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> If so, this seems self-evidently false: all edible calories start out as plant calories, and we can either eat the plants or have animals eat the plants (wasting a bunch of the input energy along the way) and then eat the animals.

I'm no expert but I believe there may actually be some limited basis to that claim: ruminants can eat things we can't (grass) and there are areas where it's more ecological to grow grass and let ruminants eat it than try to grow things that we could process. You also get the benefit of better topsoil with the help of the animal dung.

But I think the (huge) majority of the meat we eat is fed grain, where we're obviously wasting energy.

Like you say, the fact that they're mostly grain-fed makes the grass-fed angle largely moot, but in a modern context, surely if it were for some reason necessary for humans to derive calories from grass, there would be more efficient ways to do so than by than by feeding them to a cow over a several-month-long period and then eating the cow (e.g., enzymatically or through fermentation).

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