1. Variation in the Susceptibility to Ruminal Acidosis
G.B. Penner
Department of Animal and Poultry Science
University of Saskatchewan

2. Prevalence rates of SARA in dairy cows
12-30% prevalence across lactation
Diagnosed using ruminocentesis
n = 662
There is a limited amount of data reporting the prevalence rates for ruminal acidosis. Based on a survey conducted by Dr. Gary Oetzel, they reported that based on a pH threshold of 5.6, the prevelance rates for ruminal acidosis ranged between 12 and 30%. That study used ruminocentesis with samples collected between 6 and 10 h post-feeding with a total of 662 cows representing 55 herds. To my knowledge, this is the largest study conducted to evaluate prevalence rates but likely underestimates the true prevalence of this disorder. Furthermore, the use of ruminocentesis does not provide any information on the severity of the acidotic episode.
Krause and Oetzel, 2006

3. Consequences of ruminal acidosis
Healthy rumen conditions
Increased VFA
pH 6.0
Changes in rumen bacteria
Decreased fibre digestibility
SARA
pH 5.5
Epithelial damage
Ruminal acidosis is classified as either sub-acute or acute acidosis; however, it should be understood that the negative effects of SARA are based on the extent of the pH depression, the rate at which pH decreases, and the time that low pH is sustained. Generally, increased acid production corresponds to increased VFA concentration and if severe enough, the accumulation of lactic acid. The depression in pH causes changes in the rumen microbial population as well as the activity. This leads to decreased fibre digestibility. If pH decreases low enough, epithelial damage may occur. This predisposes cows to infection and laminitis as bacteria can cross the damaged epithelial barrier.
Increased lactic acid
pH 5.0
Acute ruminal acidosis

4. How much variation do we see?

5. Variation in the Severity of Ruminal Acidosis
Variation in the response of cows to production systems is common, our research has shown that this is also true for ruminal acidosis. In a study conducted at the Lethbridge Research Centre, we measured ruminal pH in 14 Holstein heifers during close-up dry period and into early lactation. What we found is that there was tremendous variation the severity of ruminal acidosis among these cows, even though they were fed a common diet. In fact, the minimum, and maximum time that pH was below 5.8 ranged between 0 h/d, to more than 22 h/d. This variation may be perceived as a challenge as some animals are clearly more susceptible to ruminal acidosis; however, this variation may also present an opportunity to identify animals that are more resistant.
Penner et al., 2007

6. Variation in ruminal acidosis around parturition
Fairfield et al., 2007

7. Is the Response to acidosis consistent?
Salivary buffer
Passage
Absorption

8. Severity is not entirely related to DMI
Penner et al., 2007

9. Resistant vs. susceptible heifers
Penner et al., unpublished

10. Subacute Ruminal Acidosis
Acid removal
Acid production
Neutralization
When talking about subacute ruminal acidosis we are really discussing ruminal pH and therefore the balance between acid production and acid removal. Anerobic fermentation of organic matter results in the production of short-chain fatty acids equating to a total of Mol/d and thus an equivalent amount of protons. Thus, there is a considerable amount of acid produced in the rumen and strategies to remove those acids are necessary. Acid removal includes the neutralization of acid with primarily bicarbonate, either from salivary secretions or secreted bicarbonate from the rumen epithelium. Other minor buffers include phosphate buffers and ammonia in addition to potential buffering capacity of feed. As such, subacute ruminal acidosis results when the rate of fermentation acid production exceeds the rate of acid removal.
Clearance

11. Acid Production in the Rumen
Many glucose molecules
ACID
2 acetate+ 2H+
2H+
1 Glucose pH
2 propionate
+ 2H+
2H+
2 Pyruvate
1 butyrate + 1H+
1H+
Acid production in the

12. Acid Removal from the Rumen
bicarbonate
blood
H-SCFA
SCFA-
Saliva pH
Acid
Figure XX. Main mechanisms for the removal of acid from ruminal contents: 1) saliva containing bicarbonate and phosphate buffers enters the rumen [neutralizes acid through the carbonic anhydrase reaction;step 4], 2) passive diffusion of undissociated SCFA across the rumen wall [protons may be removed with the absorption process], 3) absorption of dissociated SCFA in exchange for bicarbonate [supply of bicarbonate neutralizes acid; step 4], 4) carbonic anhydrase reaction [bicarbonate and a proton (acid) are converted to carbon dioxide and water], and 5) passage of acid out of the rumen.

13. Physiological Causes for Variation in the Susceptibility to Ruminal Acidosis
Salivary buffer
Passage
blood
Absorption
Passage
15.5%
Other
3.6%
Absorption
52.9%
Bicarbonate
28.0%
Absorption = 52.9
Bicarbonate = 28.0
Flow = ( )
Other = 3.6
Allen, 1997

14. Regulating ruminal pH Acid Production Salivary buffer Absorption
Passage
Acid Removal
Acid Production

15. Relationship between saliva production and ruminal pH
Duration pH <5.8 = 0.07 × saliva production – 4.7
P < 0.001
Bowman et al., 2003; Yang et al., 2003

16. Regulating ruminal pH Acid Production Salivary buffer Absorption
Passage
Acid Removal
Acid Production

17. Mechanisms: passive diffusion
Rumen pH
~5.0 to 7.0
Intracellular pH
~7.4
H-SCFA H+
Mechanisms involved in SCFA absorption and H+ movement across the ruminal epithelium. Note, model does not show the complexity of the ruminal epithelium including the individual strata and number of cells in each strata.
SCFA-
SCFA-
HCO3-
H2O + CO2
Carbonic anhydrase
Ruminal contents
Blood

18. SCFA absorption and H+ removal
Na+/K+ ATPase
2K+
3Na+ H+
Na+
Ketones-
Lactate- H+
H-SCFA
Metabolism H+
Mechanisms involved in SCFA absorption and H+ movement across the ruminal epithelium. Note, model does not show the complexity of the ruminal epithelium including the individual strata and number of cells in each strata.
SCFA-
SCFA-
SCFA-
HCO3-
Apical surface pH > 7.4
Leonhard-Marek et al., 2006
H2O + CO2
Ruminal contents
Blood

19. SCFA absorption and H+ removal
2K+
3Na+ H+
Na+
Ketones-
Lactate- H+
H-SCFA
Metabolism H+
Mechanisms involved in SCFA absorption and H+ movement across the ruminal epithelium. Note, model does not show the complexity of the ruminal epithelium including the individual strata and number of cells in each strata.
SCFA-
SCFA-
SCFA-
HCO3-
H2O + CO2
Ruminal contents
Blood

20. Mechanisms of SCFA absorption

21. Mechanisms: protein mediatedb b
Penner et al., 2009

22. Acetate vs. butyrate Protein mediated 40-72% Passive diffusion 31-72%
28 – 60%
Protein mediated
28-69%
Penner et al., 2009

23. SCFA absorption and H+ removal
Aschenbach et al., 2010
Na+ H+
2K+
3Na+ H+ H+
Na+
Ketones-
Lactate- H+
H-SCFA
Metabolism H+
Mechanisms involved in SCFA absorption and H+ movement across the ruminal epithelium. Note, model does not show the complexity of the ruminal epithelium including the individual strata and number of cells in each strata.
HCO3-
SCFA-
SCFA-
SCFA-
HCO3-
HCO3-
HCO3-
Na+
H2O + CO2
Ruminal contents
Blood

24. SCFA absorption and ruminal pH
Appearance of bicarbonate in luminal buffer (Ash and Dobson, 1963; Gäbel et al., 1991)
Dijkstra et al., 1993

25. Variation in ruminal pH
Penner et al., 2009

26. SCFA absorption explains variation in ruminal pH

27. Mechanism of acetate uptake and ruminal pH
Penner et al., 2009

28. Mechanism of butyrate uptake and ruminal pH
Non-responders also had higher serum butyrate
Penner et al., 2009

29. SCFA absorption and ruminal pH
Resende Júnior et al., 2006
r2 = 0.43
P <0.001
r2 = 0.45
P <0.001
r2 = 0.17
P <0.001
Gene
Pathway R
P value
Acyl-CoA synthetase
Activation of SCFA
-0.92
0.01
HMGCL
Ketogenesis
-0.75
0.08
LDHb
Lactate metabolism
-0.84
0.03 PC
Pyruvate metabolism
0.88
0.02
Penner et al., 2009

30. Take home messages
Considerable variation among cows for the susceptibility to ruminal acidosis
Variation in susceptibility is related to differences in the total absorption and pathway of absorption for SCFA
Future research is required to determine the molecular regulation for acidosis susceptibility

31. Thank You!