Grind up poor people and use the paste to fertilize a new crop of grain, with which to feed our cattle. Or just feed the paste directly to the cows.

It's the only way.

Spoiler Alert, click show to read: 
Yes, yes... I know that it's more energy efficient to eat, well, bacteria or something - all that stuff about 90% energy being lost with each step in the food chain. Actually. We should breed photosynthesising humans, if we want to maximise resoure efficiency. Some people are on the way there anyway...