Limits to sustainable animal consumption

Vegans rightly point out that meat-containing diets require more land (and thus more deforestation) since feeding animals is a net loss of protein and calories. But...

Pro-meat folks rightly point out that some land is only suited for grazing animals.

This raises a more important question:

If farm animals only ever ate...

• grasses and other plants from pasture & range (...)( especially from biodiverse natural lands that keep a lot of carbon stored in the roots of plants ), and
• the parts of food crops that humans can't eat (...)( for example corn cobs after the kernels have been removed (ruminants can digest certain kinds of fibrous matter that other species can't) ),

and never...

• human-edible food crops (...)( including corn and soy as those could be made into flour ) (...)( Grey area: Perhaps also include "spent grains" and "fryer waste oil" because the original materials were human-edible, and the downgrading was a choice ), nor
• crops grown specifically for animal feed (...)( Grey area: Perhaps also include the more superficial types of "pasture" that are monoculture-like and don't keep a lot of permanent roots. The exact definition can be specified in the answer to the question. ),

how much animal protein (...)( from meat, milk and/or eggs etc ) could be produced?

Clearly it's less than the status quo, as there are fewer sources of feed. But how much less?

Analysis in progress - someone is working on this research.
Results will be posted here soon ♥