Although there are thousands of edible plant species, only a relatively small number have been domesticated, i.e. converted to widespread usage by humans. Three crops—wheat, corn, and rice—provide nearly 60 percent of total plant calories that humans consume. Other major crops include potatoes, soybeans, cassava, sorghum, and legumes. The three top crops are grown worldwide, though certain regions are known for specific crops. For example, the United States supplies almost half of the world’s 800 million tons of corn annually, followed by China, Brazil, and Mexico. China, India, and the U.S. are the largest wheat producers, and almost 95 percent of all rice is grown in Asia. And, while 16 percent of total wheat production reaches the world’s markets, rice is primarily consumed where it is grown and only 5 percent makes it to the world market.

Wheat is one of the oldest cultivated crops, beginning around 10,000 years ago in the area known today as the “Fertile Crescent” between the Tigris and Euphrates rivers. Evidence suggests that wheat was used for making bread in Egypt by 5000 BC and its cultivation had spread to Europe by 4000 BC. Although the U.S. is the third largest wheat producer in the world, large-scale cultivation did not begin until the late 1800s when European settlement moved into the central plains. Today, approximately 700 million tons of wheat are grown annually around the world.

Rice continues to be a critical staple for nearly half of the world’s population, and for whom rice cultivation is the sole or primary source of food. Although rice is a good carbohydrate source, it does not provide adequate nutrition—an issue of increasing concern in the developing world where almost three billion people obtain most of their daily nutrients from rice. These populations can suffer from micronutrient deficiencies, most notably a lack of vitamin A.

Corn (or maize) is thought to be a domesticated version of the wild cereal grass teosinte, and was likely cultivated between three and four thousand years ago in Mesoamerica. It is still one of the most common crops grown in the Americas. Only about one percent of the corn that is grown is eaten as whole or processed grain (sweet corn, corn chips, or tamales); more than 50 percent is used as animal feed—primarily for cattle, hogs, and chickens—and the remainder is consumed either as starch or in the form of corn sweeteners. More recently, an increasing amount of land area has been dedicated to growing corn due to the demand for ethanol, a corn-based fuel. In 2007, ethanol production became the second largest use of corn grown in the U.S. The sustainability of this use is controversial.