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

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

A mathematical model of the rumen fermentation processes constructed to predict nutrient supply to the host animal was evaluated. Sensitivity analysis on high fiber, starch and protein diets indicated that the model responds appropriately to these types of diets and to changes in parameter values, and revealed that the model is sensitive to the availability of hexose for non-growth microbial processes and to the maximum storage rate of polysaccharides in amylolytic microbes, although sensitivity varied with diet composition. Of the parameters whose values were dependent on diet, the fraction of protozoa in the amylolytic microbial pool and the fluid and solid passage rates needed the most careful estimation. When model predictions of nutrient supply were compared with the experimental observations, those for duodenal flows of neutral detergent fiber, total non-ammonia nitrogen (NAN) and total volatile fatty acid rumen concentration were satisfactory for several feeding strategies. The partition of NAN flow into microbial and non-microbial NAN flow and the molar proportions of volatile fatty acid production and concentration were not predicted well. The representation of the complex interactions between rumen microbial populations and of their effects on the production of specific volatile fatty acids merits further study for an improvement in the prediction of nutrient supply.

KEY WORDS: • microbial metabolism • mathematical model • rumen • ruminants • digestion

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

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

⁵ Current address: Natural Resources Institute, Central Avenue, Chatham Maritime, Chatham, Kent ME4 4TB, U.K.

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

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