Mechanistic Equations to Represent Digestion and Fermentation
Current mathematical models use different forms of equations to represent feed degradation and microbial fermentation in the rumen. For example, the Cornell Net Carbohydrate and Protein System (CNCPS; Sniffen et al., 1992) assumes that feed digestion rate is first order with respect to substrate, but the models of Baldwin et al. (1987) and Dijkstra et al. (1992) assume Michaelis-Menten kinetics. A model proposed by Kohn and Boston (2000) suggests an alternative form of equations that incorporates the Second Law of Thermodynamics to represent fermentation processes in the rumen. The purpose of this paper is to explore the differences among these approaches, including the form of the equations, the theoretical basis and assumptions, and the behavior. This analysis will identify a number of important research questions to pursue.
KeywordsMethane Production Volatile Fatty Acid Concentration ofES Mechanistic Equation Propionate Production
Unable to display preview. Download preview PDF.
- Allen, M.S., and Mertens, D.R., 1988, Evaluating constraints on fiber digestion by rumen microbes, J. Nutr. 118:261–270.Google Scholar
- Chang, R., 1981, The Second Law of Thermodynamics, in: Physical Chemistry with Applications to Biological Systems, 2nd ed., Macmillian, New York.Google Scholar
- Dijkstra, J., Neal, H.D.C., Beever, D.E., and France, J., 1992, Simulation of nutrient digestion, absorption and outflow in the rumen: model description, J. Nutr. 122:2239–2256.Google Scholar
- Kohn, R.A., and Boston, R.C., 2000, The role of thermodynamics in controlling rumen metabolism, in: Modelling Nutrient Utilization in Farm Animals, J.P. McNamara, J. France, and D.E. Beever, eds, CABI Publishing, Wallingford.Google Scholar
- Mertens, D.R., 1993, Rate and extent of digestion, in: Quantitative Aspects of Ruminant Digestion and Metabolism, J.M. Forbes and J. France, eds, CABI Publishing, Wallingford.Google Scholar
- Neal, H.D.C., Dijkstra, J., and Gill, M., 1992, Simulation of nutrient digestion, absorption and outflow in the rumen: model evaluation, J., Nutr. 122:2257–2272.Google Scholar
- prins, R.A., 1977, Biochemical activities of gut micro–organisms, in: Microbial Ecology ofthe Gut, R.T.J. Clarke and T. Bauchop, eds, Academic Press, New York.Google Scholar
- Russell, J.B., and Strobel, H.J., 1989, Effect of ionophores on ruminal fermentation, Appl. Environ. Microbiol. 55:1–6.Google Scholar
- Sniffen, C.J., O’Connor, J.D., Van Soest, P.J., Fox, D.G., and Russell, J.B., 1992, A net carbohydrate and protein system for evaluating cattle diets: II. Carbohydrate and protein availability, J. Anim. Sci. 70:3562– 3577.Google Scholar
- Van Soest, P.J., 1994, Mathematical applications: digestibility, in: Nutritional Ecology of the Ruminant, 2nd ed. Cornell University Press, Ithaca.Google Scholar
- Wolfe, R.S., and Higgins, I.J., 1979, Microbial biochemistry of methane -- a study in contrasts. Microbial Biochem. 21:267–353.Google Scholar