Most commercial cattle aren't pasture fed, they aren't free ranging it in a field eating whatever they forage. Hobby and small farms, sure, but the operations that raise the most heads the cattle generally spend the bulk of their life standing in a dirt pen (a feedlot) eating from a trough. They're then fed a mix of corn, soy, ruffage (grasses).

Here is an example of a feedlot https://www.bilaterals.org/IMG/jpg/33a3ab67aa6bbe3184c0441d2...

The cattle come to a side and eat from troughs like this https://cdn.farmjournal.com/s3fs-public/styles/840x600/publi...

It takes 2.5~ pounds of grain and 1,850~ gallons of water to produce 1 pound of beef.