There is no one answer to you question. In general when creating pasture I have found it is almost always best to work with what's there, and build on that rather than tearing it out. Mow it down low and interseed a grass/legume mix with a no-till drill has worked here.

Turning sprayed corn and bean ground into productive mixed grass/legume sod pasture takes years. Cattle don't care about a few rocks and stumps. Smaller branches, stumps and roots will gradually disappear. This is for grazing. Maybe graze and mow for a few years then think hay on just the smoother parts of the existing field.

Good luck