We can never meet this demand by only grazing animals on pasture. The alternative is to feed them things that humans cannot eat. In the European Union, it is illegal to use sewage sludge in animal feed. Last year, five French rendering plants were fined for hosing down their floors, filtering out bits of carcass that had ended up in the waste-water treatment system, and putting them back in the pot. Toilets at the plant flushed into the same system but the results were sterilised.
Some animals eat excrement naturally and pigs are still fed it on some cattle farms.
True, the BSE crisis showed us what happens when a pathogen recycled in feed isn't killed by sterilisation. We need precautions, such as not feeding a species its own waste. If you stay alert to dangers to human health, then feeding sewage to animals is no more disgusting than the business of slaughtering them. Having killed the beast, we ought to use as much of it as we can. We should, of course, do what we can to make animal feed safer.