If we turned all of global beef demand into grass-fed, we’d literally decimate our output because there’s not enough grazable land.

Even land categorized as agriculturally grazable is often not realistically grazable (think mountains, etc)

Fun fact: the Native Americans that originally created the various and sundry types of corn that we have called themselves, "Walking Maize People." We've analyzed their bones and found that the specific type of carbon that corn "tags" as its own ion, made up about 30-40% of the carbon in their bones, and presumably their bodies.

Due to the fact that corn is added to almost everything that is in the US food chain, when similar analysis has been done to average US citizens, more like 60-70% of the carbon in our bodies comes from corn. We "paint" fruits and veggies with corn, we add corn as sugar to all soda, we add corn to some breads for no reason. We, the citizens of the US, are walking corn.