The balance of rumen microbial population was affected by fermentation substrate and rumen pH value. In the rumen, the pH suitable for the growth and reproduction of cellulolytic bacteria should be maintained above 6.0. On the contrary, when the rumen pH is below 6.0, it is helpful for the growth and reproduction of starch decomposing bacteria. Increasing the fiber content of high digestibility and the fermentation substrate of cellulolytic bacteria can increase the number of the bacteria. With the increase of cellulolytic bacteria activity, the concentration of acetic acid can be increased, thus increasing the synthesis of milk fat.
In addition, increasing the effective fiber content in the diet can also promote the mastication, rumination and salivary secretion of dairy cows, so as to increase the rumen pH value and have a positive impact on the activity of cellulolytic bacteria. On the other hand, due to its filling effect, increasing the available fiber in the diet may limit the dry matter intake of dairy cows, thus affecting milk production.
In order to avoid these negative effects, high digestibility non roughage fiber sources such as soybean husk and corn fiber can be used. Compared with available fiber, increasing starch content in the diet increased the production of propionic acid and lactic acid, and decreased rumen pH. Although cellulolytic bacteria can tolerate slightly lower rumen pH (5.8-6.0) in the short term, the low rumen pH caused by high starch diet will have a negative impact on fiber fermentation and milk fat synthesis in the long run. Feeding a higher proportion of non roughage fiber can alleviate intestinal filling, but long-term low pH value will limit the activity of cellulose decomposing bacteria, which will affect the digestion of non roughage fiber.
Therefore, in order to maintain rumen fermentation pad and avoid high rumen pass rate, it is necessary to balance the effective fiber content in the diet when increasing the non roughage fiber content in the diet.