I do not believe this is true. In fact, I feel the opposite is true. Raising cattle is not a closed system. You have to invest far more calories in the creature than you will ultimately harvest from it.
Now, I am not a strict vegetarian by any means. I eat fish I catch myself. I eat the deer and boar that my father hunts. And I do eat cattle that are properly grazed by honest to god cattle ranchers, people for whom I have a great deal of respect.
I do not eat corn-fed beef. And most beef is corn fed. I think it is a complete waste of corn.