Although rice originally flourished in the dry climate of Central Asia, it spread to the flood plains of tropical regions, resulting in evolution of varieties with the capacity to grow with roots submerged in water. The African and Asian varieties that are flooded during the growing season are more productive than the varieties that are not flooded, partly because the submerged roots easily extract needed nutrients from the water. Flooded rice, also known as lowland rice, is grown in paddies, which are fields that contain water enclosed by low walls of earth called bunds. Paddy rice fed by rainfall alone accounts for about 50 percent of all rice grown worldwide, while paddies flooded by a combination of rainfall and irrigation provide about 35 percent of rice produced. The major rice-producing countries, including China, India, and Vietnam, primarily cultivate paddy rice.Other African and Asian rice varieties, known as upland rice, are grown in regions of low rainfall that do not provide enough moisture for paddies, or in tropical rain forests where high humidity prevents the ripening of other edible grains. Upland rice is less productive than paddy rice, partly because it depends on nutrients that are dissolved in the soil moisture. When soil moisture is low, few nutrients are available, compared to the storehouse of nutrients typically found in paddy waters. Upland rice accounts for about 15 percent of world rice production, and is particularly important in Laos, where it accounts for between 20 and 22 percent of the rice harvest.
Rice, grown in more than 100 countries, is particularly productive in tropical regions with abundant moisture, but it also grows successfully under widely different climate conditions. Rice farmers choose varieties adapted to the region’s length of growing season, soil, altitude, and, for paddy farmers, the depth of water in the fields. Paddy rice farmers in developing countries usually sow seeds in small seedbeds, then hand-transplant the seedlings into flooded fields that have been leveled by water buffalo or oxen-drawn plows. One advantage of transplanting seedlings instead of planting from seed is that the young plants help limit weeds by shading them from needed sun. In industrialized countries, seed is sown with a planting drill or cast from an airplane into machine-leveled fields that are then flooded. Herbicides are the primary method of weed control.
Depending on the rice variety and the climate, rice grains are ready for harvest in three to six months. In developing countries, farmers harvest rice with sickles or knives, tie it in bundles, and let it dry in the field. They then remove the grain from the plant, a process called threshing, by hitting the plant against a slatted screen or walking animals over it. Farmers in industrialized countries use combines, which are machines that move through fields and harvest, thresh, and clean the grains. The grain is then dried in sheds with heated air.
Rice is susceptible to a range of diseases and pests, which annually destroy about 55 percent of rice crops. The most common diseases are caused by the fungi sheath blight and rice blast, and the stalk borer is a common insect pest. Weeds compete with rice for nutrients and water and are a serious problem, especially in upland rice farming. Rodents and birds also feed on rice grains before they are harvested. Disease-causing fungi, insects, and a variety of other pests infest rice during storage and transport.
When rice is processed, the hull is removed, exposing the bran. Rice at this stage is brown rice. The fibrous bran of brown rice is rich in oil; protein; the B vitamins thiamin, riboflavin, and niacin; and the minerals iron, phosphorus, and potassium. To make white rice, the bran is removed. White rice is less nutritious than brown rice and, when feasible, is enriched with the addition of vitamins and minerals to increase its nutritive value. Without the tough bran layer, white rice cooks faster and stores longer than brown rice, so it is often preferred in regions where fuel is limited and refrigeration is not readily available. Polished rice is made by passing white rice kernels through a machine with a brush that smoothes and shines them.
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