, 1.4 5.7% increase over no infusion

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

Protein

Microbialprotein

Metabolizableprotein

Dietary protein

NPN

NH3

Urea

– Amounts recycled» General estimates

% dietary N recycled = 15-20% (Approximately ½ as urea)» 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• Saliva

– 15 to 50% of total recycled N– Factors

» Blood urea concentration» Saliva flow

• Gut wall– Major route– Factors

» Increased ruminal [NH3]Increases urea transferase 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 needed to transfer NH3 across rumen wall

» Decreased ruminal pHConverts NH3 to NH4

+ in the rumenOnly NH3 can cross the rumen wall

• Marini et al. (2003) % CP N recycled (saliva) N recycled (Gut wall) g/d % of total g/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

• Minerals– 700 kg cow producing 190 l saliva/day will

secrete:• 1100 gm NaHCO3

• 350 gm Na2 HPO4

• 100 gm NaCl

– Minerals recycled in saliva• Na• P• S

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– Include:

• Palatine glands• Buccal glands• Pharyngeal glands

– Properties• Vary mucus saliva• Isotonic with plasma• Saliva is strongly buffered with HCO3

- and HPO4-2

• Low flow when not stimulated

• Mixed glands– Include

• Submaxillary• Sublingual• Labial

– Properties• Very mucus saliva• Hypotonic to plasma• Poorly buffered• Variable flow

The salivary glands

Composition of saliva• Composition from different glands

HCO3- HPO4

-2 Cl- Na+ K+

Parotid 95 75 13 186 5Inferior molar 134 48 10 175 9Palatine and Buccal 109 25 25 179 4Submaxillary 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• 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 flowResting 36Eating 27Ruminating 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

Diet Saliva secretion (gm/gm feed consumed)Dairy cubes .68Fresh grass .94Silage 1.13Dried grass 3.25Hay 3.65

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 rangeHPO4

-2 (second H+) 7.1 6-7HCO3

- (first H+) 6.4 5.5-7Acetate 4.8 Propionate 4.9 5-6Butyrate 4.8 Lactate 3.9Glutamate 5.6Aspartate 5.2 5-6Alfalfa protein isoelectric point 5.5 NH3 9.3Cation exchange capacity

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

Forage Mechanical pulp NDF

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

• The rumen is well-buffered for acid, but poorly for alkali

• Buffer curve

40 20 0 20 40 60 80 100 120

9

8

7 6

5

4

pH

1N KOH added 1N HCl added

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 digestionEffective fiber is the NDF remaining on a 1.18 screen, as a % of total DM

eNDF pH % of maximum fiber digestion24 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 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

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 20:80

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 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 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

» Recent theory

Reduced pH increases production of trans fatty acids from polyunsaturated fatty acids

Trans fatty acids inhibits 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

eNDF Theoretical 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:• 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 microorganisms

Dilutes VFAs

Increases liquid turnover » Motility

• Long-term health problems– 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

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

NDF 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 ADF

NDF ADF CF rFeed volume .78 .62 .71

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

rChewing 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

0 10 20 30 40 Week of lactation

Body weight, lb

Milk production, lb/day

Feed intake, lb/day

• 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, % 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• 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 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

» 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

Use of buffers in ruminant diets• 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