Dairy Cows are Different
Dairy cows are unique among farm animals in the amount of acid produced in the digestive tract and found in commonly consumed feeds. Each time dairy cows approach the feed bunk, they expose themselves to acid from several sources. This acid can be present in the feed when consumed, or can be generated from the feed during digestion. For example, corn silage ferments during storage, yielding a pH below 4. Its acid content is very high. Therefore, consumption of silage by the cow causes rumen pH to immediately drop. Moreover, in addition to the natural acid content of silage, more acid is produced when nutrients, such as starch and cellulose in the silage, are broken down into volatile fatty acid end products.
   How much acid does a dairy cow consume? A cow consuming 22 lb of corn silage (dry matter basis) with a pH of 4, would ingest approximately 1 mg of hydrogen ion (think of this as acid). Although 1 mg may not appear to be much acid, if it was mixed with 40 liters of water (approximate blood volume of a cow), the pH of the solution would be 4.6. This value is well below the normal blood pH of 7.4 (Rebhun, 1995) and is incompatible with life. Clearly, the natural acid content of some feeds represents a significant acid challenge to the cow.
   Despite the importance of the acid content of feed during consumption, the release of acids from that same feed during digestion is even more substantial. Van Soest (1994) summarized data indicating that almost 1.8 kg of acetate and from 1 kg to 2.2 kg of propionic acid are absorbed daily from the dairy cow's rumen. For perspective, to consume 1.8 kg of acetic acid, a cow would need to drink 36 liters of typical fruit vinegar. The acid load presented to the dairy cow via the diet, through inherent acid content as well as digestion-related acid production, is large enough to cause significant health problems.

Result of Acid Load
As acid levels derived from feed and acidic end products of digestion increase in the rumen, the pH of rumen contents decreases. Acid buildup, if severe enough, can cause several problems. As pH falls below 6.0, fiber-digesting microbes become less and less active, thereby reducing the extent of fiber digestion. In addi-tion, sustained low pH can actually erode the rumen lining, causing it to become more permissive to the passage of bacteria from the rumen to the blood stream. This, in turn, makes the cow more susceptible to liver abscesses. Histamine production also may be increased in response to bacterial invasion, resulting in dilation of blood vessels in peripheral tissue such as the hoof. This increases the likelihood that laminitis may develop.

Natural Defense Mechanisms
Rumenal Defenses
Because acid loads can negatively affect cow health, the cow possesses an intricate system of defense mechanisms to handle these challenges. This protection begins in the rumen, where numerous chemical buffers exist which can react immediately with acid as it enters or is produced in the rumen. Compounds, such as bicarbonates and phosphates, provide a large reservoir of buffering capacity, i.e., the ability to resist change in pH. The hydrogen ion from acid can be absorbed by these chemical buffers very rapidly. During this absorption or neutralizing process, the hydrogen ion becomes bound with a harmless compound (typically, water), eliminating its negative effects on the rumen.

   One characteristic of these chemical buffering systems is they tend to disappear after they have neutralized acid. End products produced during the neutralization often are released as gas or absorbed from the rumen. Consequently, these systems need to be replenished regularly to be effective in stabilizing rumen pH.

   The cow relies upon salivary production of chemical buffers to replenish the buffering capacity of the rumen. Although saliva production is low during initial consumption of feed, much saliva is produced and swallowed during cud-chewing. High-fiber diets that encourage cud-chewing also promote substantial production of saliva and chemical buffers, thereby providing greater protection to the rumen against acid challenges. If a cow receives adequate long-stemmed fiber (e.g., hay), even when consuming a high-grain diet, she is better equipped to neutralize the significant acid production likely to occur during digestion of grain.

Systemic Defenses
Although cows work hard to main-tain the rumen acid-base balance as constant as possible, the acid-base status of the blood is even more critical. Normal blood pH is approximately 7.4, and variation of more than 0.2 to 0.3 pH units can make the cow severely ill. Many functions in the body, including the action of enzymes and the flow of ions into and out of the cells, are very sensitive to changes in pH. Consequently, the cow possesses a series of defense mechanisms to maintain acid-base status.

   When acid enters the blood, a chemical buffering system similar to that in the rumen immediately reacts with the acid to neutralize it. In add-ition, the cow increases respiration rate and, in essence, blows acid out of the body through the lungs (in the form of carbon dioxide). If these processes fail to completely neutralize the acid, the cow's kidneys begin to excrete acid. Finally, for constant, long-term acid challenges, such as those observed for high-grain diets, the cow will begin to dissolve carbonate and phosphate buffers stored in the bones. These buffers are released into the blood, where they enhance the existing chemical buffer system. All of these systems are carefully coordinated to maximize the ability of the cow to maintain a constant acid-base status.

Feeding to Prevent Acid Overload
Many feeding strategies exist to reduce the likelihood of dairy cows becoming acidotic. As mentioned previously, maintaining adequate fiber content and fiber length in the ration will encourage cud-chewing and the release of buffers in saliva. Nutritionists recommend feeding a minimum of five lb of fiber greater than 1¹/2 inch in length per cow daily. Feeding smaller meals more frequently through-out the day will lessen the rate of digestion-related acid release in the rumen. Including feed ingredients in the ration, such as alfalfa hay which appears to possess a natural buffering capacity, will enhance the chemical buffering system in the rumen. And finally, the chemical buffering system of the rumen may be strengthened directly through the addition of buffers to the ration.

   Numerous compounds have been tried, evaluated, and sold as rumen buffers for dairy cows. Some, like calcium carbonate and dolomitic limestone, actually have very little buffering activity within the range of pH encountered in the rumen. Others, like magnesium oxide, are not buffers at all but rather acid neutralizers. The difference is that buffers act to maintain a constant pH, whereas acid neutralizers can only elevate the pH as they absorb acid. Both effects typically are beneficial to the dairy cow, but the processes are different.

   Some raw, directly mined minerals, such as trona, have natural buffering capacity due to their sodium sesquicarbonate content, and are being sold with very little additional refinement. But the primary feed additive buffer on the market is chemically synthesized sodium bicarbonate. This buffer is most stable and active at a pH of approximately 6.1; this pH also happens to support vigorous fiber digestion in the rumen.

   Because various types of stress can cause fluctuation in the rumen environment, and because buffers help stabilize the rumen, buffers should be added to the ration whenever dairy cows are facing unusual or stressful conditions. Heat stress, stress from high milk yield, calving stress and nutritional stress from poor-quality or inconsistent feed ingredients, as well as from high-grain diets, pose stressful conditions. Recommendations for inclusion of buffers in the ration depend on type of buffer and the ration. A standard rule of thumb for sodium bicarbonate inclusion is 0.75% of the total diet dry matter or 30 lb per ton of grain mix. Please note that cows should not be fed buffers at any time during the dry period, due to negative effects on calcium metabolism.

Summary
Any internal or external stressor can cause fluctuations in the rumen environment. The bacteria in the rumen that digest feed thrive upon both a stable environment, and an environment with a pH that is compatible with maximum microbial growth and activity. Good dairy management can control some undesirable rumen variation simply by keeping cows as comfortable and stress-free as possible. However, producing milk is hard work, and will always involve some degree of stress. Producing large quantities of milk by processing substantial amounts of grain daily causes even greater stress. Good nutrition management includes the addition of feeds to the ration to enhance a cow's ability to produce buffers, as well as the addition of buffers directly to the feed. The combination of these buffer sources enables a cow to remain healthy while digesting large quantities of feed and producing substantial amounts of milk.