Squishy, your train of thought is like that of a biologist with common sense. I have had similar discussions with my students trying to impress upon them that many breeds kept to day are higher maintenance and lack the ability to thrive on lower quality forages of their ancestors, even though sometimes breeds appear to be the same. Another key concern how many acres is required per animal. This varies with season, locality and often with management system employed. What type of grazing system and what kind of protection from elements are also factors influencing forage abundance and nutritional requirments. All said and done, one needs to estimate much lower production levels than typical with importations (to farm) of feeds, fertilizers and energy today.