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Simulation of Nutrient Digestion, Absorption and Outflow in the Rumen: Model Description^{1 ,2}

AFRC Institute of Grassland and Environmental Research, Hurley, Maidenhead, Berkshire SL6 5LR, U.K.

A mathematical model is described that simulates the digestion, absorption and outflow of nutrients in the rumen. The model consists of 17 state variables, representing nitrogen, carbohydrate, lipid, microbial and volatile fatty acid pools. The flux equations are described by Michaelis-Menten or mass action forms with parameters calculated from the literature. Several specific areas of improvement in representation of rumen processes were reconsidered during model development. These included microbial substrate preference, differential outflow and chemical composition of rumen microbes, recycling of microbial matter within the rumen, uncoupling of fermentation with respect to nitrogen availability, reduced microbial activity at reduced rumen pH and pH-dependent absorption of volatile fatty acids and ammonia. The model was used to examine the effects of the diet on the profile of nutrients available for absorption and was shown to respond appropriately to different intake and nitrogen levels. The validity of the improvements and the predictions of nutrient supply on a variety of dietary inputs are tested in a companion paper.

KEY WORDS: • rumen • computer simulation • mathematical model • ruminants • microbial metabolism

¹ Preliminary results were presented in part at the 41st Annual Meeting of the European Association for Animal Production, July 8–12, 1990, Toulouse, France [Dijkstra, J. & Neal, H.D.St.C. (1990) A rumen simulation model of nutrient digestion and outflow. Summaries, Vol. 1, pp. 206] and at the 20th Meeting of the AFRC Modellers' Group, April 6, 1990, London, U.K. [Neal, H.D.St.C. & Dijkstra, J. (1990) Representation of microbial metabolic activity in a model of rumen fermentation. J. Agric. Sci. 115: 148].

² One of the authors (J. D.) was in receipt of a bursary from the British Council, Amsterdam and of financial assistance from the Stichting "Fonds Landbouw Export Bureau 1916/1918," Wageningen. These funds are gratefully acknowledged. Part of this study was supported by Ministry of Agriculture, Fisheries and Food commissioned research.

³ Current address: Wageningen Agricultural University, Department of Animal Nutrition, Haagsteeg 4, 6708 PM Wageningen, The Netherlands.

⁴ Current address: Department of Agriculture, University of Reading, Earley Gate, Reading, Berks RG6 2AT, U.K.

⁵ Current address: Department of Animal Science, ETH—Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland.

⁶ Current address: AFRC Institute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon EX20 2SB, U.K.

Manuscript received 21 October 1991. Revision accepted 1 July 1992.

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