Cattle feed is more than grass

depends on the phase of development. Most beef is pasture fed for a significant period, maybe supplemented with extra feed in the winter, but still grass, etc. It's only near the relatively short end they are consolidated & finished at the feed lot.

Correct, but many lands like parts of Alberta and Texas are not very good for growing foods humans can eat. So you may as well get marginal additional calories out of the non-productive land via cellulose digesters.