Actually grazing cattle don't kill plants, they just trim off the ends.
Depends on plant species (not all survive trimming well) and cattle density (trampling certainly kills plants). However, most meat and dairy are not sustained purely by grazing. That said, harvesting grain to feed to cattle doesn't have to kill plants either, unless we consider the embryo in a seed to have the same moral status as a mature plant.
In practice, growing grain to feed to cattle to feed to humans will involve killing a lot more weeds than growing grain to feed straight to humans.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.