Rumen Dilution Rate Has Important Influences on Fermentation and Microbial CeIIYieId
Small particles, including microbes, leave the rumen with the liquid phase. Therefore, high dilution rates result in rapid removal of microbes and reductions in microbial cell concentrations.
Because high microbe concentrations suppress microbial cell division, the growth of microbes is stimulated by high dilution rates. High growth rates are nutritionally desirable because a larger portion of the energy available to the microbes is used for growth instead of for maintenance, as occurs in older, relatively stable microbial populations. Thus, high dilution rates usually increase Yλjp values, provided that adequate protein is available to support cell growth.In addition to its effect on Ta17>, the dilution rate may affect the microbial makeup of the rumen biomass and also may have some influence on the fermentation pattern. The rate of microbial washout increases with the dilution rate. At high dilution rates, microbial species with slow growth rates diminish in population size because their replication rate is not great enough to match the rate at which they are removed. Thus, selection pressure favors species with faster growth rates during times of high rumen dilution rates. Exceptions to this pattern occur because some microbes are able to attach themselves to the particulate matter in the solid and slurry zones. Such microbes then exit the rumen according to the kinetics of particle size reduction rather than dilution rate. In general, the changes occurring in the rumen microbial population with high dilution rates appear to favor acetic acid production and to increase the acetic/propionic acid ratio.