Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/4/2000
Publication Date: N/A
Citation: Interpretive Summary: Feeding of diets rich in concentrates to dairy cattle increases the acidity of the rumen, which decreases the ability of rumen bacteria to digest cellulose and other types of fiber in the diet. However, the quantitative relationship between rumen pH and fiber digestion has not been established. We removed bacteria from the rumen and measured the rate at which cellulose ewas degraded as rumen pH decreased (i.e., as acidity increased). We found that the rate of cellulose digestion is not directly related to pH, but instead is highly dependent on the pH at which the fermentation was started. Rates of cellulose digestion were unexpectedly high, below pH 6.0 (the minimum pH for growth of cellulose-digesting bacteria), as long as the fermentations were begun at pH values above 6.0. By providing quantitative data relating fiber digestion to pH, this work will allow researchers to develop more mathematically accurate models of ruminant digestive metabolism. In addition, our work suggests that producers may be able to increase fiber digestion, and thus milk production, in dairy cattle by a slight, temporary increase in rumen pH at the time of feeding (for example, be adding alkaline buffers to the diet).
Technical Abstract: In vitro fermentations of pure microcrystalline cellulose by mixed ruminal microorganisms were conducted at 39 oC under conditions where pH declined within ranges similar to those observed in the rumen. A ruminal inoculum concentration of 15% (vol/vol) was sufficient to establish digestion of 12.5 g cellulose/l at first-order rate constants (k) of up to 0.11 h**1. Values for k were successively lower at initial pH values of 6.86, 6.56, and 6.02, but in each case the value of k was maintained over a pH range of 0.3 to 1.2 units, as the fermentation progressed. Plots of k versus initial pH were linear, and k displayed a relative decrease of approximately 7% per 0.1 unit decrease in pH. At high cellulose concentration (50 g/l) and an initial pH of 6.8, cellulose digestion was initially zero-order, the absolute rate of digestion declined with pH, and digestion essentially ceased at pH 5.3 after only 30% of the added cellulose was digested; further incubation resulted in a loss of bound N and P, suggesting that at low pH cells lysed or detached from the undigested fibers. Adherence of mixed ruminal microorganisms prior to the start of fermentation was lower at pH 5.5 than at pH values greater than 6.0. Pure cultures of ruminal cellulolytic bacteria also were able to ferment cellulose to a minimum pH of 5.1 to 5.3, and the extent of fermentation was increased by coculture with noncellulolytic bacteria.