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1007 PD: Buffers and their pH systems – the rumen

Carl Old Published on 27 September 2007

In almost all biological systems, it is important that pH not deviate much from a fixed value. For example, for blood to carry oxygen from the lungs to tissue, pH must be maintained very close to 7.4. When rumen pH is either too high or too low, microbial fermentation and absorption of end products of that fermentation are less than optimal. Buffers, and other compounds, are added to rations for ruminant animals to aid in maintaining both blood and rumen pH in the desired ranges.

What is pH?

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Maintenance of blood pH, in terms of animal survival, is extremely important. Supplying oxygen to tissues and temperature regulation are the only functions that take precedence over maintenance of proper acid-base balance. While it is extremely important to recognize this fact, the first part of this article will focus on the role of buffers in the digestive tract.

The term pH is commonly used to describe the acidity or alkalinity of solutions. In regards to this discussion, the item of interest, however, is not pH, but what it represents, which is the concentration of hydrogen ions. Use of the term pH to describe acidity may be slightly confusing as it is not a numeric scale but a logarithmic scale. A change in rumen pH from 6.0 to 6.5 may appear to be slight, only 8 percent, but represents a 316 percent reduction in hydrogen ion (acid) concentration.

What are buffers?

Buffers are defined as compounds that resist change in the pH of a system. While rumen pH can vary dramatically, the normal range may be considered to be from 5.7 to 6.7. In this range, bicarbonate is the primary buffering system in the rumen, although there are other minor buffers as well.

Bicarbonate, as a buffer, is most effective at a pH of 6.37 and is effective at a range of from 4.67 to 8.07. Commonly used compounds (such as sodium sesquicarbonate, potassium carbonate, sodium carbonate, magnesium oxide, calcium and magnesium carbonates) are more properly termed alkalizing agents based on their mode of action. Practically speaking however, this distinction only applies to magnesium oxide as all other compounds mentioned add to the rumen bicarbonate pool.

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A byproduct: Volatile fatty acids

Volatile fatty acids formed during rumen fermentation are waste products produced by bacteria. Rumen fermentation is an anaerobic process and, as a result, conversion of carbohydrates in feed to microbial cells is greatly exceeded by the amount of these various waste products. When these waste products are absorbed and utilized by the host animal, the amount of energy to provide for cell maintenance and growth greatly exceeds that available to rumen bacteria. Most common among the volatile fatty acids produced during fermentation are acetic acid, propionic acid and butyric acid. It has been estimated that if the rumen were not buffered, the pH may drop to approximately 3.0.

Dissociation constants vary for common volatile fatty acids. This means that not all acids produced during rumen fermentation produce the same level of acidity. If propionic acid has a relative rank of 1.0, then butyric acid and acetic acid are 1.09 and 1.30, respectively. Lactic acid, found in silage and produced in relatively large quantities when animals are not well adapted to high-grain rations, is much more acidic than the volatile fatty acids. Lactic acid is 10.3 times more acidic than propionic acid, which can lead to problems when animals consume large amounts of silage. None of these organic acids can compare to hydrochloric acid, which is more than 70,000 times as strong an acid as propionate.

Neutralizing acid

Buffers also vary in their ability to neutralize or completely consume acid. Based solely on chemistry, one can rank buffers on a scale of from one to 10, 10 being the best. Table 1 shows a comparison of theoretical acid-consuming capacity and measured acid-consuming capacity. It should be noted that while magnesium and calcium compounds rank higher than sodium and potassium compounds, there is much more variability in quality for the former. Some calcium and magnesium buffers and alkalizing agents are relatively poor acid consumers, while others are quite good. Generally speaking, these are unrefined products and can vary based on the particular deposit from which they are mined. Potassium and sodium buffers and alkalizing agents are usually refined products and, as such, are more consistent in performance. Unrefined trona ore, predominantly sodium sesquicarbonate, tends to be less variable in performance than mined calcium or magnesium products. In general, products should be chosen based on consistency of measured results.

Quantities of buffers added to rations depends on a number of factors: rate and extent of rumen carbohydrate fermentation, quality and quantity of fermented feeds (such as corn silage) and passage rate are some of the most important. It is possible to calculate the amount of buffering required if ration composition and kinetics of rumen degradation are known.

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Plant cell walls and starch are carbohydrates varying dramatically in rate and extent of rumen degradation. If one assumes rumen losses of plant cell walls are 40 percent, then for a cow consuming 50 pounds of dry matter (DM) with 28 percent plant cell walls, theoretical production of acetic acid, propionic acid and butyric acid from cell wall fermentation are 2.0, .90 and .80 pounds, respectively. Bacterial waste, as volatile fatty acids, are 3.7 pounds and .65 pounds of microbial cells are produced from 14 pounds of cell walls. If the same ration contained 35 percent starch and that starch was 90 percent fermented in the rumen, theoretical production from that portion of the feed yields 10.5 pounds of volatile fatty acid and about 2.0 pounds of microbial cells.

Rations higher in fiber require less acid neutralization partly because of higher salivary secretions and lower rates of acid production. Feed fiber, especially that found in legumes, can remove acid much in the same way a water softener removes calcium from water (ion exchange). Total ion exchange capacity of most rations is limited; the equivalent of a fraction of an ounce of sodium bicarbonate. Amounts of buffers added to the ration can be calculated based on ruminal acid production, salivary bicarbonate production and feed pH. Excessive acid neutralization can be as deleterious as insufficient buffering, as dissociated volatile fatty acids are not absorbed as well as undissociated volatile fatty acids. When rumen pH rises too high, absorption of volatile fatty acids across the rumen wall ceases, as will rumen fermentation. At a pH of 6.0, approximately 95 percent of acetic acid is dissociated, as are 93 percent of both propionic and butyric acids. It is interesting to note that volatile fatty acid absorption across the rumen wall is more rapid shortly after a meal, before salivary secretion increases.

Since estimates regarding production of volatile fatty acids and microbial cells have been made, a brief (unrelated to buffers) yet important discussion follows. Plant cell walls are important in overall rumen function; however, the role of rumen fermentable starch cannot be overemphasized. As can be seen from the previous example, the contribution of starch fermentation to microbial cell growth is much greater than plant cell wall fermentation. At amounts that might be found in a typical dairy ration, starch has the potential to grow three times the amount of microbes and nearly five times the amount of propionic acid as plant cell walls. The implications of this, as regards milk production, are clear.

Regulating blood pH

While rumen pH can vary over a broad range, blood pH does not. Under conditions commonly found in the rumen, acid content, as measured by pH, can vary 10 fold. Blood acid content is highly regulated and varies by no more than 10 percent from the average. Normal blood pH is 7.4; animals are alkalotic when pH is greater than 7.45 and acidotic when pH is less than 7.35. Metabolism must be altered to correct either condition as blood pH outside the range of from 6.8 to 7.8 results in death.

Regulation of blood pH is not as simple as the situation in the rumen. Hydrogen ions (acid) in blood are positively charged and in order to maintain a zero charge, one of two events must occur. Introduction of acid (positively charged) must be accompanied by the addition of a negatively charged ion (anion) such as chloride or bicarbonate, or the loss of positively charged ions (cations), such as sodium or potassium. Potassium, sodium and chloride are classified as dietary fixed ions; they are quantitatively absorbed from the gut, are not metabolized and excesses are excreted in urine.

Combustion of feed indicates effects on acid-base balance; ash from cereal grains is acid, while that from forages is alkaline. Cattle are much more tolerant of alkalosis than acidosis and, as such, require a slight dietary excess of positively charged fixed ions. The magnitude of this excess is determined by a number of factors including metabolic state.

Growth is a state when animals are in a negative acid balance; while catabolic states, such as starvation, represent a positive acid state. Acid-base imbalance affects multiple metabolic processes; among these are impaired glucose metabolism and transport of compounds across cell membranes. Ultimately, under prolonged conditions of acid-base imbalance, animal health and efficiency are reduced.

Modern management practices increase energy density to improve production, primarily with increased intake of cereal grains. Until recently, no attention was paid to acid-base balance in cattle. It has been suggested that benefits resulting from the addition of buffers, such as sodium bicarbonate, relate as much to fixed ion addition (sodium) as to acid neutralization.

Sodium, potassium, chloride, phosphorus, sulfur, calcium and magnesium are commonly included in equations describing dietary acid-base status. Phosphorus, sulfur, calcium and magnesium may warrant inclusion occasionally, but these are typically added to rations to satisfy requirements. Unlike sodium, potassium and chloride, absorption of phosphorus, sulfur, calcium and magnesium is variable and often low. Sulfur is a constituent of several amino acids, and as such, metabolic state influences the contribution of sulfur to acid-base balance. Equations describing dietary fixed ion differences must be predictive of acid-base balance across all metabolic states. In addition, the simplest equation describing a system is to be used in preference to a more complex one that does not increase accuracy of prediction.

Summary

Regulation of acid-base balance in ruminants is a more complex system than that in non-ruminants. To meet the demands of high production, feeds are included in rations that can disrupt ruminal and metabolic processes. Buffers are added to rations to mitigate negative effects of acids produced during fermentation on rumen health and function. Additionally, buffers allow blood pH to remain in a range that maximizes performance and animal health. PD

References omitted but are available upon request at

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