If the World Was Vegan

Imagine that, starting tomorrow, all eight billion people stopped eating meat, dairy, eggs and fish. The farms would still be there, the fields would still be planted, and the animals would still be alive. But the question for every farmer, and for the planet, would change overnight: what should this land now grow, and what happens to the land that no longer grows anything?

This is not a prediction. No society has ever made such a shift, and it is not likely to happen suddenly. But it is a useful thought experiment, because it forces a clear look at how food systems actually work. It raises three separate questions:

1. Could agriculture produce enough food?
2. Could the land stay fertile without animal manure?
3. What would happen to ecosystems, wildlife and the land left over?

The honest answer is that the first question has a fairly strong answer, the second has a manageable but real set of challenges, and the third depends heavily on choices humans would make afterwards.

The Energy Middle-Man

Start with a simple observation from ecology. When a cow eats grass, most of the energy in that grass does not become cow. It is used to keep the animal warm, to move, to digest, to grow bones and organs that people do not eat, or it leaves as manure and methane. Only a small fraction ends up as meat or milk.

Ecologists noticed this pattern long before the modern diet debate. In 1942 Raymond Lindeman described how energy passes up a food chain, from plants to herbivores to predators, and how much is lost at each step. ^[1] From work like his came a well-known rule of thumb: roughly 10% of the energy at one level of a food chain reaches the next. The real figure varies a great deal between species and conditions, but the principle is solid. Every extra link in a food chain costs energy.

Livestock farming adds exactly such a link. Instead of people eating crops, animals eat crops (or grass), and people eat the animals. The animal becomes an energy middle-man.

How large is the loss?

For animals fed on crops, the losses can be measured directly. A 2016 study of the United States found that, averaged over all livestock, about 7–8% of the calories and protein in feed end up in human food. Beef was the least efficient at around 3%. Poultry, eggs and dairy did considerably better. ^[2] Our World in Data summarizes a similar figure for beef globally: about 2 kilocalories of beef for every 100 kilocalories of feed. ^[3]

At the scale of the whole world, a University of Minnesota study estimated that 36% of the calories produced by crops are fed to animals, but only about 12% of those feed calories return to people as meat and dairy. If crops currently used for feed and biofuels went directly to people, available food calories could rise by around 70%, enough in principle for about four billion more people. ^[4]

A broader accounting of all losses in the food system, from the field to the plate, found that people eat only about 6% of the dry plant biomass grown on agricultural land. The single largest loss is livestock production. ^[5]

The formal picture

The middle-man effect can be written as a simple relationship. If an animal converts feed energy into food energy with an efficiency $\eta$ (a number between 0 and 1), then:

where $E_{\text{feed}}$ is the energy in what the animal eats and $E_{\text{food}}$ is the energy in the meat, milk or eggs produced. With $\eta = 0.03$, as for beef in the US study, about 33 calories of feed are needed per calorie of beef. With $\eta = 0.1$, the figure is 10.

This is the heart of the vegan land-efficiency argument. It is well supported for animals fed on crops that people could have eaten. But the equation hides an important question: what, exactly, are the animals eating?

The Counterargument: What Livestock Actually Eat

In 2017 researchers at the UN Food and Agriculture Organization (FAO) published a detailed analysis of what the world's livestock eat. They found that about 86% of livestock feed, by dry weight, is material people cannot or do not eat: grass and leaves, crop residues such as straw and stalks, and by-products such as the pulp left after making sugar or oil. Grains made up about 13% of the global livestock diet. On their accounting, producing a kilogram of meat requires on average about 2.8 kg of human-edible feed in ruminant systems (cattle, sheep, goats) and 3.2 kg in pigs and poultry. ^[6]

The same study noted that about 77% of the 2.5 billion hectares used for livestock is grassland, and much of it could not be converted to cropland because it is too dry, steep, rocky or cold. ^[6] Our World in Data similarly reports that roughly two-thirds of pasture is unsuitable for crops. ^[3]

This changes the equation. Split the feed into two parts: food that people could eat ($C_{\text{edible}}$) and material they cannot ($C_{\text{inedible}}$). Today, the food supply from animals is:

If animals are removed and the edible feed is eaten directly by people, the net change in food supply is:

In words: a vegan world gains the edible feed that no longer passes through an inefficient animal, but loses the food that animals used to make from grass, straw and waste. Whether the result is positive depends on how much of the feed was edible. For grain-fed pigs, poultry and feedlot cattle, the first term dominates and the gain is large. For sheep grazing a mountainside, the first term is close to zero and the result is a small loss of food.

Both sides of the debate are therefore partly right. The middle-man argument is strong for crop-fed animals. The counterargument is strong for animals on land and feed that people cannot use.

There is one more twist. Land that cannot grow crops is not useless if it stops feeding animals. It could return to forest, scrub or wild grassland and store carbon. One study estimated that shifting global food production towards plant-based diets by 2050 could allow natural vegetation to regrow on former farmland and remove 332–547 billion tonnes of carbon dioxide from the atmosphere. ^[7] The value of marginal grazing land therefore depends on what one compares it with: no food at all, or a restored ecosystem.

Could Agriculture Feed Everyone?

The land arithmetic

About half of the world's habitable land is used for agriculture. Livestock, counting both grazing land and cropland grown for feed, occupies close to 80% of that agricultural land, yet meat, dairy and farmed fish provide only about 17–18% of the world's calories and 37–38% of its protein. ^[8,9]

The most comprehensive analysis to date, by Joseph Poore and Thomas Nemecek, consolidated data from about 38,700 farms in 119 countries. They found that even the lowest-impact animal products usually have larger environmental impacts than plant-based substitutes. ^[8] Using their data, removing animal products from the global diet would reduce the land needed for food by about three-quarters, from roughly 4 billion hectares to about 1 billion. ^[3]

A key point often missed is that this vegan world would need less cropland than exists today, not more, because so much current cropland grows animal feed. The fields that now grow maize and soy for livestock could grow food for people instead.

What studies suggest

Several independent analyses reach similar conclusions on food quantity:

• Replacing all animal products in the US diet with nutritionally comparable plant foods would, on one estimate, free enough cropland to feed around 350 million additional people. ^[10]
• A global modelling study found that a vegan diet produced the largest reductions in food-related greenhouse gas emissions (up to about 70% by 2050) and among the largest reductions in diet-related deaths (about 10%), compared with continuing current trends. ^[11]

On calories and protein alone, the evidence is fairly strong: global agriculture could feed everyone on plant-based diets, using less land than today. This is an accepted conclusion of the modelling literature, not a speculative claim. But models are not the real world, and the conclusion comes with conditions.

The conditions

• The crop mix would have to change. Much more land would grow pulses, vegetables, fruit, nuts and oilseeds, and less would grow feed maize and soy. This requires new supply chains, processing and farmer knowledge.
• Regions differ. Some countries could not grow a full plant-based diet locally and would depend more on trade.
• Livelihoods matter. In many dry regions, herding animals on land unsuitable for crops is the main source of food and income. For these people, the loss of livestock would not be offset by fields that cannot exist there.

Is zero animals the land-use optimum?

Several studies argue that the diet using the least land is not strictly vegan, but contains a small amount of animal food made from leftovers.

• A study of the Netherlands found that land use per person was lowest when around 12% of dietary protein came from animals, which were fed only on by-products and grass from land unsuited to crops. ^[12]
• A global review estimated that livestock fed only on leftovers, by-products and grass could supply about 9–23 grams of protein per person per day, a meaningful share of the 50–60 grams people need. Compared with no animal foods, such a diet could free about a quarter of global arable land. ^[13]
• A global model in which animals were fed only grass and by-products found it could provide enough food for 2050 while cutting arable land use by 26%, but only with a 71% reduction in the protein people get from livestock. ^[14]

These are model results, and they depend on assumptions about how much grassland would otherwise be used. But they point to a consistent pattern: the land-efficient diet is overwhelmingly plant-based, with at most a small animal component fed on things people cannot eat. Current Western diets are far from this point; the gap between today's diet and either a vegan or a "circular" diet is much larger than the gap between those two.

Nutrition: Enough Food Is Not the Same as Enough Nutrients

Producing enough calories and protein is only part of feeding people. The diet also has to provide vitamins, minerals and essential fats.

A 2017 study modelled the complete removal of animals from US agriculture. It found that a plants-only system would produce 23% more food, but when the authors built the cheapest possible diets from what was produced, those diets met fewer nutrient requirements. Agricultural greenhouse gas emissions fell by 28%. ^[15] The study was widely cited, and also widely criticized. Other researchers argued that its scenario was unrealistic, for example because the model kept growing crops suited to animal feed and used an optimization method that picked poor diets, and that a thoughtfully planned plant-based food system would not face the same shortfalls. ^[16] The disagreement is less about whether vegan diets can be adequate and more about whether a rapid, unplanned switch would produce adequate diets in practice.

The clearest case is vitamin B12. It is made only by microorganisms, not by plants or animals, and animal foods are the main natural dietary source. A review of studies measuring B12 status found high rates of deficiency among vegetarians and especially vegans, across ages and regions, and concluded that supplements are needed. ^[17]

Professional bodies take a consistent position: appropriately planned vegetarian and vegan diets are healthful and can be suitable for all stages of life, provided attention is paid to nutrients such as B12, iron, zinc, iodine, calcium, vitamin D and omega-3 fats. ^[18] The EAT–Lancet Commission's reference diet for human and planetary health is not vegan but is mostly plant-based, allowing small amounts of animal foods and noting their value for groups with high nutrient needs, such as young children in low-income settings. ^[19]

Summary: a vegan world is nutritionally feasible with B12 supplementation or fortification and careful food planning. It would require a deliberate public health effort, not merely a change in farm output.

Could the Land Stay Fertile?

This is where the scenario becomes genuinely complicated.

Why fertility needs replacing

Every harvest removes nutrients from the soil, mainly nitrogen, phosphorus and potassium. They must be returned, or the soil gradually becomes less productive. For thousands of years, animals helped do this. They grazed on land that was not cropped, and their manure carried nutrients back to the fields.

It is important to be precise about what manure does. Manure recycles nutrients; it does not create them (except for nitrogen that legumes in animal forage pull from the air). The nitrogen and phosphorus in manure came from the feed the animals ate. Removing animals removes a transport and recycling system, not a source of new fertility.

Where fertility would come from

Modern agriculture already depends mostly on non-animal nutrient sources. The industrial Haber–Bosch process, which turns nitrogen from the air into fertilizer, is estimated to have supported the food supply of nearly half the world's population by 2008. ^[20]

In a vegan world, fertility would come from a combination of:

• Synthetic nitrogen fertilizer, which is effective but energy-intensive and a source of pollution when overused.
• Legumes. Beans, lentils, peas, clover and similar crops host bacteria in their roots that fix nitrogen from the air. A plant-based diet relies heavily on such crops, so part of the fertility problem partly solves itself.
• Green manures and cover crops, grown to be ploughed back into the soil.
• Compost from crop residues and food waste.
• Recycled human waste, which contains most of the nutrients people eat.

At the same time, a vegan world would need less cropland overall, and no longer fertilize crops grown for animals. Livestock and crop systems are currently the largest cause of human disruption to the global nitrogen and phosphorus cycles, with large nutrient surpluses leaking into rivers, lakes and coastal seas. ^[21] In the grass-and-by-product feeding model described above, nitrogen and phosphorus surpluses fell by 46% and 40%. ^[14] Removing livestock would not automatically solve nutrient pollution, but it would remove one of its largest sources.

The phosphorus problem

Phosphorus is the harder case. Unlike nitrogen, it cannot be drawn from the air. Most phosphorus fertilizer comes from mined phosphate rock, a finite resource concentrated in a few countries. ^[22] Manure currently returns a large share of phosphorus to farmland. Without animals, closing the phosphorus loop would depend much more on recycling human excreta and food waste, which most sanitation systems today are not designed to do.

Soil organic matter

Fertility is not only about nutrients. Healthy soil holds organic matter, which improves its structure, water retention and biological life. Grass leys (fields rotated into grass for a few years) and manure are traditional ways of building organic matter. Stockless systems can use cover crops, deep-rooted green manures and crop residues instead, and such farms exist. However, evidence on how well they maintain soil organic matter across all climates and soil types, at the scale of global agriculture, is still limited.

Assessment: preventing soil degradation in a vegan world is technically plausible but not automatic. It would require deliberate management, better nutrient recycling, and probably continued use of synthetic fertilizers. The claim that land would inevitably become infertile without animals is not supported; the claim that it would stay fertile without effort is equally unsupported.

What Would Happen to Ecosystems?

The animals themselves

Livestock are not a natural part of wild ecosystems. By mass, the world's farmed mammals, about 630 million tonnes, outweigh all wild land mammals, estimated at about 20 million tonnes, by roughly thirty to one. ^[23] In any realistic transition, farm animals would not be released. Fewer would be bred, and their numbers would decline over years. Some breeds kept only for food might become rare.

The land left over

The largest ecological effect would come from the roughly three billion hectares of land no longer needed. ^[3] If much of it were allowed to return to natural vegetation, the gains for carbon storage could be very large. For high-income countries alone, one estimate put the potential at nearly 100 billion tonnes of carbon dioxide equivalent, about 14 years of current global agricultural emissions. ^[24] Such land could also become habitat for species that have lost ground to agriculture.

However, "abandoned" is not the same as "restored." Some grasslands are semi-natural ecosystems shaped by thousands of years of grazing, and they support distinctive plants, insects and birds. Grasslands provide many services beyond food, including carbon storage, water regulation and pollination, and in some places these depend on continued grazing or other management. ^[25] Without grazing, some of these areas would become scrub or forest, which would benefit some species and harm others. In fire-prone regions, uncontrolled regrowth can increase wildfire risk. Conservation grazing using wild or semi-wild herbivores, or animals kept for land management rather than food, could fill some of this role. How much of that is desirable is partly a scientific question and partly a question of values.

Wildlife in crop fields

A common objection is that plant agriculture also kills animals: harvesting, ploughing and pest control kill mice, rabbits, birds and insects. This is true. A careful review concluded that very little is known about the scale of these deaths, and that current estimates are too uncertain to settle whether vegan diets cause more or fewer animal deaths than some mixed diets. ^[26] One point does follow from the land arithmetic above: because a vegan world would use far less cropland in total, it would probably involve less of the field work that causes such deaths, not more. This is an inference, not a measured result.

Ecosystem imbalance

Could removing livestock unbalance ecosystems? For wild ecosystems, the direct effect would mostly be positive, since livestock compete with wild herbivores for grazing and often drive habitat loss. The risks are more local: loss of grazing-dependent habitats, possible overpopulation of wild herbivores in areas without predators, and disruption in regions whose landscapes and economies are built around pastoralism. These are real management problems, but there is no evidence that the absence of livestock would cause a general ecological collapse.

The broader ethical motivations behind veganism, such as animal suffering, are a separate question, touched on in Suffering. This article addresses only the physical feasibility of the scenario.

Limitations of the Scenario

This analysis is based on models and on current data, and it deliberately ignores much of the real world:

• Speed. A sudden global shift would disrupt markets, jobs and supply chains far more than a gradual one. Most studies model gradual change towards 2050.
• Economics and culture. Livestock provide income, savings, draught power, fuel, leather and wool for hundreds of millions of people. Food is also deeply tied to identity and tradition.
• By-products. In the US alone, livestock recycle over 43 billion kilograms of human-inedible by-products each year, and their manure supplies about 4 billion kilograms of nitrogen. ^[15] These materials would need other uses or disposal routes.
• Oceans. This article focuses on land. Removing fishing and aquaculture raises separate ecological and nutritional questions.
• Model assumptions. Results depend on assumed yields, waste levels, diet composition and what happens to spared land. Different, reasonable assumptions give noticeably different numbers, though rarely a different direction.

What the Scenario Reveals

The vegan-world scenario is useful less as a policy proposal than as a way of revealing how the food system works.

Animals fed on crops convert plants into human food inefficiently, and livestock use most of the world's farmland while supplying a minority of its calories and protein. On calorie and protein arithmetic, a plant-based world could feed everyone with far less land.

Models can also produce nutritionally adequate vegan diets and fertile soils, but only with deliberate B12 supplementation, diet planning, nutrient recycling, legumes and synthetic fertilizer. Some find that a small livestock sector fed only on grass and waste could use land slightly more efficiently than eliminating animals altogether.

The arithmetic cannot decide what happens to the billions of freed hectares, how phosphorus is recycled, how grazing-dependent ecosystems and herding communities are protected, or how a transition could be made fair.

The most robust conclusion is not that the world must become vegan, nor that it cannot. It is that the current balance, with high meat consumption fed largely by crops, is the least efficient part of the system, and that both the vegan ideal and its strongest critics agree that moving away from it would free land, reduce pollution and feed more people.

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