Is subacute ruminal acidosis a pH related problem? Causes and tools for its control

Abstract

Subacute ruminal acidosis (SARA) is a common digestive disorder in cattle fed high concentrate diets, and it is generally defined as a reduction in ruminal pH below 6.0 that results in lower fibre digestion, changes in rumen fermentation profile, irregular intake and reduced performance. However, the true cause of the observed effects in the rumen is unclear. Feeding high concentrate diets rich in non-structural carbohydrates and low in fibre results in a reduction in ruminal pH and the development of SARA. These two events (higher concentrate and low pH) occur at the same time and, therefore, are confounded. The question is if the effects reported are due to the reduction of pH or to the type of diet fed. The answer to this question is not only academic, because if the effects are pH dependent, then the terminology acidosis and the use of buffers and alkalizers are justified. However, if the effects are due to the type of diet, then buffers would have a limited effect and we should be looking at different types of solutions and terminology. Few experiments have addressed that objective, but the limited evidence available indicates that the effects typically attributed to SARA are due to a combination of pH and type of diet fermented. Although buffers and alkalizers help in the control of SARA, alternative strategies dealing with the control of the fermentation pathways should be investigated. Streptococcus bovis is considered the main lactic acid producer in the rumen and develops efficiently at relatively low pH. Megasphaera elsdenii and Selenomonas ruminantium are major lactic acid utilizers. Strategies to reduce lactic acid producers and (or) increase lactic acid utilizers may be helpful in the control of SARA in a way different from pH control. Feeding M. elsdenii to beef cattle reduced rumen lactic acid concentration and increased pH. Yeast and malic acid stimulate lactic acid uptake by S. ruminantium. Also, vaccination or oral supplementation of polyclonal antibodies against S. bovis reduced bacterial counts and lactic acid concentration, and increased ruminal pH. Evidence suggests that SARA is not only a pH-dependent pathology, but it is also the result of changes in the microbial population secondary to the type of diet fed. Therefore, we propose to re-name SARA as a " high-concentrate syndrome" , and suggest that a combination of pH modulating strategies and microbial population control are required to attempt to reduce its consequences. © 2011 Elsevier B.V.