Cassava in the 21st Century
In 2002, Africa was the largest producer of cassava. Asia ranked second, and South America occupied third place. Asia boasted the highest yield per acre and Africa the lowest. In 2008 the leading producers were Nigeria, Thailand, Indonesia, and Brazil. Nigeria, the Congo, and Tanzania produce the majority of Africa’s cassava. In East and West Africa, less than half of the harvest goes to make flour and pellets. The leading exporters are Thailand, Vietnam, Indonesia, and Costa Rica. Thailand, Vietnam, and Indonesia account for the majority of the world’s exports in the form of starch and pellets. Worldwide 80 countries, all of them in the tropics, grow cassava. As a source of calories, cassava ranks third behind rice and corn in the tropics. Cassava ranks second to corn in tonnage. In southern China, cassava ranks fifth in tonnage behind rice, sweet potato, sugarcane, and corn. Unable to meet demand through domestic production, China imports cassava from Vietnam and Thailand. China converts a portion of its harvest to ethanol much as the United States converts corn to ethanol, a practice that may increase demand for the root. Brazil harvests the majority of the Latin American crop. Brazil, Colombia, Cuba, Haiti, Paraguay, Peru, and Venezuela produce virtually all Latin American cassava. According to one authority, cassava yields more calories per acre than any other crop, though this honor may belong to sugarcane. Cassava may be the world’s least expensive “source of starch.” Today, the root supplies one-sixth of the daily calories of the people of Madagascar, Ghana, Nigeria, Liberia, the Congo, Uganda, Tanzania, and Mozambique. In Ghana and Nigeria, per person consumption of cassava has increased in the last 40 years, whereas consumption has declined in the Congo, Tanzania, and Uganda.
Intolerant of frost, cassava is grown between 30° north and 30° south and at elevations no higher than 6,000 feet above sea level. Cassava needs a temperature between 64°F and 77°F and 2 to 200 inches of rain per year. Tolerant of acidic and alkaline soils, cassava may be grown in soil with a pH between 4 and 9. Cassava yields best in sandy loam. High humidity favors the growth of roots. Because cassava tolerates low rainfall, it is a famine food, supplying calories and some nutrients when other crops fail. Some Africans eat cassava every day, sometimes at breakfast, lunch, and dinner. In Africa, nearly half of the population eats cassava as the primary food. One might question whether cassava deserves to be a dietary staple. Seventy percent water, the dry matter of the root is carbohydrate, 64-72 percent of its starch. The root has only 1-2 percent protein, an amount that compares poorly with the protein in legumes and grains. Cassava has vitamin C and calcium, but little thiamine, riboflavin, and niacin. So deficient is cassava in iodine that women in the Congo who eat primarily the root develop goiter. After the harvest, roots begin to rot within two days, making it imperative that they be processed or eaten quickly. Perhaps because cassava has toxins, is vulnerable to insects and diseases, deteriorates rapidly unless processed, and has little protein, researchers have not lavished the money on the root that corn and soybeans have received. Cassava, unlike corn, has few uses aside from the feeding of humans. The people of Africa eat most of the harvest. Less than 10 percent goes to livestock and industry. Industry uses cassava starch in the manufacture of clothes, adhesives, packaging material, food products, pharmaceuticals, and batteries. In Africa as elsewhere, corn rather than cassava feeds livestock. In an effort to increase the use of cassava as livestock feed, Nigeria refused to import corn in 1985. In the 1980s, high grain prices led Europeans to import cassava from Asia and Latin America to feed livestock. After 1992, the decline in grain prices caused European stockmen to jettison cassava for corn.
Small farmers grow most cassava, using it for their own sustenance. Most cassava, destined for the dinner table, never enters the market. Farmers who grow corn and cassava do not harvest cassava unless corn yields poorly. In this circumstance, farmers harvest cassava to stave off hunger. Farmers may leave cassava in the ground as long as four years without a loss in quality. In the Congo, farmers have adopted cassava because of its drought tolerance. Cote d’Ivoire, Ghana, and Uganda grow sweet varieties of cassava, whereas the Congo, Nigeria, and Tanzania grew bitter varieties. Most cassava varieties are bitter, perhaps because they are more resistant to insects and disease than sweet varieties. Today, high- yielding cultivars increase yields 40 percent over traditional varieties.
Some farmers fallow cassava land, especially where population is sparse. Throughout Africa, farmers grow cassava in preference to yams. It competes well with millet, banana, and yams. In West Africa, farmers intercrop cassava with yams. In Nimbo, Nigeria, farmers plant yams, corn, and melon in April and cassava in June. After the harvest, farmers fallow the land three years. In Uganda, banana is the primary crop and cassava secondary. Farmers intercrop cassava with corn, beans or peas, millet, and sesame. They plant cassava in March, harvesting it in November. Fallowing the land four months, they replant it to cassava. Where the soil is poor, farmers plant cassava rather than banana. Throughout the tropics, farmers intercrop cassava with beans, peas, soybeans, mung beans, peanuts, banana, plantain, rice, millet, sorghum, yam, and sweet potato. In the Congo and Tanzania, people eat cassava leaves as a vegetable, though they must be cooked to destroy the toxins. Leaves have more protein than the root as well as vitamins A and C, calcium, and iron. Despite these nutrients, Ugandans consider the leaves a food of the poor and so will not eat them. Although cassava is a subsistence crop, it has grown in importance as a cash crop in recent years. In Africa and South America, middlemen buy cassava from farmers, transport it to market, and sell it for profit. In India, Brazil, and Nigeria, women do much of the work of tending cassava. In Nigeria, half of all working women are cassava farmers, though they earn little money. Whereas men clear the land, plow it, and plant cassava, women weed the land, harvest the roots, and process them. Men prefer to take wage labor rather than to grow cassava. Women’s contribution to the cultivation of cassava is especially large where it is a subsistence crop. Men do more work where cassava is a cash crop. Cassava’s popularity may be ebbing. As its price has increased, the poor have had to buy cheap rice.
Cassava is a staple in the cuisine of several people. Rwandans combine cassava and beans. Liberians make gari foto from cassava, onion, tomato, and egg. Fufu, another Liberian dish, combines cassava, vegetables, and meat or fish. The people of Thailand coat fish, shrimp, or squid with cassava starch, frying the dish. In Kerala, India, cassava and fish are a popular combination. Cassava bread is widespread throughout the Caribbean. Puerto Ricans make chili de yucca from cassava and beans. Guatemalans make cassava souffle. Peruvians combine cassava, cheese sauce, and chili peppers. Colombians make yucca frita by frying slices of cassava much as Americans make fries from the potato.
Diseases, Pests, and Weeds
Pathogens and pests are numerous in the tropics, and cassava suffers from several of them. Fungi afflict many plants. In the case of cassava, however, viruses appear to be the principal threat. Widespread in the Americas is Cassava Common Mosaic Virus, which has reduced yields more than 30 percent in Brazil, Peru, Colombia, and Paraguay. Worrisome is a new variant of the virus, which appeared in Venezuela in 1995. In Africa, the threat comes from African Cassava Mosaic Disease, which is the continent’s most damaging disease and the most significant insect-borne disease of cassava. Scientists initially blamed the disease on African Cassava Mosaic Virus, which the whitlefly Bemisia tabaci transmits to cassava. Scientists identified African Cassava Mosaic Virus as early as 1891. As late as 1959, the virus was virtually the only threat to cassava in Africa. Since then bacterial blight, the mealybug, and the cassava green mite have assailed the root. African Cassava Mosaic Virus is not alone in causing African Cassava Mosaic Disease. East African Mosaic Virus and South African Mosaic Virus also cause the disease. Often more than one virus attacks a plant. One study found the disease in more than 80 percent of cassava plants in West Africa. Uganda has suffered severe infections of the disease. Bemisia tabaci has migrated to Cuba and Brazil, leading scientists to fear that the disease will follow. Latin American cultivars appear to be especially vulnerable to African Cassava Mosaic Disease. The bacterium Xanhomonas axonopodia causes cassava bacterial blight, which has led to the total loss of the cassava crop in the worst infections. The disease spreads through stem cuttings, which, when sown, germinate infected plants. In 1972, scientists pinpointed the disease in Nigeria. During the 1990s, the bacterium spread throughout Africa. Damp weather hastens the spread of the bacterium, which reduces yields and sometimes kills plants by defoliating them. Nigeria has reported severe losses from the disease.
In Africa, the most serious pests are the mealybug and the cassava green mite. Native to South America, they are troublesome because they have no natural predators in Africa. In 1973, the mealybug migrated to the Congo, from where it has spread throughout Africa. The bug feeds on the stem, petiole, and leaf. Conscious of the lack of natural predators, scientists in 1981 released a species of wasp that feeds on mealybugs. By 1990, the wasp had established colonies in 24 African countries. First reported in Uganda in 1971, the cassava green mite, native to Colombia, reached West Africa in 1979. Sucking sap from cassava leaves, it weakens plants. In the 1990s, scientists identified three species of mite that prey on the cassava green mite and released them in Benin in 1993. Despite this effort at biological control, green mites remain entrenched in Africa, threatening the cassava crop. In Latin America, mites of several species, cutworms, scales, and lace bugs plague cassava. The worst pests are the hornworm and stem borer. The stem borer has caused severe losses in Colombia. Insecticides are effective, but many farmers cannot afford them. Genetically engineered cassava holds the promise of helping these farmers. Genetically engineered varieties have genes from the bacterium Bacillus thuringiensis, which code for the production of a chemical toxic to insects. This toxin does not harm humans, making genetically engineered cassava safe to eat. The success of Bt corn leads one to hope for similar success from genetically engineered cassava. Also damaging are termites, which feed on cassava stems, and rodents, which devour roots.
Cassava does not compete well against weeds. Their presence lowers yields 40 to 70 percent. To combat weeds farmers have increasingly relied on herbicides in Africa and South America, though the poor cannot afford them. The derivation of herbicide-resistant cassava may not, in increasing the reliance on herbicides, help poor farmers. It is unclear whether herbicide resistant cassava will be as successful as herbicide resistant soybeans.
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