True, if you limit the 3% to wild grass. But most cows are still primarily fed grass until their final few months when they are fattened on grain.
However most cattle are certainly finished on grain without a doubt. Grain that could possibly put to better uses (or not grown and save the fuel and water).
Here's another one:
https://www.reference.com/pets-animals/cows-eat-a677816334f1...
The largest number of cattle, currently, if you counted them all up, are still on pasture.
Not that this detracts from your point, mostly a minor semantic quibble.
If a group of kids were fed healthy, home-made meals all week and then get a piece of chocolate cake on Sunday night, it's clearly misleading to say "100% of these children were raised on chocolate cake!".
Statements about how "inefficient" cows are in making meat re calories ingested, usually fail to mention that for most of the cow's life they are eating grass on a range, which is not used for anything else. We didn't need to make that grass, it's sitting there. Statements on the use on land assume we would have condos and coffee shops sitting there, but for the cows on it.