Mathematical modelling and integration of rumen fermentation processes

In ruminants, the profile of nutrients available for absorption generally differs considerably from that ingested. These differences result from the metabolic activities of the rumen microorganisms. The main aim of the present study, was to model the rumen fermentation processes, to achieve the integration and quantification of these processes, and with the long term aim of improving the level and efficiency of animal performance with respect to desired products and waste. The dynamic, mechanistic models developed in this study addressed specific aspects of rumen function which were not included in previous models, but have been recognized as being of major importance in the transformation of ingested to absorbed nutrients. In particular, the representation of microbial metabolism has been improved. Results of model evaluation showed that the outflow of nutrients from the rumen (NDF, starch, soluble sugars, nitrogen) was predicted well on a wide range of dietary inputs. However, the type of VFA was not predicted satisfactorily. Therefore, an experiment was conducted to study absorption rates of VFA from the rumen. It was found that VFA absorption is affected to a different extent by rumen volume, pH and VFA concentration. These factors should be taken into account if production of individual VFA in the rumen is to be predicted accurately. In another model developed in the present study, major aspects of rumen protozoal metabolism have been represented mathematically. The mathematical integration of protozoal, bacterial and dietary characteristics provided a quantitative understanding of mechanisms of protozoal responses and their effects on nutrients available for absorption to changes in dietary inputs. The prediction of responses of the amount of product (milk, meat, wool) and the product composition should recognize metabolism of individual substrates within the rumen or available after absorption. The models developed in this study provide a basis for the estimation of the profile of nutrients available for absorption, but further consideration of the prediction of type of VFA formed in the rumen should have a high priority.