Chickens eat the same basic diet as humans, i.e. mostly grains and legumes with sources of vitamins and minerals, while otherwise foraging for the rest. High-tech nutritionally-complete chicken feeds are important for factory-farmed chickens as they can’t forage. Traditionally, however, chicken feed was more supplementary, consisting of grain, scraps, and calcium.
Chickens tend to eat around an eighth of what a human eats, or around 125g of feed per day. As chicken feed is basically grain, this would require around 170m2 to produce organically, or one twenty-fourth of an acre.
With an average weight of 3kg, and an average meat yield of 65%, each bird will then produce around 2kg of meat and 44g of liver. Assuming a ‘harvest’ every 12 weeks, each twenty-fourth will therefore produce around 8.6kg of meat per year and 190g of liver.
As there is around 6.9μg of B12 in the meat of a chicken and 7.4μg in the liver, each chicken contains around 14.3μg of B12, and so each twenty-fourth will produce around 61.5μg of B12 per year.
A yearly B12 requirement of 550μg would therefore translate to nine twenty-fourths of an acre, or 38 birds per year. A quarter of an acre would still be needed for the rest of the human’s nutrition.
Chickens shouldn’t be kept on the same land that is being used to grow their feed, as they would trample on it and uproot it. They would therefore need more land for pasture, where the traditional stocking density is around 50 birds an acre, or a fifth of an acre in this case. Each human would therefore need around four-fifths of an acre in total.
Feeding the world in this way would require the annual slaughter of around 300 billion birds.