It wouldn't matter how environmentally unfriendly it was to cultivate land as long as livestock are fed cultivated crops, since they require more than one pound of food to produce one pound of meat.

If x = environmental impact of cultivating crops

then R*x = environmental impact of raising livestock

where R = pounds of feed required to produce one pound of meat

Now if we raised our meat differently, such as only on grass-fed pastures, then things might look a lot different. But in the US at least, factory farming and animal feed is the norm.

I'm not too fond of factory farming, but that has little to do with veganism. In any case, your math would be correct only if 1 pound of vegetables consumed by a person is the same as one pound of meat. I guess even vegetarians would admit that's wrong (I think humanoids started eating meat because it was the only thing providing enough concentrated nutrition to support a massive brain, at least without eating all day long).

Moreover, it's quite conceivable (again, no proof, but there's little proof af anything in this field) that herbivores would convert one pound of corn much more efficiently than we would. This might mean that humans are better off wasting energy by feeding vegetables to animals and then eating the animals, than eating the vegetables directly. Maybe we can't extract the nutrients from the vegetables as well as herbivores do, so that seemingly wasted energy is actually a net gain.

Energy is always lost as you move up the food chain, because no predator can capture 100% of the energy stored in their prey.

Consuming plants is not as efficient as photosynthesizing, and consuming animals is not as efficient as consuming plants.

In this case, the cows burn energy from the food they consume for as long as they're alive, meaning it's a physical impossibility for the meat produced from them to contain as much energy as the food they consumed.

Unless you're claiming that raising livestock violates the laws of physics, but that's an entirely different argument.

No, he's suggesting that it's possible humans digest plants much less efficiently than herbivores do, and less efficiently than humans digest meat. E.g., if the herbivore can capture 90% of the energy in a plant, and we can capture 80% of the energy in the herbivore, that may be a better deal than if we can only capture 10% of the energy in the plant directly. In the case of cows eating grass, that doesn't sound that far-fetched. They have the benefit of multiple stomachs evolved to break down fibers, and we don't. Surely you don't think that every animal digests plants more efficiently than they digest meat? What about obligate carnivores, which can extract roughly 0% of the energy from plants?