A stepwise adaptation was carried out on eight sheep through diets containing 10, 24, 44, 60 to a final diet containing 71% maize meal and molasses. The numbers of protozoa in the rumen increased in proportion to amount of readily fermentable carbohydrate fed, up to and including the 60% grain and molasses diet, while the numbers of total culturable bacteria remained essentially constant. However, the proportions of amylolytic and lactate-utilizing bacteria increased, and there was an orderly shift from acid-sensitive to more acid-tolerant species, particularly amongst the lactate-utilizers in response to the gradual decrease in the ruminal pH. Up to this stage the protozoa probably controlled the rate of fermentation by engulfing starch grains and bacteria and were thus able to maintain the pH of the rumen above 5·5. Lactic acid appeared only transiently and the peak values tended to diminish as adptation progressed.
The first day the final diet was fed the ruminal pH decreased to 5·4 or below for several hours. Within 7 days the number of protozoa had decreased by 50–80% and the number of total culturable bacteria increased sharply. Conditions in the rumen became unstable: peak values of D- and L- laotic acid increased by ca. 0·5 HIM, the acetate/propionate ratio decreased to ca. 2 and peak glucose concentration increased t o 3·2–9·5 mM. One animal refused all food for 1 day. Acid-tolerant species of lactate-utilizing bacteria multiplied rapidly in response to the increased production of ruminal lactic acid and the ratio of amylolytics to lactate-utilizers decreased from a mean of 10·7 to 3·6. This controlled the increase in lactic acid and the decrease in ruminal pH, allowing the ciliate protozoa to proliferate and regain control of the fermentation.
The types of cellulolytic bacteria did not change during the experiment. Despite their acid sensitivity, the number of cellulolytic bacteria per gram of ingesta was of the same order after 54 days on the 71% grain and molasses diet (0·5–13·3 × 107) as on the initial high roughage diet (3·2–7·6 × 107).
Three sheep which bloated showed no marked chemical or microbiological differences from the non-bloating animals.