This is not exactly correct. The "grain" that is fed to animals in feed lots (or most other places) is typically not straight corn or whatever. It's an engineer

This is not exactly correct. The "grain" that is fed to animals in feed lots (or most other places) is typically not straight corn or whatever. It's an engineered pellet ration or a milled grain ration. In both cases it has additives in it - added nutritional components, and often also low-level antibiotics, just depending on what the folks running the feed lot prefer. I have a sheep farm. We have to be very careful not to confuse the sheep grain ration with the chicken feed. They look exactly the same, but chicken pellet has copper in it, and sheep can't metabolize copper. We also have to be careful not to accidentally buy or feed goat ration for the same reason. That said, "grass fed" is complicated too. In a lot of places the cattle are grazing on unimproved rangeland. It isn't even fertilized. But in some places the farmers will explicitly plant forage crops such as wheatgrass; these will typically be fertilized, and possibly treated with herbicide or pesticide. Fertilizer is usually an industrial nitrogen fertilizer, but it can also be composted manure. I know of at least some farms near where I grew up that were at points treated with concentrated sludge from sewage treatment plants, in which case you would definitely expect to get heavy metal accumulation, even in straight grass fed animals. Happily my family never went that route.