Industrial agriculture doesn't rely on naturally occurring grass, corn and soybeans are also used, and feed is grown for animals using similar methods as would be used growing food for human consumption. Relying on more sustainable methods would still require a drastic reduction in meat consumption.

Also, the point I was responding to was, "What about the poor tomato plants?" If we're still pretending to do the "plants rights activist" thing, then I don't see why grass would be more acceptable than tomato plants.

Note that in the US, animals are also allowed to be fed feces as a cost-cutting measure, in spite the health risks

It's not required, but it does add decent fertilizer so I'm sure it's preferable.