Submitted to: FEMS Microbiology Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 28, 2008
Publication Date: March 20, 2009
Citation: Russell, J.B., Muck, R.E., Weimer, P.J. 2009. Quantitative analysis of cellulose degradation and growth of cellulolytic bacteria in the rumen. FEMS Microbiology Ecology. 67(2):183-197. Interpretive Summary: In ruminant animals, feedstuffs are digested in the rumen prior to gastric and intestinal digestion, and the animal is dependent on fermentation products (e.g. volatile fatty acids and microbial protein) for much of its nutrition. Ruminal fermentation has confounded diet formulation, and empirical approaches commonly used with simple stomached animals have, in most cases, been inadequate to provide a realistic prediction of animal performance. More than 20 years ago, the Cornell Net Carbohydrate Protein System was devised and its rumen sub-model had mechanistic rumen sub-model. The rumen sub-model of the CNCPS is, however, now decades out of date and much more powerful computer programs are now available. This manuscript describes one aspect of ruminal fermentation, cellulose digestion and the growth of cellulolytic bacteria. This STELLA-based model evaluates all of the major aspects of ruminal cellulose degradation: 1) ingestion, digestion and passage of feed particles, 2) maintenance and growth of cellulolytic bacteria, and 3) pH effects. Research to model ruminal fermentation has the potential to allow farmers, ranchers and nutrition consultants to devise cheaper, more efficient and environmentally friendly rations.
Technical Abstract: Ruminant animals digest cellulose via a symbiotic relationship with ruminal microorganisms. Because feedstuffs only remain in the rumen for a short time, the rate of cellulose digestion must be very rapid. This speed is facilitated by rumination, a process that returns food to the mouth to be re-chewed. By decreasing particle size, cellulose surface area can be increased by up to 10 6-fold. The amount of cellulose digested is then a function of two competing rates, namely the digestion rate (Kd) and the passage rate of solids from the rumen (Kp). Estimation of bacterial growth on cellulose is complicated by several factors: 1) energy must be expended to maintain as well as grow the cells, 2) only adherent cells are capable of degrading cellulose and 3) adherent cells can provide non-adherent ones with cellodextrins. Additionally, when ruminants are fed large amounts of cereal grain along with fiber, ruminal pH can decrease to the point where cellulolytic bacteria no longer grow. A dynamic model based on STELLA software is presented. This model evaluates all of the major aspects of ruminal cellulose degradation: 1) ingestion, digestion and passage of feed particles, 2) maintenance and growth of cellulolytic bacteria, and 3) pH effects.