Initial pH as a Determinant of Cellulose Digestion Rate by Mixed Ruminal Microorganisms In Vitro

¹ Universidad Catolica Argentina, Buenos Aires, Argentina
² Department of Bacteriology, University of Wisconsin
³ Department of Bacteriology, University of Wisconsin and US Dairy Forage Research Center, USDA-ARS, Madison, WI 53706

In vitro fermentations of pure cellulose by mixed ruminal microorganisms were conducted under conditions in which pH declined within ranges similar to those observed in the rumen. At low cellulose concentrations (12.5 g/L), the first-order rate constants () of cellulose disappearance were successively lower at initial pH values of 6.86, 6.56, and 6.02, but in each case the value of was maintained over a pH range of 0.3 to 1.2 units, as the fermentation progressed. Plots of versus initial pH were linear, and 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. Pure cultures of ruminal cellulolytic bacteria also were able to ferment cellulose to a minimum pH of 5.1 to 5.3, but the extent of fermentation was increased by coculture with noncellulolytic bacteria. A model is proposed in which the first-order rate constant of cellulose digestion is determined by the pH at which the fermentation is initiated, and end product ratios reflect greater activity of the noncellulolytic population as pH declines.

Key Words: cellulose • digestion kinetics • microbial adhesion • ruminal pH

Submitted on June 12, 2000
Accepted on October 3, 2000

This article has been cited by other articles:

				D. Bossen, D. R. Mertens, and M. R. Weisbjerg Influence of Fermentation Methods on Neutral Detergent Fiber Degradation Parameters J Dairy Sci, April 1, 2008; 91(4): 1464 - 1476. [Abstract] [Full Text] [PDF]

				J. J. Couderc, D. H. Rearte, G. F. Schroeder, J. I. Ronchi, and F. J. Santini Silage chop length and hay supplementation on milk yield, chewing activity, and ruminal digestion by dairy cows. J Dairy Sci, September 1, 2006; 89(9): 3599 - 3608. [Abstract] [Full Text] [PDF]

				Q. Zebeli, M. Tafaj, H. Steingass, B. Metzler, and W. Drochner Effects of Physically Effective Fiber on Digestive Processes and Milk Fat Content in Early Lactating Dairy Cows Fed Total Mixed Rations J Dairy Sci, February 1, 2006; 89(2): 651 - 668. [Abstract] [Full Text] [PDF]

				R. Imamidoost and J. P. Cant Non-steady-state modeling of effects of timing and level of concentrate supplementation on ruminal pH and forage intake in high-producing, grazing ewes J Anim Sci, May 1, 2005; 83(5): 1102 - 1115. [Abstract] [Full Text] [PDF]

				C. C. Taylor and M. S. Allen Corn Grain Endosperm Type and Brown Midrib 3 Corn Silage: Site of Digestion and Ruminal Digestion Kinetics in Lactating Cows J Dairy Sci, April 1, 2005; 88(4): 1413 - 1424. [Abstract] [Full Text] [PDF]

				C. Leonardi, K. J. Shinners, and L. E. Armentano Effect of Different Dietary Geometric Mean Particle Length and Particle Size Distribution of Oat Silage on Feeding Behavior and Productive Performance of Dairy Cattle J Dairy Sci, February 1, 2005; 88(2): 698 - 710. [Abstract] [Full Text] [PDF]

				K. S. Schwartzkopf-Genswein, K. A. Beauchemin, T. A. McAllister, D. J. Gibb, M. Streeter, and A. D. Kennedy Effect of feed delivery fluctuations and feeding time on ruminal acidosis, growth performance, and feeding behavior of feedlot cattle J Anim Sci, November 1, 2004; 82(11): 3357 - 3365. [Abstract] [Full Text] [PDF]

				L. R. Lynd, P. J. Weimer, W. H. van Zyl, and I. S. Pretorius Microbial Cellulose Utilization: Fundamentals and Biotechnology Microbiol. Mol. Biol. Rev., September 1, 2002; 66(3): 506 - 577. [Abstract] [Full Text] [PDF]