Week 8 (Unit 16)
Cassava, Sweetpotato, and Yams
Tropical root and tuber crops
Many people in the tropics and subtropics rely on root and tuber crops as their primary source of carbohydrate. According to IFPRI's projections for 2020, demand for root and tuber crops will increase by 55 percent in the developing world, with greatest increases in sub-Saharan Africa and Asia.
In this lecture we will discuss the most widely grown tropical root and tuber crops - cassava, sweetpotato, and yams. Many other species are important food sources at the local or regional level, because they may be adapted to particular evironmental conditions or fill specific niches in the farming system. The carbohydrate supplied by these crops is mostly starch, which is stored in underground plant organs, including enlarged roots, corms, rhizomes, or tubers.
Several edible aroid species (members of the Araceae family) are good examples of crops with specific adaptation. Cocoyam (Xanthosoma sagittifolium) is known by several common names including malanga, yautia and tannia. Taro (Colocasia esculenta Schott) is grown primarily in Hawaii and is well-known in its cooked form as 'poi', the traditional staple food of native Hawaiians. Both aroids produce edible corms and are known for their adaptation to humid, shaded conditions. They are among the most shade tolerant of terrestrial food crops.
Cassava is adapted to the tropics and subtropics, and can be grown in very diverse environments. It is particularly versatile in terms of its requirements for water, producing a crop in regions with at little as 500 up to 5,000 mm (20-200 inches) of rainfall per year. It is frequently grown in areas that are too humid for grain crops, yet it also has excellent drought tolerance. At maturity, the crop can survive many months without rain.
Cassava is also tolerant to low soil fertility and acidity - conditions that often exist in high rainfall areas in the tropics. It is frequently grown on marginal lands, and can produce yields of 5-6 t/ha on soils that would not support other crops. A total biomass of 45 t/ha in 12 months can be expected under good conditions. Maximum yield potential is about 30 t/ha root dry matter in 12 months. Cassava's potential is greatest on light, well-drained soils that are rich in potassium.
Some cassava cultivars can be harvested in 8 months, but roots can be stored for up to 2 years in the soil - this flexibility makes it an important food security crop. Environmental stress can trigger remobilization of starch from the tuberous roots. Unlike potatoes, the enlarged roots of cassava are used primarily for storage and do not play a role in reproduction.
"Bitter" types of cassava produce cyanogenic compounds. It is possible that these compounds provide protection against predators (rodents, etc.).
Cassava is usually propagated by stem cuttings, preferably when they are fresh from mature plants. It is possible to obtain ~100% regeneration from healthy cuttings. Germination from seed is only ~30%, and plants generated from seeds produce a taproot of poor quality. Best yields are obtained when cassava is planted at the beginning of the rainy season.
Cultivars can also be distributed for planting as disease-tested in vitro meristem cultures (micropropagation).
Cassava is most often grown in intercrop systems with vegetables, plantation
crops (such as coconut and coffee), yam, sweet potato, melon, maize, rice,
groundnut, and other legumes.
More than 600 million people depend on cassava in Africa, Asia and Latin America. Cassava is mostly grown by poor farmers and is vital for both food security and for income generation. Presently cassava is the fifth leading food crop in the world. About 203 million Mt are produced annually on 18 million ha. The average yield is 10.8 t/ha. Nigeria is the leading producer.
Most of the world production is from traditional systems without many inputs. Only ~11% of total production is traded on world markets. In sub-Saharan Africa it is grown mainly by women and used mostly for food. In Asia and Latin America, the roots also provide raw material for small-and large-scale processing into livestock feed and starch.
Reasons why cassava is often the preferred crop in parts of Africa
Brazil is a major center of diversity for cassava germplasm, and Central America is a minor center. Wild relatives of cassava range from the southern US to northern Argentina; some are in danger of extinction.
Cultivated types possess only 25% of the diversity of the wild progenitors. Lack of diversity hinders genetic improvement. Intercrosses between cultivated cassava and wild Manihot species occur in nature, and can be done artificially. During the 1990's, many new accessions from Latin America were introduced to Africa through a joint CIAT/IITA project. The Geographic Information System was used to identify target areas that had similar agroclimatic features to the areas of adaptation for each accession.
Core collections are maintained at CIAT (Colombia), in Brazil, and at
IITA (Nigeria). Preserving collections is expensive because cassava is
To develop cassava with adaptation to the multitude of environments and end-user preferences in Africa, there is a need for local breeding efforts involving both farmers and end-users.
Diseases of cassava
Major diseases of cassava in Africa:
Resistance to ACMV and CBB has been obtained through wide crosses with a wild relative and more recently from land races.
Insect pests of cassava
Landrace varieties with resistance to cassava green mite have also been identified.
The variegated grasshopper is another important pest of cassava in Africa.
Cassava roots are high in starch (~30%), but are low in protein. Cassava is a good source of vitamin C, carotenes, calcium, and potassium. The starch is highly digestible.
The young leaves can also be eaten. Cassava leaves are rich in protein and vitamins A and B. They are consumed as a vegetable or in sauces in some parts of the world.
Removal of cyanide
Some varieties can be eaten raw or boiled ("sweet" types), but many contain cyanogenic glucosides ("bitter" types) and require further processing. Release of hydrocyanic acid occurs with injury. Cultural practices and environmental conditions influence levels of hydrocyanic acid in the plant. Levels will increase with nitrogen fertilizer and drought stress.
Konzo disease is associated with cyanide poisoning, and can be fatal. It is generally not a problem, but does occur when there is civil unrest or famine, in which case people look for immediate food sources and cassava may not be processed properly. Leaves from varieties with cyanogenic potential must also be processed.
In 1996, IFAD and FAO initiated work to develop a global strategy for
development of cassava. They asked the question, "Can cassava, a
traditional subsistence food crop, become the raw material base for an
array of processed products and industrial development that will effectively
increase the demand for cassava and thereby contribute to agricultural
transformation and economic growth in developing countries? If so, how?"
(GDSC) document was completed in 1997 and updated in 2000. The
website for the GDSC initiative contains good descriptions of the
status of cassava production and utilization in important cassava-producing
countries around the world.
In tropical areas, sweetpotato flowers and cross-pollinates easily. In more temperate climates plants do not flower. Sweetpotato is a hexaploid with 6x = 90 chromosomes.
Sweetpotato plays an important role throughout the developing world as a food security crop. When other crops fail due to natural disasters, war, pests and diseases, or lack of inputs, sweetpotato is there to prevent starvation. Once the crop begins to produce edible roots, it will continue to produce for months, even when there is a drought. The crop can be left in the ground and harvested as needed.
China is clearly the main producer of sweetpotato in the world, producing more than 80% of the total crop. More than 100 other countries grow sweetpotato, but only those countries shown in the table below produce more than one million metric tons. Average yields worldwide are about 14.5 Mt/ha. Over 95% of the crop is grown in developing countries. It is typically grown by small-scale farmers, often in marginal areas.
Sweetpotato will grow between 15°C and 35°C, but produces lower yields at the extremes of that range. The crop responds to higher moisture levels, but can tolerate drought and produce edible roots under low fertility conditions. It grows best on a well-drained, sandy soil. Excessive soil moisture may inhibit root development and cause mature roots to rot.
Seeds can be produced in tropical climates, but due to the cross-pollinating nature of the plant, the seeds do not breed true. Propagation is by vegetative means, from transplants produced by bedding mother roots, or from rooted cuttings from the vines. Tip cuttings have the advantage of being relatively free of insect pests and diseases. To produce transplants, storage roots are placed in beds and allowed to sprout before planting. The sprouts are then either cut or pulled from the mother root and transplanted in the field.
Just as for potatoes, it is important to cure sweetpotatoes for about ten days after harvest to heal cuts and bruises. After curing, the roots should be stored at a temperature of 55°F to 70°F. They can be stored for up to 12 months.
The world collections of sweetpotato are held at the International Potato Center in Lima, Peru. Despite the vast genetic diversity that exists in sweetpotato, producers in the US generally grow one of two cultivars, 'Jewel' and 'Beauregard'.
Pests and diseases
Major uses of sweetpotato:
In Asia, about half of the crop is used for animal feed. In Africa, yields are much lower, but almost all of the crop is used for direct human consumption.
Although sweetpotato is a diverse crop with much variability, it can be divided broadly into two groups. In developing countries where the crop is used as a staple food, most sweetpotato cultivars have white to cream colored flesh, are high in dry matter, have a somewhat bland flavor and are not too sweet. In developed countries where the crop is generally consumed as a vegetable or as a dessert, most cultivars have orange flesh, are fairly moist when cooked, and have a very strong, sweet flavor.
Traditional sweetpotato varieties in sub-Saharan Africa are white-fleshed. An initiative called VITAA is now underway to introduce orange-fleshed varieties, as a means for combating Vitamin A deficiency. These varieties are high in beta-carotene, a precursor of Vitamin A in the body. Vitamin A deficiency is a leading cause of blindness and mortality among children in Africa.
Sweetpotato is high in energy and vitamins. In addition to beta-carotene, other nutrients supplied by sweetpotato are Vitamin C, iron and potassium.
Raw sweetpotatoes contain trypsin inhibitors which may reduce one's ability to utilize protein. These inhibitors are completely denatured with cooking.
Have you ever tried sweet potato pie? It tastes a lot like pumpkin pie. Thirty-seven different recipes are available from the RecipeSource.
Many D. cayenensis-D. rotundata cultivars are dioecious, which ensures cross-pollination. The inflorescences are spikes that emerge from the leaf axils. Most yams are pollinated by thrips, but other insects can also be effective pollinators.
D. cayenensis-D. rotundata types are perennials with an annual cycle. Typically, yam requires about 12 months from planting to harvest. Towards the end of the rainy season, vegetative growth slows down, and tubers are initiated. The balance between vegetative growth and tuber production depends on the amount of sunshine. Long sunny periods favor vegetative growth, and short sunny days favor tuberization.To maximize photosynthesis and obtain high yields, the crop should be planted before the onset of the rains. The above ground parts of the plant die back at the end of the rainy season, followed by a tuber dormancy period. Yams have an advantage over cassava and sweet potato because they can be stored from four to six months at ambient temperatures. Once sprouting begins, tuber quality decreases rapidly.
Yams are generally propagated from pieces of the tuber called "yam seeds". Yam seedlings rely on the parent tuber for energy and nutrients during the inital growth phase, until their root system is developed.
Yam is often intercropped with other crops such as maize. Much of the crop in West Africa is produced without fertilizer. It is generally grown early in the cropping phase (time between fallows), when the land is still fairly fertile. It is often planted on mounds to promote tuber growth and facilitate harvest.
Constraints for yam production
One major constraint for large-scale, commercial production of this crop is the quantity of tubers needed for seed. About 30% of a crop must be set aside for this purpose. A farmer may need a truckload of yam tubers to plant a field that could be seeded with a bag of maize.
Another constraint for yam production is the need for staking material. For best yields, the vines should be staked, and allowed to grow to a height of several meters. Good staking material may not be available, or may be costly to obtain.
Yam tubers grow deep in the ground, so digging them out at harvest is very labor-intensive. It is estimated that about 40% of the total costs of yam production are for labor.
Yams are affected by many pests and pathogens including insects, nematodes, fungal and bacterial diseases, and viruses. The high moisture content of yams (70–80%) makes them especially susceptible to attacks by microorganisms while in storage.
Production of yams in Africa is largely confined to the ‘yam zone’, comprising Cameroon, Nigeria, Benin, Togo, Ghana, and Côte d’Ivoire, where approximately 90% of the world’s production takes place. According to FAO statistics, an average of 38.8 million tons of yam were produced each year in 2000-2002, 96% of this in Africa. Worldwide, the average yield was about 9.2 t/ha. The leading producer was Nigeria. Of the 4.2 million hectares of land planted with yam annually, more than 68% of this area was in Nigeria.
Utilization of Yam
Virtually all yam production is used for human food.
In terms of nutritional value, yams resemble potatoes to some extent. They are about 70% water and 27% carbohydrate (mostly starch). They provide good amounts of Vitamin C and potassium. Yam is slightly higher than potato in dietary fiber.
Fresh yam may be boiled or grilled and eaten. In West Africa, it is boiled and pounded in a large mortar and pestle to produce a thick paste. Pounded yam is a preferred food in the yam belt of West Africa, and is often served on special occasions, or to honor one's guests. Many cultures have yam festivals, to celebrate the harvest of the new crop.
Fresh yam tubers are also peeled, chipped, dried, and milled into flour that is used to prepare a dough called 'amala' or 'telibowo'.
In the USA, the name 'yam' is often used to refer to orange-fleshed sweetpotatoes. True yams can be found in "international" grocery stores that market ethnic foods. Some West African shops in the US also sell dried, instant pounded yam flour.
Have you reviewed the Group Projects and made comments to the authors?
Take the quiz on this Unit on the Blackboard.
CIAT. 2004. Cassava: A Crop for Hard Times and Modern Times.
CIAT. 2004. Solutions that cross frontiers. Integrated cassava based
cropping systems in Asia: Farming practices to enhance sustainability.
CIP. 2004. About Sweetpotato. http://www.cipotato.org/sweetpotato/
Collins, W.W. 1995. Sweetpotato.
DeVries, J., G. Toenniessen. 2001. Securing the harvest: biotechnology, breeding and seed systems for African crops. CABI Publishing, New York, NY.
Hamon, P., R. Dumont, J. Zoundjihekpon, N. Ahoussou, and B. Tio-Touré. 2001. Yams. In A. Charrier, M. Jacquot, S. Hamon, and D. Nicolas (eds) Tropical Plant Breeding. SPI and CIRAD, France.
IITA. 2004. Cassava. http://www.iita.org/cms/details/cassava_project_details.aspx?zoneid=63&articleid=267
Leihner, D. 2002. Agronomy and Cropping Systems. In Hillocks,
R.J.; Thresh, J.M.; Bellotti, A.C. (eds.) Cassava: Biology, Production
and Utilization. CABI.
Levetin, E. and K. McMahon. 2005. Starchy Staples. Chapter 14 in Plants
and Society, 4th edition. McGraw-Hill, New York, NY. Additional on-line
notes and references:
O'Hair, S.K. 1995. Cassava.
O'Hair, S.K. 1990. Tropical Root and Tuber Crops. p. 424-428. In:
J. Janick and J.E. Simon (eds.), Advances in new crops. Timber Press,
Raffaillac, Jean-Pierre, and G‚rard Second. 2001. Cassava. In A. Charrier, M. Jacquot, S. Hamon, and D. Nicolas (eds) Tropical Plant Breeding. SPI and CIRAD, France.