Summary: A container with nylon and stainless steel mesh screens (10 μm and 20 μm pores, respectively), which allowed the passage of bacteria and soluble substrate but restricted the passage of protozoa, was used to investigate the phenomena of sequestration, migration and lysis of protozoa in the rumen of steers. After feeding, the concentration of Isotrichidae in the rumen increased 8·7-fold within 40 min and then decreased 88% by 4h; however, the concentration of Isotrichidae inside the containers remained almost constant. Fluctuations in concentrations of Isotrichidae were shown to be due to migration and sequestration within the rumen. Ophryoscolecidae did not exhibit the phenomena of sequestration and migration. When steers were fed once a day, about 50% of the decrease in the concentration of Ophryoscolecidae immediately after feeding could be attributed to the dilution effects of feed and water and/or passage out of the rumen. The remaining 50% of the decrease appeared to be from cell lysis, resulting in a wasteful recycling of protozoal protein. In an experiment to determine the effects of feed restriction on concentrations of Ophryoscolecidae, it was shown that decreased concentrations associated with substrate restriction are due to cell lysis rather than to passage out of the rumen.


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