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United States Department of Agriculture

Agricultural Research Service


item Sauvant, D
item Mertens, David

Submitted to: Institut Natinal de la Recherche Agronomique Productions Animales
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: A database containing more than 400 observations from over 80 experiments published in research journals was used to determine the dietary characteristics that promote optimal pH or acidity in the rumen (stomach) of cows. The health and optimal digestive function of cows depends upon having ruminal pH that are slightly acidic. As the production of cows increase they need to be fed more grain to obtain the energy needed to mak milk. However, grains in the diet result in more acid being produced in the rumen when the feeds are digested by microbial fermentation. In addition, cows chew less when they eat grains compared to when they eat hay and produce less saliva that helps to neutralize the acid that is produced in their rumens. We observed that it is desirable for cows to maintain a ruminal pH that is higher than 6.25 (neutral = 7.0). Using the database, we were able to develop recommendations for dietary characteristics that will help cows with moderate levels of milk production to maintain a desirable pH. It is important that the ration contain at least 35% fiber and that 40% of the particles in the diet be .75 inches long to stimulate adequate chewing by the cow. The ration should not contain more that 45% grain or 25% starch to ensure that too much acid is not produced by the fermentation of the feed in the rumen. High producing cows may need rations containing more grain and less fiber to meet their energy requirements. These cows may need to be fed buffers (such as bicarbonate) to maintain adequate ruminal pH. Research indicates, in addition to nutrient concentrations, dairy cow diets need to meet recommendations that ensure that a desirable ruminal pH is maintained. Diet characteristics recommended can be used by farmers and nutritionists to ensure diets will promote optimal health and productivity.

Technical Abstract: Ruminal acidosis is a major concern in modern nutrition of ruminants because increases in production are associated with increases in the proportion of concentrate in diets. Feeding more concentrates has two ramifications: 1) concentrates have more fermentable organic matter than forages and results in more acid production in the rumen and 2) concentrates stimulate less chewing than forages resulting in a reduction of salivary flow which recycles phosphate and bicarbonate buffers. Ruminal acidosis can lead to several production disorders such as negative digestive interactions, reduced milk fat content, and digestive and metabolic pathologies. A database of published observations was summarized to determine the minimum average pH threshold for optimal ruminal function. The database was also used to develop equations for predicting ruminal pH based on dietary characteristics. These equations were used to define recommendations for dietary characteristics. Minimal dietary constraints that allow pH to be maintained about the threshold of 6.25 were: 35% total NDF, 25% forage NDF, or 40% of particles >2 mm in ration DM. Maximum constraints recommended are 45% concentrate, 25% starch, or 20% ruminally degraded starch. In addition the ration should stimulate >40 minutes of total chewing per kilogram of DM intake. High levels of intake induce a shorter transit time and a lower pH in the rumen. Thus, the mean pH is typically below the threshold of 6.25 when the level of DM intake is >2.5% of live weight. In conclusion, it is important to formulate rations not only to meet the classical recommendations for nutrient requirements but also to meet the characteristics that affect ruminal pH.

Last Modified: 07/25/2017
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