Variation in the Susceptibility to Ruminal Acidosis G.B. Penner Department of Animal and Poultry Science University of Saskatchewan Email: greg.penner@usask.ca

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

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

How much variation do we see?

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

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

Is the Response to acidosis consistent? Salivary buffer Passage Absorption

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

Resistant vs. susceptible heifers Penner et al., unpublished

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 80-90 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

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

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.

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 = (8.9+3.1+2.1+ 1.4) Other = 3.6 Allen, 1997

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

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

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

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

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

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

Mechanisms of SCFA absorption

Mechanisms: protein mediated b b Penner et al., 2009

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

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

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

Variation in ruminal pH Penner et al., 2009

SCFA absorption explains variation in ruminal pH

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

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

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

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

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