Calorie Expansion and Compression
Fertilizer multiplies the calories a field grows. Feeding them to animals hands most of them back.
Two forces act on the world’s food supply between the field and the plate, and they pull in opposite directions. The first is expansion: fertilizer, mostly nitrogen made from air and natural gas, multiplies what an acre yields. The second is compression: roughly a third of the calories that come off the world’s fields are fed to animals, and most of them are burned as body heat and movement rather than stored as meat, milk or eggs. The size of each, and where the second one is smaller than it looks, explains a good deal about who buys corn and soybeans and why.
Expansion: what fertilizer adds
The longest-running answer is a wheat field in Hertfordshire. Broadbalk, at Rothamsted, has been sown to wheat every year since 1843, and the experiment was paid for by John Bennet Lawes, whose fortune came from patenting superphosphate in 1842. The strip that has never received fertilizer or manure still yields about 1 tonne a hectare, close to what it gave Lawes. Plots on the same field given full nitrogen, phosphorus and potassium gave 6.6 tonnes in the 1980s, and plots rotated with other crops now give over 10, with modern varieties that can use the extra nitrogen.
Across farming as a whole the effect is smaller than Broadbalk’s, because few fields have gone a century without nutrients, but it is still large. A review of long-term trials in the United States and England (Stewart and colleagues, 2005) attributed at least 30 to 50 percent of crop yield to commercial fertilizer. Vaclav Smil and Jan Willem Erisman put the population it supports at about half of everyone alive, which is consistent with it. The fertilizer family covers how that nitrogen, phosphate and potash are made and traded.
Compression: what feeding gives back
The fullest global accounting (Cassidy, West, Gerber and Foley, 2013, covering 41 crops) found that 55 percent of crop calories are eaten by people directly, 36 percent are fed to animals, and 9 percent go to biofuels and industry. Of the calories fed to animals, about 12 percent come back as food. So the 36 percent of crop calories that enter a feed trough returns about 4 percent of the total to the human diet, and roughly a third of everything the world grows is spent keeping livestock warm and moving. The same study estimated that sending those calories to people directly would raise the food supply by about 70 percent, enough in principle for around four billion more people.
The loss varies more than tenfold by animal. A beef animal is large and slow-growing, and most of what it eats goes to maintaining its own body over two years or so before slaughter, while a laying hen of under 2 kilograms produces about ten times her own weight in eggs each year. Measured against everything the animal eats:
| Product | Calories kept per 100 calories of feed |
|---|---|
| Milk | 24 |
| Eggs | 19 |
| Poultry | 13 |
| Pork | 8.6 |
| Lamb and mutton | 4.4 |
| Beef | 1.9 |
Energy conversion efficiency, global averages, counting all feed including grass and crop residues. Source: Alexander et al. (2016), Global Environmental Change, via Our World in Data. A separate US study (Shepon, Eshel and colleagues, 2016) puts the average across US livestock at 7 to 8 percent, with beef at 3 percent for both calories and protein.
Has it changed?
Less than the growth of meat eating would suggest. Feed took about 42 percent of world cereal use in 1961, peaked near 49 percent in the early 1970s, drifted down through the 1990s and 2000s, and sits near 40 percent today. The tonnage fed to animals has grown enormously over the period, but cereal output grew alongside it, so the share has stayed in a band. Over the same years US ethanol took a rising share of the corn crop, which competes for the same bushels.
Share of World Cereals Fed to Animals
percent of cereal use; Share of total world cereal use allocated to animal feed, 1961 to 2023. Source: FAO food balance sheets via Our World in Data. The step down at 2010 coincides with FAO changing its balance-sheet method, so part of it is a break in the series rather than a change in what animals were fed.
Why the luxury framing overstates it
Calling feed a luxury use assumes every calorie a cow eats could have fed a person, and most of them could not. The FAO’s own accounting (Mottet and colleagues, 2017) puts world livestock feed at about 6 billion tonnes of dry matter a year, of which 86 percent is material people do not eat: grass, crop residues such as straw and stover, and by-products such as soymeal and distillers’ grains, the protein left after the oil or the ethanol has been taken out. Livestock in that study eat about a third of world cereals. Counting only the feed a person could have eaten, a kilogram of boneless meat takes about 3 kilograms of edible feed, 2.8 for cattle and sheep and 3.2 for pigs and poultry.
That is how beef can sit at the bottom of the table above and level with pork and chicken here. A grazing animal turns grass on land too steep, dry or poor to plough into food a person can eat, which adds calories to the human supply rather than taking them away. The waste concentrates in one place: grain and soymeal fed to animals in feedlots and sheds, which is where most of the world’s corn and nearly all of its soybean meal ends up.
What it means for the grain market
Feed is the largest single claim on corn and almost the only claim on soybean meal, so the meat in a diet is a leveraged bet on grain. A household that adds a kilogram of pork to its diet adds several kilograms of feed demand. China’s pig herd is the largest example: China imports about 100 million tonnes of soybeans a year, about three fifths of world soybean trade, overwhelmingly to crush for pig and poultry feed. The ratios on this page are the multiplier between a change in diet and a change in grain demand, so the mix of meat matters as much as the amount: poultry has been the fastest-growing meat for decades, and it turns feed into food about seven times as efficiently as beef.