The role of dynamic modelling in understanding the microbial contribution to rumen function

Abstract

Mechanistic models of microbial metabolism in the rumen aim at an improved understanding and integration for research purposes or at an improved prediction for practical purposes. The standard way of representing such models is the rate : state formalism. The system is defined by a number of state variables and a set of differential equations describe the change of the state variables with time. Three different types of solution to these dynamic models are distinguished, and examples of these solutions are described to illustrate the applications and contributions of dynamic modelling in the study of the rumen microbial ecosystem. Type I solutions are obtained when the system is in steady state and the differential equations are solved by setting the differentials to zero. An application of the type I solution is the indirect approach to quantifying the fibrolytic anaerobic fungi in the rumen. The solutions of the model describing the alternate life cycle of rumen fungi, with its free-swimming dispersal and particle-attached stages, appear to be consistent with ruminal and faecal observations. Type II solutions are obtained when the system is not in steady state but the differential equations can be integrated analytically. An application of this type of solution is the quantification of the growth and growth yield in batch cultures. Such models help to quantify the degradation of substrates in the rumen and to elucidate the interactions between groups of rumen micro-organisms. Type III solutions are obtained when the system is not in steady state and when the differential equations have to be solved numerically. Applications of the type III solutions are the rumen simulation models that describe substrate degradation, endproduct formation and microbial metabolism in an integrated manner. To illustrate this type III solution, a model of lactic acid metabolism in the rumen is defined, and its contribution to understanding of the paths and rates of lactic acid disappearance described. It is essential that the models are based Nutrition Research Reviews (2002), 15, 67-90

Nutritional Value ofAcacia AmentaceaandParkinsonia TexanaGrown in Semiarid Conditions

“…However, this is not always the case, e.g. when the capacity of microorganisms that metabolise excess soluble material is at its maximum (mean of both species=23%), the onset of degradation of the insoluble fraction could take longer and the value of L would be higher (Dijkstra et al, 2002). This could be the case of P. texana, whose L increased (from 0.868 to 1.2 h) in presence of PEG.…”

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

“…Synchronisation of energy from degraded carbohydrate and various N sources will theoretically increase EMPS by proper matching of nutrients and avoidance of energy spilling, but benefits of synchronisation in vivo are equivocal (Firkins et al, 2006). Recycling of urea-N to the rumen and temporary storage or depletion of intracellular polysaccharides will assist in overcoming instantaneous deficiencies (Dijkstra et al, 2002). Overall, the practical impact of energy spilling and the need to account for it in models of rumen metabolism seems to be limited in most situations in dairy cattle nutrition.…”

“…These systems have several drawbacks that are well recognised (e.g. Baldwin, 1995;Dijkstra et al, 2002;Hanigan, 2005). Single energy or protein values for a feed quote static conditions, whereas the energy or protein supply depends on a number of interrelated factors, including site of digestion and level of feed intake.…”

Nitrogen Utilization of Perennial Ryegrass in Dairy Cows

“…The addition of WSC (inulin/sucrose, 80/20, g/g) to fresh perennial ryegrass in a RUSITEC system over 10 days showed a linear decrease in pH and NH 3 with increasing WSC levels and an increase in the efficiency of microbial protein synthesis at 1.5 times the WSC content in grass (Lee et al 2003). However, the increased dilution rate at higher WSC concentrations in these in vitro systems may have contributed to this rise in microbial efficiency, since the fractional dilution rate is one of the major determinants of microbial efficiency (Dijkstra et al 2002). In steers fed ryegrass cultivars differing in WSC concentration (83 g/kg DM), the ruminal NH 3 content was lower with the high-WSC cultivar, but the in situ efficiency of microbial protein synthesis and flow of N to the duodenum as proportion of N intake were not different (Lee et al 2002).…”

“…Besides, the supply of soluble N (peptides, amino acids, NH 3 ) and (or) CHO may exceed the maximum capacity of microorganisms to use all these substrates immediately, and therefore may not be metabolized by rumen microorganisms (Dijkstra et al 2002;Bach et al 2005). Moreover, feed intake level, the storage of CHO in rumen microbes (Dijkstra 1993), the recycling of N within the rumen due to bacterial lysis and protozoal predation (Dijkstra et al 1998;Koenig et al 2000), and recycling of urea N within the body to the rumen (Lapierre and Lobley 2001) have a (large) effect on the efficiency of microbial protein synthesis.…”