SUMMARY: A chemotactic migration of Isotrichidae, primarily and , into sheep rumen contents was observed at feeding time, followed by an abrupt decrease in relative numbers, presumably as a result of sequestration. An increase in the numbers of isotrichids was also observed before feeding, and was not a response to handling or sampling of the animals. When feeding was delayed for 4.5 h, isotrichid numbers increased markedly, falling again when feed was provided. The size of the decrease in numbers after feeding was directly related to feed intake. In general, chemotactic migration of the Isotrichidae was observed 12 h after feeding, while migration without feeding occurred after 22-24 h. In animals normally fed once daily, the relative number of isotrichids in the rumen contents of an unfed animal about 3 h after the regular feeding time is probably the best estimate of total isotrichid numbers. Two migrations of the same isotrichids appeared to occur in animals fed twice a day. It is proposed that feeding level controls the amount of storage polysaccharide in the protozoan cell, which in turn controls migration of Isotrichidae into the rumen. The isotrichids appeared to sequester by settling in the ventral rumen.


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