why rumination is necessary in grass eating animals

Grass-eating animals such as cows quickly swallow grass and store it in a separate portion of their stomach called the rumen.

Food gets partially digested in the rumen. This partially digested food is called cud. The cud later returns to the mouth in the form of small lumps. Animals then keep chewing on these lumps. This is known as rumination. It is important for complete digestion of partially digested food.