Why can certain omnivores digest grass but not humans?

Rats cannot digest grass any more than we can.

Eating is not the same as digesting.

Humans eat grass-like stuff all the time - scallions, lettuce, etc.,

but we only DIGEST part of it.

The rest passes right through our digestive system.

And this undigestible material is necessary to physically stimulate our digestive tract.

This is why cats or rats sometimes eat grass - it cleans them out and makes them feel better

All plant material contains lots of cellulose - that's the "fiber" that we hear about all the time.

Cellulose makes up the cell walls of all plant cells. Wood is almost pure cellulose.

But no animal is able to produce the enzymes that break down cellulose.

The only organisms that can produce those enzymes are some microorganisms and fungi.

The guts of cows and other grass-eaters contain some of those microorganisms that digest the cellulose for them. Likewise termites, who consume wood, have similar microorganisms living in their gut, digesting the cellulose for them.

Being able to digest cellulose means that these animals can get their energy supply from cellulose (which we cannot do), so they can live on materials made up mostly of cellulose - such as grass or even wood..

Well, in order to digest grass you would need to be able to break down the cellulose in the cell walls of the plant cells. The only thing that can break down the cellulose is an enzyme called cellulase. Animals like cows have microbes in their stomach that can produce cellulase to digest all the grass they eat. We don't have the microbes needed so we cannot digest any cellulose... Hope this answers your question! :)