2. Topics
• What is proper rumen function?
• Regulation of ruminal pH
• Consequences of low ruminal pH
• Additives: buffers, antibiotics, etc.
• More than pH: osmolality
• Recommendations
3. Proper rumen function
• Rumination
– Particle size reduction
– Maintain pH: salivation
• Motility
– Maintain pH: absorption of fermentation acids
– Mixing
• Passage: Release of small digested particles
• Inoculation of newly consumed feed
• Maintenance of epithelial integrity
– pH
– Osmolality
4. Eating, ruminating, and ruminal pH
7.0
6.8
6.6
6.4
6.2
Ruminal
pH
pH
chewing
Allen, 1997, J. Dairy Sci. 80:1447
30
20
10
0
Feed
Remaining
(kg)
eating
6
4
2
0 8 10
Time after feeding (h)
6. Removal of H+ from the rumen
Route of removal eq/d %
Absorbed as VFA 39.2 55
Incorporated into H2O 20.1 29
Flow from the rumen as:
H2PO4
- 6.6 10
VFA 2.3 3
NH4
+ 1.5 2
Particulate matter 1.0 1.4
Free H+ neg neg
Total 71
Allen, 1997 J. Dairy Sci. 80:1447
7. Variation in ruminal pH explained by diet
characteristics
Allen, 1997 J. Dairy Sci. 80:1447
Percent of total variation explained
8. Effective fiber and salivary buffer flow
Allen, 1997, J. Dairy Sci. 80: 1447
Fine, 20% FNDF Coarse, 24% FNDF
Total chewing, min/d 462 659
Ruminating, min/d 288 402
Eating, min/d 174 257
Saliva flow, L/d 256 271
Buffer flow, eq/d 38.9 41.2
Diet effects:
Cow effects: unknown
Effective fiber has additional effects by direct buffering of rumen
mass and stimulating motility and VFA absorption.
9. Buffer systems in the rumen
• Saliva
– Bicarbonate, pKa = 6.1, effective pKa~ 7.0
H+ + HCO3
- <-> H2CO3 <-> H2O + CO2 <-> CO2 (gas)
– Hydrogen phosphate, pKa = 7.2
H+ + HPO4
-2 <-> H2PO4
-
• VFA, pKa = 4.7 - 4.8
• lactate pKa = 3.8
• Digesta: BC of feedstuffs extremely variable (Jasaitis et al., 1987)
– legume forages, high protein feeds > grass forages, low protein feeds > cereal
grains
– Most buffering is from pH 4 to 6 (Wohlt et al., 1987)
4 5 6 7
pH
RUMINAL DIGESTA
10. Effects of supplemental buffers
Erdman, 1988 J. Dairy Sci. 71:3246
• Studies show variable effectiveness at increasing ruminal
pH, fiber digestibility, milk fat yield, and DMI
• More effective on low fiber diets
• Dependent upon forage type (e.g. corn silage > alfalfa)
14. Rumen motility decreased
• Cause
– Low rumen fill
– Hypertonic fluids at abomasum
– Butyric acid
– Stimulation of CCK by dietary fat
• Effects
– Decreased absorption of VFA
– Increased tonicity of rumen fluid
– Decreased passage of VFA, water
– Increased liquid pool size in rumen
– Systemic dehydration
15. Low pH decreases rate of fiber digestion
Oba and Allen 2003 J Dairy Sci 86:184
Closed: 32% starch
Open: 21% starch
Circle: HMC
Triangle: DRY
R = 0.58, P < 0.001
17. SARA vs. acute acidosis
• Different
– SARA = chronic condition
– Acute acidosis = difficult recovery
• pH nadir
– SARA > 4.9-5.0
– Acute < 4.9
• Lactic acid
– Acute: accumulates in rumen fluid
– Rare in dairy cattle: transient
18. Lactic acid accumulation in the rumen: main players
• S. bovis: starch fermenter, can grow rapidly, tolerates pH 5, Gram +,
sensitive to virginiamycin
– Normal growth: produces acetate, ethanol, and formate
– Rapid growth or low pH: produces lactic acid
• M. elsdenii: lactic acid utilizer, grows more slowly, tolerates pH 5.2
– Produces acetate and butyrate
• Lactobaccilli: ferment sugars, starch, Gram +, sensitive to virginiamycin,
tolerates pH << 5.0
– Produces L-lactic acid and D-lactic acid
Russell, J. B. and M. S. Allen (1983)
19. Ruminal pH and individual VFA concentration
Data from Oba and Allen, 2003 J. Dairy Sci. 86:195
20. 0
5
10
15
20
25
4.5 5 5.5 6 6.5 7 7.5
Concentration,
mM
Rumen pH
Data from Oba and Allen, 2003 J. Dairy Sci. 86:195
Relationship between ruminal pH and lactate
concentration
21. Feed consumption (kg as fed)
Oba and Allen, 2003. J. Dairy
Sci. 86:174-183
Diet:
32% starch, 23% NDF
High-moisture corn
Corn silage and alfalfa silage
Lactic acid is transient:
Rapidly metabolized at pH
~5.2 and above
22. Effects of feed additives on rumen function
• 8 heifers per treatment fed 2.5% of BW/d DMI of a TMR (62:38
F:C) and feed additives for 20-d
• Treatments
– Control
– Virginiamycin, 10 g/d
– Monensin, 2.2 g/d + Tylosin, 0.44 g/d
– Monensin, 2.5 g/d + live yeast (S. cerevisiae), 25 g/d
– Sodium bicarbonate, 200 g/g + Magnesium oxide, 30 g/d
• Challenge on d 21: wheat @ 1% of BW and fructose @ 0.1% of BW
• No effect of treatment on ruminal pH or concentrations of total
VFA, propionate, L-lactate, D-lactate or total lactate.
Golder et al., 2014. J. Dairy Sci. 97 :985–1004
23. Survey of ruminal acidosis in Australia
• 8 cows (3 primiparous & 5 multiparous) from each of 100 dairy
herds in NSW and VIC
• Ruminal pH determined by ruminocentesis 2-6 h after
milking/feeding, rumen fluid collected by stomach tube for VFA
concentrations
• Herds were separated into 3 groups by cluster analysis based on
ruminal pH, VFA, lactic acid, and ammonia concentrations:
– “Acidotic” (10%): low rumen pH, higher total [VFA]
100% monensin (252 mg), 50% virginiamycin (157 mg)
– “Sub-optimal” (30%): higher rumen pH, higher total [VFA]
66% monensin (258 mg), 29% virginiamycin (187 mg)
– “Normal” (60%): higher rumen pH, lower total [VFA]
61% monensin (240 mg), 7% lasalocid (265 mg), 25% virginiamycin (157 mg), 4% tylosin (120 mg)
Bramley et al., 2008. J. Dairy Sci. 91:308–321
24. Survey of ruminal acidosis in Australia (cont.)
Bramley et al., 2008. J. Dairy Sci. 91:308–321
Results “Acidotic” “Sub-optimal” “Normal”
NDF, % 30.4c 35.7b 36.1a
NFC, % 40.3a 33.7b 33.5b
Starch 22.7a 16.8b 16.9b
DMI, kg/d 19.4 18.7 19.2
Milk (L/d) 30.0a 27.9b 29.7a
Milk fat (kg/d) 1.00 1.04 1.05
Milk protein (kg/d) 0.94 0.87 0.91
Ruminal pH 5.7c 6.2b 6.3a
Total VFA, mM 101a 95a 63b
D-lactate, mM 0.34a 0.28b 0.12c
What is the evidence that lactic acid is a problem in dairy herds?
25. Importance of rumen fill
• Reduce risk of DA’s
• Maintain rumen pH
– Direct buffering
– Stimulate rumen movement, acid absorption
– Stimulate chewing, salivary buffer secretion
26. Epithelial damage: rumen ulcers
• Cause
– Low intracellular pH of ruminal epithelia, cell death
• Affected by both high VFA concentration and low
ruminal pH (Plaisier et al., 2022 J. Dairy Sci. 105:7141-7160)
– Rapid change in ruminal osmolality
• Effects
– Colonization by Fusobacterium necrophorum
– Access to blood
• Liver abcesses, lung abcesses
• Endotoxin absorption
• Histamine absorption
27. Rapidly fermented starch:
rumen fluid becomes hypertonic
• Net water flow from rumen to blood
– Keratinized layer: barrier to water flow
• Water flow from blood to rumen
– Dehydration
– No barrier: cell disruption?
Mooney and Allen, unpublished
rumen plasma n=2131
n=2217
H2O
H2O
29. Summary
• Concentration gradient is an important factor affecting rate of VFA
absorption and rumen pH
• Rumen motility increases the concentration gradient
• Rumen motility is increased by fibrous mass and inhibited by
butyric acid and increased osmolality
• Rumenitis/ulcers likely from rapid increase in ruminal
fermentation, increased VFA concentration AND lower pH
• Major risk factors for acidosis include:
– Low ruminal digesta mass (buffer capacity)
– Highly fermentable ration, feeding grain separately
30.
31. Does high potassium reduce saliva flow? We don’t know, but:
• Potassium enters the rumen via feeds
• Sodium enters the rumen primarily via saliva
• Saliva composition is relatively constant
• Grazed pastures are high in potassium
• Increased osmolality reduces rumination
• Electroneutrality is maintained in the rumen
31
Mooney and Allen, unpublished
1 cow sampled every 20 min for 24 h (n = 72)
Na+ = 140.1 – 1.079 K+, P < 0.0001
8 cows, 4 diets, sampled every 20 min for
24 h (n = 2,304)
Editor's Notes
Russell, J. B. and M. S. Allen (1983). Physiological basis for interactions among rumen bacteria: Streptococcus bovis and Megasphaera elsdenii as a model. Current Perspectives in Microbial Ecology. American Society of Microbiology.
