Ruminal pH regulation and nutritional consequences of low pH

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Abstract

Volatile fatty acids (VFA) and lactic acid can build up in the rumen and reduce ruminal pH. Low ruminal pH for prolonged periods each day can affect feed intake, microbial metabolism and feed digestion, and has also been related to inflammation, diarrhea and milk fat depression. This paper considers aspects of pH regulation, as well as the effects of ruminal pH on rate of substrate degradation and on the profile of VFA available for absorption. Removal of VFA from the rumen by passage in the liquid phase and by absorption through the rumen wall are major processes that influence ruminal pH. The buffering capacity (BC) of rumen fluid is variable and is generally assumed to depend primarily on bicarbonate. Bicarbonate-dependent absorption is not just a primary absorption pathway of VFA but can also secrete bicarbonate at a capacity equal to that from saliva, thus removing protons from the rumen by neutralization. In addition, the inherent BC of the diet is involved in pH regulation, largely explained by the cation exchange capacity of feedstuffs. Empirical models to predict ruminal pH have had limited success. The inclusion of dietary characteristics in those models is needed to improve prediction accuracy. Representations of the effect of pH on fiber degradation adopted in models of ruminal function differ widely and include linear decline, saturation-type and sigmoidal relationships. In comparison with pH effects on degradation of fiber in sacco, most representations tend to overestimate the inhibiting effect of pH. Because the products of fiber hydrolysis are a major source of energy for microbial growth in the rumen, proper understanding and representation of fiber degradation at low pH is vital to predict microbial protein supply and VFA production satisfactorily. Variation in VFA profile is associated with variation in methane production, nutrient partitioning and milk composition. Various ruminal bacterial species have been observed to shift pathways in response to changes in pH while fermenting the same substrate. Mechanistic rumen models have adopted VFA stoichiometric coefficients related to type of substrate present in the feed or fermented in the rumen, but the majority of models do not include the effect of pH on VFA profile. In conclusion, ruminal pH is a major determinant of the profile of nutrients available for absorption. Shifting focus to factors other than salivary bicarbonate secretion will aid in better understanding ruminal pH regulation. Improved models to predict effects of ruminal pH on microbial metabolism and VFA profile will enable further optimization of dairy cow nutrition.

Introduction

Fermentation of feedstuffs in the reticulo-rumen (hereafter: rumen) produces volatile fatty acids (VFA) and lactic acid. These acids can accumulate and reduce ruminal pH if rumen buffering cannot keep pace with their accumulation (Plaizier et al., 2009). Low rumen pH for prolonged periods each day can negatively affect feed intake, microbial metabolism, and nutrient degradation, and low ruminal pH is related to inflammation, laminitis, diarrhea and milk fat depression (Stone, 2004, Krause and Oetzel, 2006, Enemark, 2008), although the extent of problems that occur and the precise mechanisms by which low ruminal pH increases these disorders have not been fully characterized. High yielding dairy cows fed energy dense rations rich in rapidly fermentable starch or sugars at high feed intake levels are particularly susceptible to acidosis, although goats, sheep and other ruminants are also prone to the disease (Braun et al., 1992). Subacute ruminal acidosis, which induces an elevation in rumen temperature (AlZahal et al., 2008), affects from 10% to 40% of dairy cattle in a herd, resulting in large financial losses and major concern for animal welfare reasons (Garrett et al., 1999, Kleen et al., 2009, Tajik et al., 2009).

Despite the impact of low ruminal pH on farm profitability and animal welfare, current empirical feed evaluation systems are unable to address the complex interrelationships in the rumen that result in acidosis following the ingestion of certain feeds. These systems aim to match nutrient requirements with nutrient intake at pre-defined production levels but are not suitable to predict the responses to dietary changes in terms of production level and product composition, digestion of nutrients and excretion of waste end-products to the environment, and nutrition related disorders. The change from a requirement to a response system to meet the needs of various stakeholders requires prediction of the profile of absorbed nutrients and its subsequent utilization for various purposes (Hanigan, 2005, Dijkstra et al., 2007, Kebreab et al., 2009). Mechanistic approaches to feed evaluation may address the relationships between host animal, feed and rumen micro-organisms, and demonstrate a greater capacity for describing ruminal fermentation processes that determine ruminal pH (Baldwin, 1995, Dijkstra et al., 2008). The development of these models to address prediction of ruminal pH requires concepts and data on the dynamics of nutrient degradation, microbial fermentation and acid removal from the rumen. Therefore, this contribution considers aspects of pH regulation, as well as effects of ruminal pH on rate of substrate degradation and on type of VFA formed.

Section snippets

Ruminal pH regulation

Fermentation of feed by microorganisms in the rumen produces VFA and sometimes lactic acid. Ruminal pH will drop when VFA or lactic acid accumulate in the rumen. To prevent this drop in pH related to production of acids in the rumen and acids ingested with the feed, the acids have to be removed from the rumen or buffered. Volatile fatty acids are removed from the rumen by passage in the liquid phase and by absorption through the rumen wall (Aschenbach et al., 2009). Acids accumulated in the

Ruminal pH and substrate degradation

The primary fibrolytic bacteria are intolerant to low ruminal pH, and rumen fiber digestion rapidly decreases when pH drops below critical values (Erdman, 1988, Russell and Wilson, 1996). In contrast, low pH values can facilitate the growth and activity of amylolytic bacteria (Mackie et al., 1978). High dietary levels of rapidly fermentable carbohydrates that results in high amounts of VFA produced and/or insufficient buffering of the rumen contents, both leading to low pH values, can thus

Ruminal pH and type of VFA produced

Volatile fatty acids produced through ruminal fermentation are of paramount importance, providing some 0.7 of the ruminant's energy supply (France and Dijkstra, 2005). The proportions among individual VFA are of particular interest because different VFA arise from variations in substrate intake and bacterial populations, and the individual VFA have distinct metabolic fates. In particular, propionic acid is a substrate for gluconeogenesis and is the main source of glucose in the ruminant.

Conclusion

Ruminal pH is a key determinant of the profile of nutrients available for absorption. Rate of clearance of VFA and secretion of buffers into the rumen have a major impact on ruminal pH, and more quantitative information is required on VFA absorption mechanisms. Reduction in ruminal pH is associated with decreased fiber degradation, decreased acetate to propionate ratio, and decreased CH4 formation. Quantitative understanding and description of the processes involved in pH regulation, and in the

Funding

JF and SL gratefully acknowledge receipt of collaborative grant no. SAB2010-0151 from the Spanish “Ministerio de Educación” under the program “Programa Nacional de Movilidad de Recursos Humanos del Plan Nacional de I-D+i 2008-2011 – subprograma EXTESP-EDU”. Dairy Farmers of Canada is thanked for financial support.

Conflict of interest

None.

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    This paper is part of the special issue entitled: Rumen Health: A 360̊ Analysis, Guest Edited by A. Van Vuuren, S. Calsamiglia and Editor for Animal Feed Science and Technology, P. Udén.

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