The U.S. feeds 322 million persons with 335 million acres of “cropland used for crops”. But some of that land is misused or underutilized. We waste 34 million acres growing corn for ethanol. We grow too much corn and soy for livestock feed. We export too much corn and soy to other nations for livestock feed. If we stop exporting corn and soy, and cut the production of domestic livestock feed in half, we would free up 85 million acres. We devote too much land to grow sugarcane and sugar beets for refined sugar. If we stop growing sugarcane, and cut sugar beet production in half, we free up another 18 million acres. About 11 million acres in the U.S. is used for cotton production. We could cut that acreage in half, freeing another 5.5 million acres. Cotton is useful, but world stocks are at an all-time high; we are producing more cotton than we need.
Total misused or underutilized land: 142.5 million acres.
The U.S. uses about 1.04 acres per person of cropland to feed the population. But 42.5% of that land (142.5 million acres) is underutilized or misused. So we really only need 0.575 acres per person.
There are 795 million hungry persons in the world (per the 2015 World Hunger Report). At 0.575 acres per person, we would seem to need 457 million acres to provide them with a full diet.
However, most of the hungry have some food. They usually have a staple crop that is high in carbs, moderate in protein, low in fat. I estimate (in my book Hunger Math) that their current diets meet 65.5% of their total caloric needs. The unmet need is 34.5% of a normal diet. Let’s recalculate the above estimate of land, taking into account that estimate: 795 million hungry persons, times 34.5% (unmet need), times 0.575 acres per person, gives us 158 million acres of land needed to end world hunger.
Of course, that land must be added to the existing world agricultural system. And it would be best if most of the additional food were grown near the hungry, perhaps by the hungry. But the U.S. certainly could improve the situation by freeing up over 140 million acres for production of the foods needed by the hungry, especially vegetable oil and plant protein sources (legumes, grains, nuts/seeds).