In order to studythe effect of pH on defaunation in the rumen, four rumen fistulated steers werefed a basal roughage diet for a 4-week adaptation period followed by 17 weeksof feeding with three diets and two feeding levels of high concentrate diet.Rumen outflow fluid rate was evaluated in both ration levels. Rumen protozoapopulation was monitored weekly and when animals became defaunated, protozoawere reinoculated with rumen contents from one of the faunated steers. At everytwo weeks, during all the experimental period, rumen pH was measured in allanimals at 0, 4, 8 and 12 h after feeding. It was observed an individual animalinfluence on the establishment and maintenance of the rumen ciliate protozoapopulation. In all sampling times, mean rumen pH values were higher in faunatedsteers than in the defaunated ones. No differences were observed in rumen outflowfluid rates between the two ration levels. Extended periods of low rumen pHare probably more detrimental to the survival of ciliate protozoa in the rumenthan other factors.
