Simulation of Nutrient Digestion, Absorption and Outflow in the Rumen: Model Description ,

Abstract

A mathematical model is described that stimulates 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.

In vivo production and molar percentages of volatile fatty acids in the rumen: a quantitative review by an empirical approach

“…In current mechanistic models (Baldwin et al, 1987;Dijkstra et al, 1992;Pitt et al, 1996), separate coefficients for roughage and concentrate diets are considered (Murphy et al, 1982;Bannink et al, 2006;Sveinbjö rnsson et al, 2006) and their use requires accurate knowledge of five fermented substrates (soluble carbohydrates, starch, hemicellulose, cellulose and protein), although the precision of the analytical determination of some of these substrates (i.e. hemicellulose) remains questionable.…”

Predicting the profile of nutrients available for absorption: from nutrient requirement to animal response and environmental impact

“…Evaluation of this representation showed good accuracy when compared with observed data, but application to predict EMPS in the rumen would require quantitative data for parameterisation and these are not available. Baldwin (1995) and Dijkstra et al (1992) used a different representation and adopted a Michaelis-Menten relationship in which the fermentation of carbohydrates to VFA for non-growth purposes was limited by increased availability of Nsources. A similar representation can be applied for other nutrient sources that may impair the biosynthetic use of energy in carbohydrates.…”

“…Pirt (1975) as affected by maintenance requirement, availability of precursors for microbial growth, and microbial composition. Default situation calculated according to Dijkstra et al (1992) assuming a maintenance coefficient of 0.05 g carbohydrate per g microbial dry matter (DM) per h, a microbial crude protein content of 0.6 g/g microbial DM, and ammonia and preformed amino acids to deliver 0.80 and 0.20 g nitrogen (N) per g required N. Changes in assumptions: (i) maintenance requirement, maintenance requirement raised to 0.15 g carbohydrate per g microbial DM per h; (ii) precursor availability, ammonia and amino acids to deliver 0.40 and 0.60 g N per g required N, respectively; (iii) microbial composition, microbial CP content raised to 0.80 g/g microbial DM at the expense of storage polysaccharides in microbial DM.…”

“…However, substrate degradation and bacterial growth have been shown to increase with addition of AA or peptides in fibrolytic and amylolytic bacteria (Bach et al, 2005). In various models, the theoretical improvement in EMPS when all N required is provided by AA rather than ammonia varied widely: 19% (Russell et al, 1992), 59% (Dijkstra et al, 1992) and 77% (Baldwin, 1995). A major part of the variation in these values can be explained by differences in the units in which EMPS is expressed (i.e.…”

“…An example of further development of a nutritional model to predict faecal and urinary composition was presented recently by Reijs et al (2006). They expanded the mechanistic rumen model of Dijkstra et al (1992) with static equations to describe digestion processes in the intestines and to describe excretion of various urinary components. The model predicts OM, C and N excretion of various faecal and urinary components.…”