GRASS-fed is the way to go. I checked with S3Meats, in Kansas, from where I have purchased grass-fed, grass-finished (NO CORN because leaving it out means their herd hasn't seen a vet in five years). Here's what they replied to me this morning:

"We dont feed our cattle, they graze cover crops on our fields. If those rations are limited we will give them hay that is directly harvested from our other fields.

Thanks for asking."

Their fields are so pristine, I suggested they make honey, too, but it is a small, family-run enterprise. The father did the delivery and he knows exactly what's NOT in his cattle. Clean Operation, all the way.