Then they're sent to a feed lot for some period of time to fatten up.

Ah, from the article you linked: "However, comparisons of FCR among different species may be of little significance unless the feeds involved are of similar quality and suitability."

So cattle might be more efficient than appears at first glance, since as ruminants they're initially fed stuff you can't feed other animals (ADDED: including humans), especially "on marginal land where agriculture isn't ideal", or perhaps as part of a crop rotation cycle. Alfalfa is a 3-for, a rotation cycle that's not e.g. corn or soybeans, a nitrogen fixer, and tasty food for cows.

And all the above isn't the best for our topic at hand, for that alfalfa and such isn't stuff we can digest, nor is silage from corn leftovers, so it mostly feedlot cow food we could redirect to humans.