Well maybe - but what percentage of a typical "grass fed" cow's calories are from the feedlot they spend the last N months of their lives in (where they are fat

Well maybe - but what percentage of a typical "grass fed" cow's calories are from the feedlot they spend the last N months of their lives in (where they are fattened up)? If humans eating plant derived food is 5x more efficient that using it to raise meat is (from memory this is low), then for your argument to carry weight, we'd want > 80%. How common is that?