The forestomach of ruminants and camelids does not only allow a differential excretion of fluids and small particles but also a differential excretion of small and large particles. The question whether larger particles of different size classes are also retained for different time periods, or whether simply a particle-size threshold exists above which all particles of a size higher than this threshold are retained in an undifferentiated manner, has not been addressed so far. We determined the mean retention time (MRT) of different-sized large particles (10 and 20mm) in three banteng (Bos javanicus) on two forage only diets, grass and grass hay. We used cerium-mordanted fibre (10mm) and lanthanum-mordanted fibre (20mm) as particle markers, mixed in the food. Average total tract MRT for large and very large particles at the grass diet was 58 and 56h and at the grass hay diet 66 and 64h, respectively. Very large particles moved slightly faster than large particles through the gut of the banteng. Three interpretations are possible: Very large particles are resubmitted to rumination sooner than large particles; ingestive mastication of the particle markers could have reduced the difference in the size of the particle markers; alternatively, particle retention may be governed by a threshold, above which all particles of a size higher than this threshold are retained in an undifferentiated manner. In order to test these possibilities, experiments with fistulated animals would have to be performe
