TECHNICAL PAPER # 17
 
                    UNDERSTANDING THE PRODUCTION
                            OF THE MAJOR
                 TROPICAL/SUB-TROPICAL ROOT CROPS
                          CASSAVA, POTATOES,
                 SWEET POTATOES, YAMS AND COCOYAMS
 
 
 
 
                                By
                         Dr. Nail H. Ozerol
 
 
                        Technical Reviewer
                       Dr. Herbert F. Massey
 
           
                                VITA
                    1600 Wilson Boulevard, Suite 500
                      Arlington, Virginia 22209 USA
                 Tel:  703/276-1800 * Fax:   703/243-1865
                       Internet:  pr-info@vita.org                           
 
                Understanding the Production of the Major
                     Tropical/Sub-tropical Root Crops       
                            ISBN:  0-86619-217-4
               [C] 1984, Volunteers in Technical Assistance
                
                              PREFACE
 
This paper is one of a series published by Volunteers in Technical
Assistance to provide an introduction to specific state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their situations.
They are not intended to provide construction or implementation
details.  People are urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
 
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis.  Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.  VITA staff included Leslie Gottschalk
and Maria Giannuzzi as editors, Julie Berman handling typesetting
and layout, and Margaret Crouch as project manager.
 
VITA Volunteer Dr. Nail Ozerol, the author of this paper, is the
director of N. H. Ozerol & Associates, a consulting firm specializing
in nutrition and preventive medicine.   He was formerly on
the faculty of the Department of International Health, College of
Medicine, Howard University, and that of the Center of Nutrition,
Meharry Medical College.  He has consulted on nutrition in several
African countries.  Dr. Herbert F. Massey, reviewer of this
paper, has been a VITA Volunteer for 14 years.   He is the director
of International Programs for Agriculture at the College of
Agriculture, University of Kentucky.   He has consulted on agricultural
teaching and/or research projects in several tropical
countries throughout the world.
 
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries.   VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations.  VITA maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
 
               UNDERSTANDING THE PRODUCTION OF THE MAJOR
                    TROPICAL/SUBTROPICAL ROOT CROPS:
         CASSAVA, POTATOES, SWEET POTATOES, YAMS, AND COCOYAMS
 
                    By VITA Volunteer Nail H. Ozerol
 
I.  INTRODUCTION
 
Root crops is a general term commonly used for a wide variety of
food plants that have an underground storage organ known as a
root, tuber (rhizome), corm, or bulb.   Root crops are rich in
starch, and low in protein and oil.   They are excellent sources of
calories.  Some are consumed as major staples, such as cassava,
potatoes, sweet potatoes, yams, and the aroids (cocoyams).
Others, such as carrots, onions, garlics, parsnips, and radishes,
are used as fresh vegetables.
 
Historically, governments and academic centers have paid relatively
little attention to root crops as compared to grain crops.
These crops have been regarded as inferior food, and produced and
consumed only by the subsistence farmers in the developing parts
of the world.  In recent years, however, the tropical root crops
have been "rediscovered" by the research communities and others
who are concerned with the food and nutrition problems of low income
people.
 
The tropical root crops, in general, have a great potential in
meeting basic food and energy needs of the developing world, and
therefore deserve to be fully explored in rural development
projects and strategies.  Reliable estimates suggest that annual
tropical root crop production is in the range of 170 million
metric tons, roughly equivalent, in calorie content, to 50 million
metric tons of grain.  There is now a sharp increase in
scientific research and investigation in every aspect of this
crop in certain well-established research centers, such as:
International Institute of Tropical Agriculture (IITA), Ibaden,
Nigeria; International Center for Tropical Agriculture (CIAT),
Cali, Colombia; and International Potato Center (CIP), Lima,
Peru.  The following factors have been responsible for the growing
international interest in the food potentials of these crops:
 
     o   A growing interest in and appreciation of a large group
        of rural poor who depend on these crops for their basic
        calories.
 
     o   Increased population growth, and the relative rise in the
        prices of fossil-based energy have contributed a great
        deal to interest in root crops as source of food and energy.
 
     o   World food shortages, and the ever-increasing need to
        explore new frontiers in order to alleviate world hunger.
 
ADVANTAGES AND DISADVANTAGES OF ROOT CROPS
 
Root crops have the following advantages:
 
     o   They are rich in starch and calories.
 
     o   They grow well in a wide range of soil types so long as
        there is adequate rainfall.
 
     o   They require relatively little care in terms of labor and
        other inputs used in their production.
 
     o   Unlike cereals, they can be stored without processing or
        drying in a highly humid environment.
 
     o   Relatively few serious pests and diseases plague root
        crops compared with those associated with cereals and
        legumes.
 
     o   Some root crops, such as cassava, can be left in the
        ground as food resources until required.
 
The disadvantages of root crops are as follows:
 
     o   They are low in protein and oil.
 
     o   Their awkward shapes and large size make them prone to
        bruising in transit and secondary infection by micro-organisms.
 
     o   They are bulky to handle in trade, marketing, and storage
        due to their high moisture content.
 
MAJOR USES OF ROOT CROPS
 
The following three major uses of root crops, both tropical and
subtropical, are now universally recognized.
 
Root Crops as Food
 
Root crops are a major source of food and calories in many tropical
countries.  The Food and Agriculture Organization estimates
for 1974 suggest that root crops provide 20 percent of the total
caloric intake for 11 countries, and nearly 40 percent or more of
all calories in Zaire, Ghana, and Togo.   Again, they are basic
calorie sources in Brazil and Indonesia and also provide several
other nutrients.
 
Root crops are usually prepared to be eaten in another dish, such
as in various stews.  The high starch content in the crops helps
to thicken the liquid base, so that it adheres to the meat or
vegetables in the stew.
 
Root Crops as Feed
 
The use of root crops as feedstuffs in developing countries is
expanding.  Recent studies in Venezuela have demonstrated that
high yields of good-quality protein are obtainable from cassava
leaves at reasonable cost for use in livestock feeding.   Similarly,
cassava leaves have been used for commercial exports as
livestock feeds in Thailand.   Almost all of Thailand's cassava
root crop is exported as dried chips, sometimes pelleted, primarily
for use as animal feed.  In general, cassava products can be
successfully substituted for alternative sources of feed for
different species of livestock in both tropical and subtropical
countries.
 
Root Crops as Substrate
 
Among the most interesting technological developments in the use
of root crops are the fermentation processes for the manufacture
of sugar, ethyl alcohol, and single-cell protein.   Cassava, especially,
among a few others, has been used extensively as a substrate
(raw material) for the production of ethyl alcohol.
 
II.  MAJOR ROOT CROPS
 
The five major root crops of the tropics and subtropics are
cassava, potatoes, sweet potatoes, yams, and cocoyams.   These and
other important root crops are listed in Table 1.
 
                      Table 1.  Important Root Crops
 
Common Name                    Genus                        Family
 
Beet                      Beta vulgaris               Chenopodiaceae
 
Carrot                    Daucus carota               Umbelliferae
 
Cassava                   Manihot esculenta           Euphorbiaceae
 
Cocoyam, Asiatic          Colocasia esculenta         Araceae
 
Cocoyam, Tropical
  American                 Xanthosoma sagittifolim      Araceae
 
Horseradish               Rorippa armoracia           Cruciferae
 
Jerusalem Artichoke       Helianthus tuberosus        Compositae
 
Onion                     Allium cepa                 Liliaceae
 
Parsnip                   Pastinaca sativa            Umbelliferae
 
Potato                    Solanum tuberosum           Solanaceae
 
Radish                    Raphanus sativus            Cruciferae
 
Rutabaga                  Brassica napobrassica       Cruciferae
 
Sweet Potato              Ipomoea batatas             Convolvulaceae
 
Yam                       Dioscorea                   Dioscoreaceae
 
 
CASSAVA
 
Cassava (Manihot esculenta, Euphorbiaceae) is a perennial shrub
native to South America that is now grown throughout the tropics.
Other common names for cassava are tapioca, mandioca,
manioc, sagu, and yuca.  Cassava was brought into cultivation by
the American Indians probably 4,000 years ago, was later introduced
to West Africa in the sixteenth century, and then spread to
other tropical regions of the world.
 
<Figure 1>

36p05.gif (600x600)


 
For a variety of reasons, recent growing research interest in
root crops has focused mainly on cassava.   It is a major source
of calories for some 300 million people in the developing countries
of the world.  It is one of the world's most efficient
plant converters of solar energy to carbohydrates.   It yields
more calories of food per unit input of labor efforts than any
other crop.  It is relatively resistant to insects and plant
diseases, and requires few inputs of a traditional production
system.  It is adaptable to a wide range of agro-climatic conditions,
and performs surprisingly well on acidic soils of poor
fertility.  It can be left in the ground until it is needed.
 
Fresh cassava roots compare favorably with the other root crops
in terms of calorie content, but rank at the bottom in terms of
protein.  Cassava roots are generally rich in calcium and ascorbic
acid, and contain significant amounts of thiamine, riboflavin,
and niacin.  The cassava leaves are rich in high-quality protein,
and are consumed in most of the tropical countries.
 
Production
 
Cassava is by far the most economically important of the tropical
root crops, with annual production exceeding 100 metric tons
grown on some 12 million hectares.   It is produced in more than 80
countries, but fewer then 20 countries account for 90 percent of
production.
 
A major factor behind the extensive production of cassava is its
adaptability to a wide range of soil and moisture conditions.  It
can be successfully grown in areas with rainfall ranging from 500
to 5,000 millimeters.  Except at planting, cassava can withstand
periods of prolonged drought and is, therefore, a valuable crop
in regions of low or uncertain rainfall.
 
Light, sandy loams of medium fertility give the best results, and
the crop can be grown successfully on soils with a pH ranging
from 4.5 to 9.0.(*)  Saline and swampy soils are not suitable for
cassava production.
 
The ideal temperature for cultivating cassava ranges from 18 [degrees] C to
35 [degrees] C; growth stops at 10 [degrees] C.   Cassava is perennial in the tropics,
and annual in the temperate zone.   It can be grown at altitudes up
to 2,000 meters.
 
Cassava is grown both as a single crop, and in combination with
sorghum, maize, groundnuts, cowpeas, yams, sweet potatoes, upland
rice, and certain other vegetables.
 
(*) pH indicates the acidity or alkalinity of the soil, and is based
on a scale of 0 (acid) to 14 (alkaline) with the midpoint of 7
indicating a neutral soil condition.
Under the typical slash-and-burn agriculture of the tropics,
cassava stem sections are hand-planted just before the rainy
season.  If all goes well, within as little as seven months a number
of starchy roots can be harvested from each plant; however,
the best yields are not obtained until about 16 months pass.  If
allowed to grow for too long in the soil, the roots become rather
woody and less edible.
 
Cassava is planted on ridges and on flat ground, but ridge planting
is more common.  Cuttings about 20 to 30 centimeters long are
inserted in the soil at a depth of about half their height, often
at an angle of 30 to 40 degrees.   Cuttings sprout 7-14 days after
planting, and root bulking begins during the second month after
planting.  The crop is planted in May-June, and harvested the
following September-December.   Cassava has a high potassium requirement.
If potassium is not present in the soil in sufficient
amounts, yields are reduced, and the tubers have a low starch
content and higher hydrogen cyanide (HCN) content.
 
Harvesting is done by hand by digging up the tubers after cutting
the tops off the plants.  With large-scale production, the tubers
can be mechanically ploughed up, but the yields are often reduced
because a higher percentage of tubers is left in the ground.
Once harvested, the tubers deteriorate rapidly and begin to rot
after 48 hours.  Cold storage, where possible, at 0 [degrees] C to 2 [degrees] C
and 85 to 95 percent relative humidity has been reported to
extend the storage-life for periods up to 6-1/2 months.
 
Yields vary greatly depending on the variety of cassava, soil,
climate, age at harvest, etc.   The average is about nine metric
tons of fresh roots per hectare.   Production of cassava in 1975
exceeded 100 million metric tons (fresh roots), of which more
than 40 percent was produced in Africa, about 30 percent in South
America, and the remaining in Asia.   Approximate estimates suggest
that cassava production since the early 1960s expanded by
25 percent.
 
Uses
 
Cassava is used in a number of ways.   In addition to its consumption
by humans and as feedstuffs by livestock, it is now commonly
used as a raw material (substrate) in the manufacture of various
industrial products.  Starch is the most important such product,
but in Brazil the roots are used to make alcohol.
 
As a foodstuff, cassava is consumed as a boiled or roasted vegetable
or as paste, meal, or flour.   The whole root may be boiled
and has a sticky, heavy consistency, and of itself is rather
tasteless.  In Brazil, the roots are usually shredded, then
heated and dried to make a meal known as "farinha de mandioca."
 
 
In Indonesia, the roots are sectioned, dried in the sun, and
later ground into meal.  In the making of tapioca, an important
export from Indonesia, the peeled roots are grated, soaked with
water, kneaded, strained, dried, and heated to hydrolyze the
starch to sugar, and gel particles into "pearls" while being
stirred on a grill.  In Jamaica, the roots are ground into a mush
called "bami" or formed into cakes known as "casabe."  The better
known preparations include "gari" and "fufu" in West Africa.
 
Cassava Toxicity
 
A major problem with the use of cassava is the toxicity from the
cyanide compounds found in the fresh roots.   The cyanide is
concentrated in or near the skin of the root, and is freed into
its active form when the skin is broken.   In this manner, the
cyanide compound contributes to the plant's resistance to pests.
The cyanide content, however, varies from species to species, and
changes under environmental conditions, such as humidity, temperature,
and age of plants.  However, in areas where cassava is
the staple food, chronic cyanide poisoning can result if the cassava
is not processed properly.  Chronic cyanide poisoning is
notable in some areas of Africa, particularly in Zaire.   Recent
studies in the Lake Kivu region of Zaire suggest that a cassava-based
diet inhibits iodine uptake by the thyroid gland and may
lead to goiter, birth defects, mental retardation, and other
chronic disorders.  Proper processing of cassava for consumption
is the most effective solution to the problem of cyanide poisoning
and its consequences.
 
Diseases and Pests
 
Cassava is susceptible to various pests and diseases.   Leaf
mosaic, a virus disease transmitted by white flies, is the most
serious disease of cassava.  It can be spread by infected cuttings.
 
Bacterial blight, a new and potentially disastrous disease, was
first detected in West Africa in 1972.   White thread, a root
disease, has been reported to cause crop losses of 20 percent or
more in Ghana.  Other minor diseases are brown leaf spot, white
leaf spot, and anthracnose.
 
The most serious insect pests are white flies, scale insects, and
the variegated grasshopper.  Nematodes are also very serious
cassava parasites, particularly in West Africa.   Various species
of termites have also been known to cause damage to cassava
crops, while rodents and wild animals often attack the roots.
 
POTATO
 
The common potato (Solanum tuberosum, Solanaceae) is a member of
another large and important plant family, Solanaceae, which includes,
among many others, eggplant and tomato.   The genus Solanum
includes more than 2,000 species.
 
The potato was first seen by Europeans in 1537 when the Spanish
landed in what is now called Colombia, and was brought back to
Europe by 1570.  It was cultivated throughout the continent before
1600, and in Ireland by 1663.   The cultivated potato is said to
have been first introduced into North America in 1621.
 
Potatoes are the leading starchy root crop of the subtropical
countries, and one of the eight leading staple food crops of the
world.  Annual production of potatoes is approximately twice that
of all other edible root crops combined.   However, because of its
limited climatic adaptability, less than 10 percent of production
occurs in developing countries.   The International Potato Center
(CIP) in Peru is developing new varieties of this nutritious root
crop, which perform well under a variety of soil and climatic
conditions.
 
<Figure 2>

36p09.gif (600x600)


 
Among the root crops, the potato is known for its high protein
content.  It is almost equal to rice on a dry weight basis, and
with a protein quality approaching that of beef.   With its high
yields and short maturation periods, the potato outranks all major
world food crops in protein production per unit of time.  The
food value of the potato varies depending on the variety, growth,
environmental conditions, storage, and handling.   Its composition
consists of 70 to 80 percent water, 8 to 28 percent starch, and
1 to 4 percent protein.  It also contains vitamins such as riboflavin,
ascorbic acid, and trace elements.   It is an important
source of high-quality nutrients for people in the tropical
highlands.  The potato has been a continuous object of research
and investigation all over the world, with special focus of
interest in the International Potato Center (CIP) in Peru.  The
Center is attempting to increase the tolerance of the crop to
high temperatures, and once it is accomplished, then it is likely
that larger areas of West Africa will be open to cultivation.
 
Production
 
Potatoes are grown as a single crop or in combination with
sorghum, millet, maize, cowpeas, groundnuts, sweet potatoes, and
other vegetables.  Propagation is done by tuber, either whole or
cut.  Whole tubers are less liable to rot in the soil.  Planting
material should be free from diseases, pests, and damage.  Certified
potato "seeds," free from virus, should be used when possible.
Potatoes may be planted by hand or mechanically, and the
crop is usually planted on ridges at a depth of 5 to 15 centimeters.
 
Most potato varieties have very specific temperature requirements,
thereby limiting the adaptability of this crop in tropical
regions.  Tuber formation is retarded when the soil temperature
rises above 20 [degrees] C; above 29 [degrees] C, little if any, tuberization takes
place.  Although young potato plants are very susceptible to hard
frosts, most varieties will tolerate light frosts.
 
Potatoes require a continuous supply of moisture.   Evenly distributed
rainfall is considered essential, and drought, even for
short periods, can have serious effects on yields and quality of
the crops.  Well-drained peat soils are particularly suited;
however, potatoes could grow on most soils if drainage is adequate.
A deep, well-drained loam, or sandy loam, with a pH of 5
to 5.6 is considered to be the best.   Potatoes respond well to
manures and chemical fertilizers, and good yields can be obtained
only with adequate fertility.   Fertilizer requirements vary
greatly depending on the variety and growing conditions.
 
Potatoes do not compete well with weeds, and timely, efficient
weeding, by pulling or tillage, is essential.   In temperate
zones, the crop is often repeatedly hoed, up to five times during
the growing season.  Normally, the crop is ready for harvest in
three to four months.  Harvesting should be done on a dry day,
when the tubers are mature.  The crop can be harvested by hand or
mechanically.  If it is harvested mechanically, a wide range of
equipment can be used, including diggers, spinners, and ploughs.
Harvested tubers should be stored temporarily in a shaded, dry,
and well-ventilated place for 7 to 10 days to allow the skins to
harden before the potatoes are prepared for market or storage.
Potato yields vary with variety, length of growing season, climate,
and the type of soil.  With efficient farming methods in
temperate climates, yields well in excess of 25 metric tons per
hectare are quite common.  Yields are lower in the tropics, averaging
about 14 to 15 metric tons per hectare.
 
Uses
 
Potatoes can be eaten boiled, roasted, baked, fried, or mashed.
They can be made into fried chips or crisps, dehydrated and
flaked, or made into flour.
 
Potatoes can be pulped and fermented to produce alcohol.  Potato
tubers make an excellent livestock feed and can be fed fresh or
dried and used in the form of a meal.
 
Diseases and Pests
 
Potato crops are subject to a number of diseases, some of which
are of great economic importance in both developed and developing
countries.  Brown rot, or bacterial wilt, is the most serious
potato disease in West Africa.   The disease is carried by seed
tubers.  Other bacterial diseases include soft rot, ring rot, and
late blight.  Several other diseases are also of considerable
importance.  Among these are virus diseases that can cause crop
losses.  Virus-free planting stock is essential since there are no
effective treatments for these diseases.   Finally, a number of
pests, particulary aphids and nematodes, have been found to
cause economic losses.  These pests not only harm the crop, but
also spread virus diseases such as leaf roll and mosaic.
 
SWEET POTATO
 
Sweet potatoes (Ipomoea batatas, Convolvulaceae) are widely grown
in tropical, subtropical, and warm temperate areas of the world.
They originated in tropical America and likely spread to the
Pacific before the time of European exploration.   Japan is probably
the leading sweet potato producer; it is a national staple
and largely consumed there.  In many other parts of the world,
sweet potato is utilized as feedstuff for livestock.
 
<Figure 3>

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36p12b.gif (600x600)


 
<Figure 4>
 
The sweet potato, although a perennial, is normally cultivated as
an annual crop.  The crop under normal circumstances is harvested
from three to eight months after planting, depending upon the
variety and environmental conditions.   Sweet potato varieties
vary considerably in their adaptability to soil and other conditions.
They require at least 500 millimeters of rain during the
growing season.  For good yields, an annual rainfall of 750 to
1,250 millimeters is necessary, with drier, weather as the crop
reaches maturity.  The sweet potato can tolerate lengthy dry periods
once roots are established.  However, yields are greatly reduced
if soil moisture is inadequate during the period when root
storage begins.  The sweet potato is easily adapted to a considerable
range of soils, but is sensitive to alkaline or saline conditions.
It does not tolerate waterlogging.   Warm days and nights
are essential, and a mean temperature above 24 [degrees] C is required for
optimum crop growth.  At least 25 millimeters of moisture per
week for four to five months is essential.   Increased altitude
appears to result in increased protein content of the roots.
Sandy-loam soil, with a pH range from 4.5 to 7.5, appears to be
ideal for satisfactory growth.   Tubers reach their ideal conditions
at four to seven days of short-term storage with 85 to 90
percent humidity.  After harvest, the tubers should be stored at
12 to 16 [degrees] C with 85 to 90 percent humidity, or where warm.
 
Tubers are rich in carbohydrates, vitamin A, and vitamin C, and
also contain significant amounts of calcium and iron.   Studies and
reports of the International Institute of Tropical Agriculture
indicate that the yield of sweet potatoes under favorable conditions
is between 20 and 30 metric tons per hectare, and experimental
yields greater than 40 tons/hectare have been obtained.
 
Production
 
In most parts of the world, the sweet potato generally is a home
garden crop that never goes to market.   It is mainly grown in combination
with other crops such as sorghum, millet, maize, rice,
cowpeas, groundnuts, yams, cassava, potatoes, and tobacco.  It
may be propagated by tubers, slips, or vine cuttings.   Cuttings
are the most commonly used planting material.   In slip propagation,
tubers are planted in a nursery bed.   The new plants, which
sprout from the various buds of the tubers, are known as slips.
They are separated and planted and are relatively free from soilborne
diseases, and the tubers produced are of a more uniform
shape and size.  Vine cuttings 20 to 45 millimeters long with
seven or more nodes are planted one-half to two-thirds of their
length in the soil.
 
Roots sprout from the subterranean nodes within 5 to 15 days
depending on the quality of the planting material and the environmental
conditions.  Once the crop is established, it requires
minor weeding if the land has been properly prepared, and is not
over-infested with weeds.  The growing period generally varies
from four to six months depending upon the variety.   At maturity,
the stems turn from green to brown.
 
Harvesting usually takes place during the dry season in November
and December.  Mechanical harvesting of the crop is possible but
losses can be considerable.  Sweet potato tubers are very perishable,
and the methods used to harvest the crop could have a very
significant effect upon the market quality and storage life of
tubers.
 
Uses
 
Sweet potatoes are harvested primarily for human consumption.  In
the tropics, the major proportion of the crop is eaten straight
from the ground as a vegetable, after boiling, baking, or frying.
In Malawi, they are sometimes boiled or roasted and pounded with
groundnuts to produce "futali."   In some areas, notably India and
parts of East Africa, the peeled tubers are sometimes sliced and
dried in the sun to produce chips, which are often ground into
flour.  Sweet potatoes are also a source of starch and are used as
livestock feed.
 
Diseases and Pests
 
Diseases that attack the sweet potato tubers during storage are
more serious than those affecting the crop during cultivation.
Black rot, for example, can be serious especially when the tubers
are damaged during harvesting.   The sweet potato weevil is the
major insect pest.  Its larvae feed on the roots and tubers.
Proper crop rotation helps in its control.   Chemical control also
appears to be promising.  Recently, several weevil-resistant varieties
have been isolated.
 
YAMS
 
The true yam (Dioscorea) is not to be confused with the sweet
potato, which is sometimes incorrectly termed a "yam."  The genus
Dioscorea includes several hundred species, but only a few are of
importance as food crops.  Of these, the Asiatic species Dioscorea
alata (commonly called the greater yam), and two closely related
Western African species, Dioscorea cayanensis, (yellow yam)
and Dioscorea rodundata (white yam), are the most common and economically
important.
 
Yams originated in the Far East, spread westward, and have since
evolved independently in the Eastern and Western Hemispheres.
Yams are now widely grown throughout the tropics, under both
rain forest and savanna conditions, where there is a combination
of adequate moisture and good drainage.   Yams require adequate
moisture throughout the growing period.   In several countries of
West Africa--the major production zone, with 66 percent of the
world's crop--yams are becoming increasingly expensive because of
their high labor requirements and low yield.   As a result, yams
are being partially replaced by cassava, rice, and wheat, despite
the fact that they are richer in protein than cassava.
 
<Figure 5>

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36p15b.gif (600x600)


 
<Figure 6>
 
Production
 
The average growth period is 8 to 11 months.   Yields range from 5
to 15 metric tons per hectare.   Only a very small portion of yams
enters the international market; the bulk of the yams are marketed
as fresh produce or eaten upon harvesting.   Optimum temperature
requirement appears to be around 25 to 30 [degrees] C.   Growth slows
down below 20 [degrees] C, while temperatures over 30 [degrees] C have an adverse
effect, especially if accompanied by dry conditions.   Yams require
adequate moisture throughout the growing period, and there
is a positive correlation between moisture supply, vine growth,
and tuber formation.
 
In West Africa, yams reach their highest productivity in areas
where there is a dry season of two to four months and a rainfall
of 1,200 millimeters or more during the growing season.   Good
drainage is essential for high yields and quality.   Yams perform
best in well-drained sandy-loam soils.   In heavy soils, they are
susceptible to rot, while in very sandy soils favorable moisture
conditions are difficult to maintain.   Yams are influenced by
photoperiodicity--that is, their growth is affected by the relative
amount of light they get in a day.   However, the effects of
day length on the vines and tuber production have not been
completely investigated.
 
Yams are propagated either by seed yams or sets.   Most yams produce
one or two tubers larger than the rest, and these are the
ones suitable for use as food.   They are cut off near the top,
leaving the crown with the green stem attached.   This is replanted,
and gradually grows again, producing two or more small seed
tubers.
 
The production of seed yams usually is not sufficient.   Thus,
large yams are cut into pieces known as "sets," which are then
used for plantings.  On average, seed yams and sets weigh between
170 and 400 grams.  Spacing plays an important role in the growth
of tubers:  generally, the closer the spacing, the higher the
yield.  Little or no chemical fertilizer is used on yams, though
yams respond well to phosphate and potassium fertilizers if they
are applied correctly.
 
Both seed yams and sets are planted in the middle of hills in
holes 15 centimeters deep.  Shallow planting may dry out the sets
before sprouting.  After planting, the hills are capped with a
layer of dry grasses or weeds about 30 centimeters in diameter on
the top of the hill, and kept in position by a thin layer of
soil.  In general, protected yams sprout more quickly than unprotected
yams as they do not dry out.   When young shoots appear,
long yam poles from hardwood trees are installed to support the
vines and promote healthy tuber growth.
 
During the growing season, cultivation includes weeding, hilling,
and setting the poles.  While chemical fertilizers are not
commonly used on yams, organic manure is.   Most edible yams
normally reach maturity 8 to 11 months after planting.   Harvesting
is done by hand.
 
Uses
 
Yams are a starchy staple crop, normally eaten as a vegetable,
either boiled, baked, or fried.   In West Africa, the major proportion
of the yam crop is eaten as "fufu," a stiff dough.  Yams are
sometimes dried and made into flour.   In villages, peelings and
waste from the yams are often used for feeding poultry or livestock.
 
 
Diseases and Pests
 
Among the various diseases affecting yams, shoe string and die
back are of major importance.   Varieties resistant to these diseases
are now being developed.  Witches broom has been known to
cause damage to yam crops in West Africa, and a virus disease of
the mosaic type has been reported.   Storage losses from various
fungal rot diseases are generally severe, especially when the
tubers are damaged.
 
Among the insects, yam tuber beetles are by far the most serious
pests in West Africa.
 
COCOYAMS
 
The cocoyam, commonly called taro or dasheen, is an important
staple in Southeast Asia and Polynesia.   It has many varieties.
Colocasia esculenta (Araceae family) is of Asian origin, but has
been grown in West Africa for centuries.   It is known as the "old
cocoyam," distinguishing it from Xanthosoma sagittifolium, the
"new cocoyam" of tropical America, which was introduced to West
Africa during the nineteenth century.
 
Like many plants of the Araceae family, called aroids, the cocoyam
grows from a fleshy corm (tuber) that can be boiled, baked,
or mashed into a meal.  The well-known "poi" of Hawaii is a product
of taro which has been crushed and fermented.   Cocoyams are
rich in carbohydrates and very low in protein.
 
<Figure 7>

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<Figure 8>
 
The aroids are the least significant of all tropical/subtropical
tuber crops in terms of production. However, they perform well
under hot, humid conditions, and show best results on deep loam
soils with a high water table.   The maturation period, which
varies according to the variety, ranges from 6 to 18 months.
Yields vary from 10 to 30 metric tons per hectare.   Although
accurate figures are not available, the cocoyam is a major root
crop in the forest region of West Africa, accounting for up to 75
to 80 percent of the total world output.
 
Production
 
The cocoyam is grown as a rainfed crop.   Small corms or pieces of
corms, or the tip of the main root stock with part of the original
corm attached are used for propagation.   Spacing varies widely.
The cocoyam is grown as a single crop, or in combination with
maize, rice, and vegetables.   Planting is usually done during the
rainy season.  The cocoyam can be grown on a wide variety of
soils, but deep, well-drained loams with a pH of 5.5 to 6.6 are
considered to be the best.  Yields are very low in sandy or hard
clay soil.  The more fertile the soil, the higher will be the
yield.  Cocoyams are very sensitive to waterlogging and saline
conditions.  For maximum growth, a mean temperature of 20 to 30 [degrees] C
is a must.
 
Sprouting usually takes place one to two weeks after planting.
The crop is harvested when the leaves turn yellow.   Mechanized,
commercial production of aroids under irrigation and rainfed
conditions is possible in the tropics.
 
The cocoyam's potential as a calorie source and its adaptability
to unfavorable growing conditions make it a valuable crop for
many rural development programs.   Cocoyams are now included in
many agricultural projects, and their cultivation is readily
progressing in most developing countries because of their resistance
to fungal infection, and tolerance of drought.
 
Uses
 
The corms (the underground part of the cocoyam) and the cormels
(lateral tubers) are rich in starch.   Like potatoes, they can be
eaten after being boiled, baked, roasted, or fried in oil.  The
young leaves of some cocoyam species are boiled and eaten as a
green vegetable.  The peeled tubers, after pre-cooking and
drying, can be used to produce a flour.
 
Diseases and Pests
 
Several diseases, viral and fungal in nature, have been known to
attack the aroid varieties.  Root-knot nematodes can also cause
damage if the soil is heavily infested.
 
                        BIBLIOGRAPHY
 
Austin, M.E., and Graves, B.   "Mechanical Harvesting of Sweet
     Potatoes."  Proceedings of the Second International Symposium
     on Tropical Root and Tuber Crops (1970).
 
Bustamante, A.A.  Improve Your Cassava Crop.  Oklahoma City, Oklahoma:
     World Neighbors.
 
Coursey, D.G.  "The Edible Aroids."  World Crops 20 (1968):  25-30.
 
Coursey, D.G., and Haynes, P.H.   "Root Crops and their Potential
     as Food in the Tropics.  World Crops (July/August 1970):
     261-265.
 
Francois, C.F., and Law, J.M.   Sweet Potato Storage.  DAE Research
     Report No. 429.  Baton Rouge, Louisiana:   Louisiana State
     University, 1971.
 
Goering, T.J.  "Tropical Root Crops and Rural Development."   World
     Bank Staff Working Paper No. 324.  Washington, D.C.:  World
     Bank, April 1979.
 
Hill, D.S., and Waller, J.M.   Pests and Diseases of Tropical
     Crops.   Vol. 1:  Principles and Methods of Control.  London:
     Longmans, Green and Co., Ltd., 1982.
 
International Institute of Tropical Agriculture.   Annual Report
     Ibadan, Nigeria:  IITA, 1977.
 
Janick, J.  "Plant Science."  An Introduction to World Crops.
     California:   W.H. Freeman and Company, 1969.
 
Kane, Mike.  "The Surprising Sweet Potato," Organic Gardening and
     Farming, V. 25, No. 5, May 1978.
 
Kassam, A.H.  Crops of the West African Semi-Arid Tropics.  Hyderabad,
     India:   International Crops Research Institute for the
     Semi-Arid Tropics, 1976.
 
Kranz, J.H.S., and Koch, W., eds.   Diseases, Pests and Weeds in
     Tropical Crops.  Berlin, West Germany:   Verlag Paul Parey,
     1977.
 
Lambert, Michael, ed., Taro Cultivation in the South Pacific.
     Handbook No. 22.  Noumea, New Caledonia:   South Pacific Commission,
     1982.
 
United States Department of Agriculture.   Tropical Yams and Their
     Potential:   Part 3, Dioscorea alata, Agricultural Handbook
     No. 495.   Washington, D.C.:   USDA, 1976.
 
Williams, C.N.  "Growth and Productivity of Tapioca (Manihot
     utilissima)."  Experimental Agriculture 10 (1974):  9-16.
 
                    SOURCES OF INFORMATION
 
Asian Vegetable Research and Development Center
P.O. Box 42
Shanhua, Tainan, Taiwan
 
Cameroon National Root Crops Research Institute
Institute of Agricultural Research
B.P. 13
Nyombe, Cameroon
 
Centro Internacional de Agriculture Tropical (CIAT), or
International Center for Tropical Agriculture
Apartado Aeroe 67-13
Cali, Colombia
 
Food Technology Development Center
Pertanian Bogor
P.O. Box 61
Bogor, Indonesia
 
International Development Research Centre
Box 8500
Ottawa, Canada K1G 3H9
 
International Institute of Tropical Agriculture (IITA)
P.M.B. 5320
Ibaden, Nigeria
 
International Potato Center (CIP)
P.O. Box 5969
Lima, Peru
 
Mayaguez Institute of Tropical Agriculture
P.O. Box 70
Mayaguez, Puerto Rico
 
National Root Crops Research Institute
P.M.B. 1006
Umudike, Umuahia
Nigeria
 
Philippine Root Crop Research & Training Center
Visayas State College of Agriculture
Baybay, Leyte 7127, Philippines
 
Root and Tuber Crops
Central Research Institute for Agriculture
J1.  Merdeka 99
Bogor, Indonesia
 
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