A Cow's 4 Stomachs Can Digest Grasses and Extract All Its Nutrients. Man Has 1 Stomach.

from the University of Minnesota:

Ruminant stomachs have 4 compartments: the rumen, the reticulum, the omasum and the abomasum.

Rumen microbes ferment feed and produce volatile fatty acids, which is the cow’s main energy source. Rumen microbes also produce B vitamins, vitamin K and amino acids.

In calves, the esophageal grooves allows milk to bypass the rumen and directly enter the abomasum. Rumen development occurs following a change in diet and microbial growth.