system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can horses and cows eat grass alone yet still pack on muscles while humans need more than just vegetables to bulk up?
gpt
Grass has plenty of proteins. The problem, for humans at least, is that the cell walls are cellulose which the human digestive system can't break down. Cows have several stomachs. One of them called the rumen has a wider variety of bacteria to help break down cellulose. Cows also regurgitate what they eat and then chew it some more, which is called ruminating (animals that do this are called ruminants).

Horses are not ruminants. They have single chambered stomachs, but also an enlarged and specialized cecum in the bowels where plant matter is fermented by bacteria and broken down.