How can herbivores digest cellulose?
The diet of herbivores are plant material and all plants have a cell wall present in their cell structure which contain cellulose.
Cellulose can be digested/broken down only by cellulase an enzyme ABSENT in herbivores.
So basically all herbivores have a symbiotic relationship with certain "cellulose digesting bacteria".
These bacteria have cellulase which break down the cellulose into absorbable substances (eg: glucose), glucose is then absorbed by the body and provides nutrition.