Vegetarianism is heavily promoted. But let's say all people on Earth stop eating animal products. Can we grow enough crops so all people on the Earth are provided with enough healthy, nutritious food?
The question is of course very theoretical, but without discussing future possibilities to cultivate deserts and oceans, is there enough space to grow enough crops?
The production of meat is much less efficient than the production of the crops the animals eat. If you would use all the grain to feed people directly instead of producting meat, it has been estimated that the US could feed about 800 million people with that grain.
One paper about "Sustainability of meat-based and plant-based diets and the environment" states that
For every 1 kg of high-quality animal protein produced, livestock are fed about 6 kg of plant protein.
The production of meat is clearly less efficient than directly producing and eating plants.
So it would be much easier to feed the world population on a vegetarian diet than with a meat-rich diet.
The question is invalid, as we already produce way more than enough food. There is no more "world hunger" problem due insufficient food production. Currently much greater problem is an epidemic of obesity.
The lowest calorie intake is in sub-Saharan Africa, where on average people eat 2,176 calories per person per day. Highest in US with a average of 3,654 calories. World average is 2800 calories. (source: "EarthTrends: Nutrition: Calorie supply per capita". World Resources Institute.).
Healthy calorie intakes is defined as between 2000 and 2500, USDA recommends 2000 calories. Thus even in most "hunger stricken" zones of the world, people eat on average more than USDA recommendation. World average is way above upper limit of healthy diet.
Of course these are averages, the problem isn't production, it's distribution of goods. In the most obese nation of the world, where food industry produces about 4,000 calories per person per day, there are 30 mln people experiencing hunger and few thousands who die annually of malnutrition (source: LiveStrong). Clearly, no one in right mind would argue, that there isn't enough food in US.
You also have to take in account, that significant progress has been made in last decades. According to Global Burden of Diseases, Injuries, and Risk Factors Study, in metric of increased risk of death, child malnutrition has dropped from 6th position in 1990 to 16th in 2010, with a overall risk drop of -62%, on the other hand obesity went up from 10th position up to 6th (+60% risk increase). Currently obesity kills 3 times more people than malnutrition.
This question is a very contentious one as it relies upon a lot of variables that are largely poorly understood.
The first part is arable land mass. Currently animal production is focussed on either high value grazing areas or low value extensive areas. Extensive grazing areas cannot be cropped. That area of production would have to be made up by increased crop production in other areas. 56% of Australia is extensive agriculture, worldwide it is ~5,000,000,000 hectares. That is a large amount of low production land to make up for.
http://www.nature.com/nature/journal/v478/n7369/full/nature10452.html "Simply put, we can increase food availability (in terms of calories, protein and critical nutrients) by shifting crop production away from livestock feed, bioenergy crops and other non-food applications..... But even small changes in diet (for example, shifting grain-fed beef consumption to poultry, pork or pasture-fed beef) and bioenergy policy (for example, not using food crops as biofuel feedstocks) could enhance food availability and reduce the environmental impacts of agriculture."
Also, the areas quoted vary so much, because the figures are not fully understood. Cropping land is rarely cropped every year, instead rotated or rested at intervals, dependant upon crop type, soil type and amount of water/rainfall. Some countries just don't have accurate records.
The second part is feed conversion. There are methods, such as mixed enterprise systems (crops plus grazing) and of course feedlotting that use grain feeding. The feedlotting is what people refer to the most, not understanding that cattle have a much higher energy conversion rate for vegetable matter than humans (being ruminants) and are not regularly fed for their entire lives. Thus grazing remains a large part of production.
Humans also preferentially eat higher protein foods like meat (see rise in meat demands from Asia with increasing wealth). This is because it is more calorically and nutritionally dense as a food, which is linked to satiety.
The third part is grain types. Most grains that are fed to animals are what is referred to in the grains industry as "feed grains". These are generally lower quality grains that are unsuitable for human food production. Some of the grains used cannot be eaten by humans (e.g. lupins have high alkaloid levels that give both a bitter taste and become toxic when consumed regularly). Obviously the category of feed grain varies from "could be used" through to "cannot be used" for humans. This is once again a shorfall in the production required to replace meat in the diet. Remembering that feed crops are often grown where human crops cannot be grown, or not grown regularly (e.g. see wheat classes and agronomy).
These factors combine to create quite a different picture than what is normally presented in the "can we grow enough crops to replace meat eating" discussion. There would be less land available for cropping than is available for producing a mixed diet. There would be crops produced that would not be suitable for human consumption. We would also need slightly higher production or quality of crops to make up the energy conversion gap. This all makes for a large hole in the argument.
References:
Info on ME and DE for humans: http://fao.org/DOCREP/006/Y5022E/y5022e04.htm
This paper covers come of the conversion ratios for different animals that are grain fed (Cattle 7:1, Pigs 4:1, Chicken 2:1): http://ncbi.nlm.nih.gov/pmc/articles/PMC1240832
Another reference for the protein claims: http://sciencedirect.com/science/article/pii/S0301622699000196
Meat production is highly resource intensive. A non-vegetarian meal will use several times more land, water and labour to produce than its vegetarian counterpart.
To produce 1kg of Beef approx 16,000 litres of water is needed. Compare that to 130 litres of water needed to produce 1kg of lettuce. Agriculture uses 60% of all the freshwater on the planet. So water is quite a scarce resource and meat production uses a LOT of it.
For every 1kg of meat produced several kilograms of grain have to be fed to the slaughtered animal. The land used for producing those several kilograms of grain can satisfy far more vegetarian people than non-vegetarians. Meat production is causing deforestation as we have not enough land to satisfy our needs.
If these resources were diverted to vegetable and fruit production, we could be feeding many billions of people better than we are right now.
