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The Reason Why Humans Can’t Eat Grass but Cows Can

Author: Jessica Zhang

Editors: Yanxi Chen and He-Hanson Xuan

Artist: Cici Zhang

Humans cannot digest cellulose, a main component of forage, so we cannot survive solely by consuming grass. Cows not only utilize cellulose as their primary source of energy but also produce dairy products.

Cows only have one stomach, but it is divided into four compartments: the rumen, reticulum, omasum, and abomasum. After the food goes through the esophagus and enters the first and largest compartment, the rumen, fermentation occurs. Billions of microbes living in a hospitable environment of pH ranging from 5.5 to 7.0 and temperature ranging from 37 to 40 degrees Celsius can convert cellulose, composed of beta-glucose subunits, into volatile fatty acids and gasses. The produced gasses are released through eructation, while the volatile fatty acids are directly absorbed from rumen. Sometimes after cows ingest an unsorted mixture of materials, some large part of food may return from the rumen to their mouth to be chewed again. This unique process called rumination allows cows to consume a large amount of forage in a short time.

The next component, the “stomach,” also known as the reticulum, is only partially separated from the rumen but has substantially differing functions. Its honeycomb structure allows it to trap large particles. Those large particles are then sent to the mouth again for further rumination. Reticulum is like a police guard, preventing large particles from passing through.

After the “gate” (i.e., reticulum) comes the omasum. This section of the stomach is highly folded, creating leaf-like structures and occupying a large surface area. The small bumps, papillae, on the leaves absorb volatile fatty acids. This region has significant water and electrolyte absorption due to slow water flow.

The final compartment in the four-stomach system is the abomasum. Differing from three previous stomachs, whose main functions are storage and absorption, the abomasum can secrete enzymes and hydrochloric acid, digesting the remaining food and microbes. In addition, because of the presence of the acid, the pH in this stomach is around 2.5. These characteristics make abomasum function like a human stomach.

Understanding the mechanism of the four-stomach system helps us understand how cows digest cellulose. Through this extraordinary ability, cows are able to transform grass into milk and meat effectively.



“Beef Skillathon.” Beef Skillathon, Rounds, Whitney, and Dennis B. Herd,

“General Anatomy of the Ruminant Digestive System.” Suwannee River Youth Livestock Show

and Sale, Umphrey; J.E; C.R; Staples, Sept. 1992,

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