The ruminants bring back swallowed grass into their mouth and chew it for sometime. Grass has cellulose. It is the carbohydrate which can only be digested by ruminants because they have rumen. When they eat grass, it is sent to the rumen where enzymes acts on it and softens it. Softened cellulose is called cud. It is sent back to the mouth where ruminants chew it for sometime.

Methanogens are a group of archaebacteria that occur in marshy areas where they convert formic acid and carbon dioxide into methane. Some of the methanogen archaebacteria live as symbionts (Eg. Methanobacterium) inside the rumen or first chamber in the stomach of herbivorous animals that chew their cud (ruminants). These archaebacteria are helpful to the ruminants in the fermentation of cellulose. 

In ruminants, the stomach is differentiated into separate chambers - rumen (for churning, digestion of cellulose and hemicellulose and bacterial fermentation), reticulum (traps and stores temporarily large feed particles), omasum (absorbs water) and abomasum (similar to human stomach, digests proteins). Abomasum is the true stomach. Omasum is absent in camels. In camels rumen and reticulum have diverticula or water pockets for temporary storage of food.

Ruminants (cattle, sheep, deer, giraffes) are hooved animals with a stomach divided into four chambers- rumen, reticulum, omasum, and abomasum. Symbiotic bacteria and protists living in the first two chambers digest cellulose, splitting some of it into sugars, which are then used by the host and the bacteria themselves. The bacteria produce fatty acids during their metabolism, some of which are absorbed by the animal and serve as an important energy source. Food that is not sufficiently chewed clumps together, forming a cud. The cud is regurgitated into the animals mouth, where it is mixed with saliva and chewed again. When the cud is reswallowed, the partly digested food is further broken down by the cows own enzymes.

Cud is a portion of food that returns from a ruminant's stomach to the mouth to be chewed for the second time. Cows spend nearly eight hours out of every day chewing their cud. Hence, option A is correct.

The ruminant stomach is composed of 4 separate compartments. Food passes first into the 'rumen', then 'reticulum', 'omasum' and finally into the 'abomasum'. The first three compartments helps in digestion of complex carbohydrates with the aid of microorganisms, which produce volatile fatty acids. The abomasum functions similarly to the carnivore stomach as it is glandular and digests food chemically, rather than mechanically or by fermentation like the other 3 chambers of the ruminant stomach. Hence, option A is correct.

• The animals which partially digest food have a four-chambered stomach and the process of digestion is as follows: The food is chewed and swallowed quickly and passed into the first stomach called Rumen.
•  Then the microorganisms help in breaking down the food and this partially digested food is called cud. Hence, the rumen is the stomach part in which herbivores secretes the grass which is swallowed quickly.

• The animals which partially digest food have four chambered stomach and the process of digestion is as follows: The food is chewed and swallowed quickly and passed into the first stomach called Rumen.
•  So, the animals are referred to as ruminants. Then the microorganisms help in breaking down the food and this partially digested food is called cud. Hence, Partially digested food in ruminants is cud.

Ruminant animals such as cattle, buffalo, sheep, goat and camel have a compound stomach, which consists of four chamber, viz, rumen, reticulum, omasum and abomasum. Rumen is the first and the largest of the four chambers. Rumen and reticulum harbour numerous bacteria and protozoa, which carry out extensive fermentation of cellulose. So, these two chambers function as sites for cellulose digestion in ruminants. The gastric juice containing enzymes and HCl is secreted only by the fourth chamber i.e., Abomasum.

Cellulose is broken down to short chain fatty acids in ruminants ,during cellulose fermentation by anaerobic bacteria.

Ruminants include cattle, sheep, goats, buffalo. These animals all have a digestive system that is uniquely different from our own. Instead of one compartment to the stomach, they have four. Of the four compartments, the rumen is the largest section and the main digestive center

Vertebrates lack the ability to hydrolyse the beta glycosidic bond of plant cellulose due to the lack of the enzyme cellulase. Thus, ruminants must completely depend on the microbial flora, present in the rumen or hindgut, to digest cellulose. Digestion of food in the rumen is primarily carried out by the rumen microflora. Since, the environment inside a rumen is anaerobic, most of these microbial species are obligate or facultative anaerobes that can decompose complex plant material, such as cellulose, hemicellulose, starch, and proteins. As bacteria conduct fermentation in the rumen, they consume approximately 10% of the carbon, 60% of the phosphorus, and 80% of the nitrogen that the ruminant ingests. To reclaim these nutrients, the ruminant then digests the bacteria in the abomasum. The enzyme lysozyme has adapted to facilitate digestion of bacteria in the ruminant abomasum. Pancreatic ribonuclease also degrades bacterial RNA in the ruminant small intestine as a source of nitrogen. 

• Rumination is also called second chewing. 
  
• Ruminant stomach: The stomach of a ruminant is divided into four chambers – the rumen, reticulum, omasum and abomasum. 
• The process of digestion is as follows: The food is chewed and swallowed quickly and passed into the first stomach called Rumen. The microorganisms help in breaking down the food. Digestive juices of the rumen partially digest the food. The partially digested food in the rumen is called cud. Hence, The partially digested food in the stomach of ruminants is called cud.

Rumen is a sac-like largest part of the stomach of the ruminants. It acts as a storage or holding vat for feed. It is also a fermentation vat.