The nutritional components of green grass on which they feed are enclosed within the cellulose cell walls which consist of cellulose. Like most animals, in fact, heterotrophs in general, they lack cellulose-digesting enzyme, a nutritional inability of herbivores. The herbivores therefore need to grind up their tissue first. Herbivorous mammals depends exclusively on certain microorganisms living in their gut, which produce the enzyme cellulase that breaks down cellulose in ruminants. This relationship is a symbiotic association, becauses plant materials are difficult to digest, herbivores need a long ingtestine.
Ruminants have developed a four-chambered stomach to solve the problem of cellulose digestion. The first chamber is the rumen where food mixed with saliva undergoes fermentation by  mutualistic organism (such as bacteria, protists, fungi). The partly digested food is then regurgitated and thoroughly re-chewed – a process known as rumination. The food is then re-swallowed and undergoes further fermentation. The digested food eventually passes through the third chamber called omasum where it undergoes few reaction to reach the abomasums which is the true stomach. The partly digested food is finally digested here by the host’s enzymes and henceforth undergoes digestion by the usual mammalian digestive enzymes.