system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does grass provide all the energy and nutritional needs of powerful animals like bison and horses?
gpt
Many grazing animals don't really digest the grass itself. Take a cow for instance. When a cow chews up the grass and swallows, it ends up in a stomach chamber called the rumen.

This is basically a fermentation chamber. It's filled with microbes that digest the chewed up grass. These bacteria produce all kinds of lipids (fats), proteins, and so on as byproducts of their own digestive system.

The cow then regurgitates the cud from the rumen back into it's mouth where it chews up this mash of predigested material into a much finer particulate of broken down grass, bacteria and the bacteria's beneficial byproducts.

It then swallows this cud again, this time it's much finer and more liquid. It sinks to the bottom of the rumen and instead of remaining in the rumen, it's passed on to the cow's other stomach chambers. This mass of bacteria, byproduct, and predigested grass is then properly digested by the cow.

Without the bacteria and the byproduct of the bacteria, grass wouldn't be nutritious for the cow at all. But the cow feeds the bacteria and in turn, the bacteria feed the cow.

Most grazers have a system like that. Bovines usually have multichambered stomachs while animals like horses have extra long intestines where this process takes place.