Dynamics of large ciliate protozoa in the rumen of cattle fed on diets of freshly cut grass

1. The dynamics of large ciliate (holotrich) protozoa (Isotricha and Dasytricha spp.) in the rumen of cattle given cut, fresh ryegrass (Lolium multiflorium Lam) were studied by means of a single intrarumen injection of 14C-labelled protozoa prepared in vitro by adding [Me14C]choline to rumen fluid containing protozoa and incubating at 39 degrees for 2 h. 2. An indication of the lysis rate of protozoa in the rumen was obtained from the radioactivity apparently lost through the methane pool. 3. The turnover time of the holotrich protozoa indicates that these protozoa were extensively retained in the rumen and that only a small proportion of those produced in the rumen flowed out in the digesta. This was supported by the estimation of the rate of lysis which was approximately 85% of the turnover rate in the rumen. 4. The apparent production rate of the larger protozoa indicates that they contribute only about 9% of the predicted net microbial protein synthesis in the rumen.