As one particularly loud farmer I'm familiar with is fond of saying, there are certain foods that should only be eaten if you have a gizzard.

Gizzards can't process all of the things a ruminant can, but there's some overlap.

The thing is chickens aren't vegetarians. They're omnivores. We'd be better off if we processed the leftovers through invertibrates (worms, soldier flies) and then fed the invertibrates to the chickens. Same for pigs. And both pathways would still be more efficient than cows.