This document summarizes rumen function and factors that can lead to rumen dysfunction. Proper rumen function requires rumination to break down feed particles and maintain pH, as well as motility to mix contents and absorb fermentation acids. A key regulator of rumen pH is the balance between production and absorption/passage of fermentation acids. Low rumen pH can decrease fiber digestion and passage rates. Risk factors for low pH include low rumen fill, highly fermentable rations, and feeding grains separately from forages. Low pH can damage the rumen epithelium and increase risks of acidosis.

1. Rumen Function (and Dysfunction)
Mike Allen
Department of Animal Science

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)

5. Fermentation acids: production vs. removal
FOM intake
HA
H+
Passed as:
H2PO4
VFA
Digesta
NH4
+
Absorbed as VFA
H2O

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)

11. Buffering capacity of forages (meq/kg DM)
Forage Initial pH pH 4 to 6
Fresh Italian ryegrass 6.03 386
Perennial ryegrass 6.01 388
Fresh alfalfa 6.10 487
Alfalfa silage 4.74 705
Whole corn plant 5.20 191
Corn silage 3.90 397
Erdman, 1988 J. Dairy Sci. 71:3246

12. Concentration gradient
Blood Rumen epithelium Rumen
Motility
Digesta consistency
Metabolism
Butyrate >
Propionate >
acetate
Flow

13. Effects of increased acid load
• Decreased DM intake (propionate)
• Decreased rumen contents
• Decreased motility and mixing
• Decreased buffering
– Rumen contents
– Chewing, saliva flow

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

16. Relationship between milk fat % and ruminal pH
5.0
5.5
6.0
6.5
7.0
2.0 2.5 3.0 3.5 4.0 4.5
Milk Fat %
pH
Allen, 1997 J. Dairy Sci. 80:1447
16

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

28. Recommendations
• Limit rapidly fermented starch sources
• Prevent slug feeding of starch
• Keep the rumen full

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

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)