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The effects of diffusion and viscosity on ruminant faecal excretion of markers in a simple compartmental model

Published online by Cambridge University Press:  09 March 2007

J. H. M. Thornley ,
J. France ,
M. S. Dhanoa  and
R. C. Siddons
J. H. M. Thornley
Affiliation:
Institute of Terrestrial Ecology (Edinburgh), Bush Estate, Penicuik EH26 0QB
J. France
Affiliation:
AFRC Institute of Grassland and Environmental Research, North Wyke Research Station, North Wyke, Okehampton EX20 2SB
M. S. Dhanoa
Affiliation:
AFRC Institute of Grassland and Environmental Research, Plas Gogerddan, Aberystwyth SY23 3EB
R. C. Siddons
Affiliation:
formerly AFRC Institute of Grassland and Environmental Research, Hurley, Maidenhead SL6 5LR
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Abstract

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Using equations obtained in a previous analysis, results are computed numerically which illustrate the effects of diffusion and viscosity on faecal excretion patterns of markers in ruminants. Results are first given for plug flow, a velocity gradient produced by viscosity, and diffusion, each mechanism operating alone. Plug flow gives a period during which no marker appears in the faeces, then a sharp increase in faeces marker concentration, followed by rapid washout. A velocity gradient gives a more gradual appearance of marker in the faeces followed by a slower washout. DMusiou alone (although not realistic for ruminant marker kinetics) can give early appearance of marker followed by slow washout. Combining diffusion with a velocity gradient produced by viscosity can give a range of behaviour, depending on the effective diffusion coefficient, D'; an approximate method is used to compute these solutions. Because plug flow with no velocity gradient plus diffusion gives results similar to convective flow with a velocity gradient plus diffusion, we believe it will not be possible to determine the main mechanisms defining marker outtiow patterns from observations of marker kinetics alone, and more detailed investigations will be needed. Although estimates of quantities such as mean transit time are unaffected by detailed mechanism, the interpretations of measures such as sigmoidicity, sharpness of the faecal marker concentration v. time curve, and length and nature of the washout tail are highly dependent on mechanism.

Keywords

RuminantsDigesta markersCompartmental modelDiffusionViscosity
Type
Ruminant faecal excretion of markers
Information
British Journal of Nutrition , Volume 74 , Issue 6 , December 1995 , pp. 747 - 763
Copyright
Copyright © The Nutrition Society 1995

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