How do bacteria help cows digest grass?

Cows do not have four separate stomachs; rather, they possess a single stomach divided into four distinct compartments, the largest of which is the rumen. This compartment functions as a fermentation vat where bacteria and other microorganisms play a critical role in digestion.12

Because cows lack the enzymes necessary to break down cellulose—a tough fiber found in grass—they rely on these symbiotic microbes to perform the task. The bacteria ferment the cellulose into volatile fatty acids, which the cow absorbs and utilizes as a primary source of energy.3421

Additionally, these ruminal bacteria help release and degrade protein within the plant cells and synthesize essential water-soluble vitamins, reducing the cow's dietary requirements for those nutrients. To assist this process, cows regurgitate and re-chew their food as cud, which further breaks down the plant material and ensures the ruminal fluid is thoroughly mixed with the ingested forage.412