Models describing ruminal in situ digestion as affected by the lag process and fractionation of substrate
van Milgen, Jacob
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Permalink
https://hdl.handle.net/2142/20870
Description
Title
Models describing ruminal in situ digestion as affected by the lag process and fractionation of substrate
Author(s)
van Milgen, Jacob
Issue Date
1992
Doctoral Committee Chair(s)
Berger, Larry L.
Department of Study
Animal Sciences
Discipline
Animal Sciences
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Agriculture, Animal Culture and Nutrition
Language
eng
Abstract
Models describing ruminal in situ digestion are used frequently to evaluate the nutritive value of feedstuffs. Most models assume the existence of a soluble, a potentially digestible, and an indigestible fraction. Some models include a discrete lag time to account for a delay in disappearance when substrate is incubated in the rumen. The existence of a discrete digestion lag is unlikely to occur in vivo; therefore, a fully dynamic, two-compartment model was used to describe digestion. Substrate present in the first compartment was considered unavailable for digestion. Flow of substrate from the first to the second compartment was described by a first-order process and is likely affected by hydration of substrate and initial microbial attachment.
In order to account for the accumulation of microbial mass on substrate, a three-compartment mass conversion model was used where the first, second, and third compartments represented substrate undergoing hydration, substrate undergoing digestion, and bacterial mass accumulation, respectively. Digestion end-products were viewed as having either no association (e.g., volatile fatty acids), temporary association (e.g., microbial mass), or permanent association (e.g., microbial debris) with the remaining substrate. Data for substrate remaining (the first two compartments) and bacterial mass accumulation (the third compartment) were fitted simultaneously.
The soluble fraction is defined as that soluble in water and is, by definition, an intrinsic fraction. An incubation study, using two steers fed either a concentrate or forage diet, indicated that the residue remaining after 6 weeks of incubation was affected by diet but not by animal, suggesting that both the potentially digestible fraction and indigestible fraction were not intrinsic fractions per se.
Although it is assumed that the potentially digestible fraction is homogeneous, the chemical and morphological heterogeneity makes this assumption untenable. Digestion kinetics of physically and chemically isolated fractions of wheat straw and alfalfa were different, contradicting the assumption of homogeneity of substrate. However, a stochastic model based on a gamma distribution of heterogeneous fractional digestion rates indicated that the variance of the distributed fractional digestion rates approached zero. Therefore, the results of stochastic models do not necessarily have a biological meaning.
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