Saliva and the Fiber Requirements
of Ruminants
Nutrient Requirements of Beef Cattle:Seventh Revised Edition:Update 2000. pp. 129130. 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 0:1000
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
• 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
Dietary protein
NPN
Protein
NH3
Microbial
protein
Metabolizable
protein
Urea
• Will be important on low protein diets
• An important consideration in minimizing N excretion
– 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
6
8
10
12
14
16
18
% N recycled
61
46
34
24
17
12
10
» Marini et al. (2003)
Holstein heifers fed a corn meal-molasses- citrus pulp diet
fed at 1.8 x maintenance
% CP in diet
9.1
11.8
15.7
18.6
% N recycled
30
37
25
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 pH
Converts NH3 to NH4+ in the rumen
Only NH3 can cross the rumen wall
• Marini et al. (2003)
% CP
N recycled (saliva)
g/d
% of total
9.1
0.8
3.0
11.8
1.5
3.6
15.7
3.8
10.4
18.6
5.4
13.7
N recycled (Gut wall)
g/d
% of total
25.1
97.0
39.6
96.4
32.7
89.6
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
Parotid
Inferior molar
Palatine and Buccal
Submaxillary
HCO395
134
109
6
HPO4-2
75
48
25
54
Cl13
10
25
6
Na+
186
175
179
15
K+
5
9
4
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
Resting
Eating
Ruminating
% of saliva flow
36
27
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
Dairy cubes
Fresh grass
Silage
Dried grass
Hay
Saliva secretion (gm/gm feed consumed)
.68
.94
1.13
3.25
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 range
HPO4-2 (second H+)
7.1
6-7
HCO3- (first H+)
6.4
5.5-7
Acetate
4.8
Propionate
4.9
5-6
Butyrate
4.8
Lactate
3.9
Glutamate
5.6
Aspartate
5.2
5-6
Alfalfa protein isoelectric point
5.5
NH3
9.3
Cation 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
9
8
7
pH
6
5
4
40
20
1N KOH added
0
20
40
60
80
1N HCl added
100
120
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
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
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
Milk or body weight
Synthesis, kcal /
40
100 Kcal ME
above maintenance
Body tissue
10
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
» 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
24
20
16
12
8
4
Theoretical maximum microbial synthesis, g/g CHO fermented
.4
.4
.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
– 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
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
r
.62
CF
.71
• NDF is more highly related to chewing time than CF or ADF
NDF
Chewing time .86
ADF
r
.73
CF
.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
Week of lactation
30
40
• 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 Maximum NFC, % DM
25
44
27
42
29
40
31
38
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 used
Buffer
Sodium bicarbonate
Additional effects
-
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