How do rumen microbes help an animal use plant fiber?

A cow can obtain useful food from a mouthful of tough grass with help from tiny living partners. Many of these microbes live in the rumen, one compartment of the cow's stomach. The cow swallows the plant material, but it cannot carry out every part of breaking it down on its own.

Plant fiber contains large molecules that resist the cow's own digestive processes. Rumen microbes supply enzymes that help break down parts of that fiber. They also carry out fermentation, chemical processes that change the resulting food substances into other compounds.

Some fermentation products are small compounds called fatty acids. These can be absorbed and used by the animal, including as an energy source. There are several steps between grass and a cow's movement or growth: plant material enters the rumen, microbes change it, and useful products become available for absorption.

The microbes benefit too. The cow brings them a continuing supply of material and provides a place to live. This partnership lets the animal use much fibrous food that would otherwise be difficult for it to use. It does not make every plant component digestible, but it connects the chemistry of tiny organisms to the feeding of a much larger one.