The dissociation of the fluid and particle phase in the forestomach as a physiological characteristic of large grazing ruminants: an evaluation of available, comparable ruminant passage data

Whether differences in digestive physiology exist between different ruminant feeding types has been an ongoing debate. In this regard, potential differences in ingesta retention have been understood to be of particular importance. We analyzed a data pool in which only mean retention time (MRT) data for the ruminoreticulum (RR) were collated that were obtained using comparable techniques with either chromium or cobalt EDTA as a fluid marker and/or with chromium-mordanted fiber of less than 2 mm in size as a particle marker. Data were compared using one averaged value per species. In general, the paucity of species in such a collection is striking and does not allow—in contrast to earlier statements—any final conclusions regarding the influence of body weight (BW) or feeding type on ruminant MRTs. In particular, there was no significant correlation between MRTparticlesRR or MRTfluidRR and BW, neither in the interspecific nor in the intraspecific comparisons, and no difference between the feeding types. The trend that indicates longer MRTparticlesRR in grazers is based on too few species to be conclusive. Small browsers seemed to have shorter MRTfluidRR than similar-sized grazers. In contrast, there was a trend for large grazers to have shorter MRTfluidRR than large browsers. In direct pair-wise comparisons between cattle and the browsers giraffe, moose, and okapi, the latter difference was significant. Cattle also had the highest relative RR fluid outflow rates among the species investigated. This is in accord with the observation that grazers have larger omasa, a major function of which is water-reabsorption distal to the RR. Grazers seem to have longer MRTparticlesRR per unit MRTfluidRR, and cattle are particular outliers in this respect. It is hypothesized that potentially shorter MRTfluidRR in large grazers and higher relative outflow rates are linked to a higher saliva production and a lesser viscosity of both saliva and RR fluids. A constant supply of a fluid phase of low viscosity is proposed to be the prerogative for the physical mechanisms of flotation and sedimentation that result in the stratification of RR contents and its selective particle retention typical for large grazing species