All meat cattle in the US are fed grass at some point in their lives*, it's what they are finished on that matters. They can legally say grass-fed on their website even if they finish on organic corn, but it's illegal to put grass-fed on their packaging if they do that. Considering that "grass-fed" is a value-added term, if it doesn't have grass-fed on the package, it's probably been finished on grain, even if it's organic grain and even if it's not in a feedlot. You can also finish bison on grain, it's quite common these days. You don't need to force-feed them, all ruminants will eat crap just like humans will if you put it in front of them.
*some dairy cattle never see the light of day and some do become hamburger
