Bacterial degradation of cellulose and fermentation of glucose occur in the rumen of cows, sheep and other ruminant animals. The main source of energy for the fermenters is , while the main source of energy for the ruminant is

Glucose, volatile fatty acids

Explanation:

Fermentation involves breaking down of carbohydrate anaerobically with the help of microbes. These microbes require glucose to work . The source of energy of ruminants as the volatile fatty acids which is a product of digestion and amounts to about 70percent of its energy source. The availability of this volatile fatty acids determine the fat and protein composition of the milk.

substitution is the answer in this case