[earnon]

What if the animal ate only grass?

Also, keep in mind that modern grain agriculture is 100% dependent on fossil fuels.

Back in the day, animals were raised on the farm along with vegetables. The animals ate the cellulose (inedible to us) and scraps and created protein. They also created fertilizer (today it is synthesized from fossil fuels and wreaks environmental havoc - the Dead Zone at the mouth of the Mississippi is mostly caused by nitrogen runoff from farms) and ate bugs, so there was no need for pesticides.

[zzzeek]

I can't cite an authoritative source but I think the idea is the amount of unforested, undeveloped grassland on the planet is nowhere near enough to support the current meat intake of the world population. There are actually similar issues with organic vegetables too.

Of course if people wanted to eat much less meat, and if there were a lot fewer people, things might work better sure.

[earnon]

That's probably true. However, like I said above, our agriculture is also unsustainable. Most of the increase in crop yields over the past few decades have come as a result of fossil fuel-based fertilizers. Without oil yields would plummet. Overpopulation is the problem.

Ideally, animals should be raised either on land that's not suitable for agriculture, or on the farm, eating the inedible plant parts and providing more of a closed-loop ecosystem.