First, agriculture made for animal consumption doesn't have the same quality sanitarian standards as agriculture made for humans.

Second, not all animals require vast agriculture.

We could also war animals that do not requiere such agricultural measures. I have eaten cows, pigs and chickens that were raised traditionally, out of the natural fields and the parts of the plants that we humans do not want to eat.

In fact for a environmental perspective traditional farming is far better than vegan diet.