1. Three heifers were changed from a diet of equal parts of hay and barley cubes (50:50 diet) to one entirely of barley cubes given in three equal feeds throughout the day. Feed intake was restricted to 80% of calculated appetite at the time of change and this percentage progressively decreased as the live weights of the animals increased.
2. The change of diet had no significant effect on the volume of rumen fluid but the rate of outflow from the rumen was significantly lower on the barley diet than on the 50:50 diet.
3. Animals on the restricted barley diet developed an exceptionally high rumen ciliate population and the bacterial population was shown by Gram films to include a number of organisms typical of roughage-fed animals. In culture, organisms of the genus Bacteroides were predominant but these appeared largely as cocco-bacilli in the Gram films. This microbial population was associated with a higher proportion of butyric acid than of propionic acid in the rumen fluid.
4. Occasional fluctuations in ciliate populations occurred in all three heifers. Decreases in ciliate number were paralleled by increases in propionic acid and decreases in butyric acid but not necessarily by a fall in pH. Under these conditions Gram films showed increases in bacteriodes-type rods and in certain curved Gram-negative rods.
5. Rumen ammonia concentrations were on average lower and showed a different diurnal pattern when ciliate numbers were reduced. Lactic acid concentrations were low and were not affected by the size of the ciliate population.
6. When the three heifers were given the barley diet ad lib. there was a decrease in rumen pH and a complete loss of rumen ciliates. The rumen bacterial population and the volatile fatty acid proportions were similar to those seen during decreases in ciliate number at the restricted level of intake. These changes also occurred in a fourth heifer which was changed fairly rapidly from the 50:50 diet to a restricted amount of the barley diet.
7. Two steers which had never had access to roughage were changed from ad lib. to restricted intake of the barley diet and were later given an inoculum of rumen ciliates. The rumen microbial population and the pattern of fermentation so produced were similar to those found in the heifers on the restricted barley diet.
8. Anomalous values were noted for total counts of rumen bacteria when free starch grains were present in the rumen fluid.
9. It is concluded that large ciliate populations and high proportions of butyric acid can be produced in animals fed exclusively on a barley diet by suitable adjustment of the intake and the method of feeding. It is postulated that the ciliate population may be largely responsible for the high butyric acid concentrations.