Introduction
In the Far East. cassava was not
known as a food plant until 1835. In about 1850 it was transported directly
from Brazil to Java, Singapore and Malaya. When the more profitable rubber
plantations were started on the Malay peninsula, cassava growing moved to other
parts of Indonesia where it flourished. During the period 1919-41 about 98
percent of all cassava flour was produced in Java, but during the Second World
War Brazil increased and improved its production.
Cassava was unknown to the Old World
before the discovery of America. There is archaeological evidence of two major
centres of origin for this crop, one in Mexico and Central America and the
other in northeastern Brazil. The first Portuguese settlers found the native
Indians in Brazil growing the cassava plant. and Pierre Martyr wrote in 1494
that the "poisonous roots" of a yucca were used in the preparation of
bread. It is believed that cassava was introduced to the western coast of
Africa in about the sixteenth century by slave merchants. The Portuguese
brought it later to their stations around the mouth of the Congo River, and it
then spread to other areas.
In 1854 Livingstone described the preparation of
cassava flour in Angola, and subsequently Stanley described its use in the
Congo. Cassava cultivation increased after 1850 in the east African territories
as a result of the efforts of Europeans and Arabs who were pushing into the
interior and who recognized its value as a safeguard against the frequent
periods of famine.
Now grown throughout the tropical
world, cassava is second only to the sweet potato as the most important starchy
root crop of the tropics.
The cassava plant has been
classified botanically as Manihot utilissima Pohl of the family Euphorbiaceae.
In recent publications, however, the name Manihot esculenta Crantz is being
increasingly adopted.
The plant is popularly known under a
great variety of names: ubi kettella or kaspe (Indonesia), manioca, rumu or
yucca (Latin America), mandioca or aipim (Brazil), manioc (Madagascar and
French-speaking Africa), tapioca (India, Malaysia), cassava and sometimes
cassada (English-speaking regions in Africa, Thailand, Sri Lanka).
The term cassava (manioc in
French-speaking countries) is usually applied in Europe and the United States
of America to the roots of the cassava plant, whereas tapioca denotes baked
products of cassava flour. The word tapioca derives from tipioca, the Tupi Indian
name for the meal which settles out of the liquid expressed from rasped tubers
and is made up into pellets called tipiocet.
Because it grows easily, has large
yields and is little affected by diseases and pests? the areas under cassava
cultivation are increasing rapidly. The plant is grown for its edible tubers,
which serve as a staple food in many tropical countries and are also the source
of an important starch. Its value as a famine relief crop has long been
recognized. In parts of the Far East during the Second World War many people
survived on cassava roots, and in Africa it was a principal food source for
workers in mining and industrial centres.
It is now grown widely as a food
crop or for industrial purposes. In many regions of the tropics cassava
occupies much the same position as white potatoes do in some parts of the
temperate zones as the principal carbohydrate of the daily diet. The industrial
utilization of cassava roots is expanding every year.
In the early decades of this
century, cassava was held responsible for the rapid exhaustion of forest
clearings, but later experiments in many parts of the tropics showed that it is
not a soil-depleting crop. Since the Second World War, a more balanced
appraisal of the crop has developed. More scientists, agriculturists and
sociologists have become aware of its importance in developing countries, where
it is most commonly produced. In many countries emphasis is being placed on
research for the improvement of production and utilization of cassava crops.
Cassava cultivation
The plant
The cassava plant is a perennial
that grows under cultivation to a height of about 2 4 m. The large, palmate
leaves ordinarily have five to seven lobes borne on a long slender petiole.
They grow only toward the end of the branches. As the plant grows, the main
stem forks, usually into three branches which then divide similarly. The roots
or tubers radiate from the stem just below the surface of the ground. Feeder
roots growing vertically from the stem and from the storage roots penetrate the
soil to a depth of 50-100 cm. This capacity of the cassava plant to obtain
nourishment from some distance below the surface may help to explain its growth
on inferior soils.
Male and female flowers arranged in
loose plumes are produced on the same plant. The triangular-shaped fruit
contains three seeds which are viable and can be used for the propagation of
the plant. The number of tuberous roots and their dimensions vary greatly among
the different varieties. The roots may reach a size of 30-120 cm long and 4-15
cm in diameter, and a weight of 1-8 kg or more. The plant, its flowering shoot
and its various parts are shown in Figures I and 2.
Clusters of root of the Bogor
variety, ripe for harvesting, are shown in Figure 3. A cross section of the
root is given in Figure 4. The peel consists of an outer and an inner part, the
former comprising a layer of cork cells and the phellogen. The cork layer,
generally dark-coloured, can be removed by brushing in water, as is done in the
washers of large factories. The inner part of the peel contains the phelloderm
and the phloem, which separates the peel from the body of the root. The texture
of the transition layer makes possible an easy loosening of the whole peel from
the central part, thus facilitating the peeling of the roots.
The cork layer varies between 0.5
and 2 percent of the weight of the whole root, whereas the inner part of the
peel accounts for about 8-15 percent. Generally in ripe roots this is about 2-3
mm thick. The starch content of the peel is only about half that of the core.
The peel is much firmer in structure, hindering a smooth rasping by primitive
raspers; small factories prefer to peel the roots before working them up. The
loss of starch incurred by rejecting the peel. however' is not acceptable to
the larger factories. which remove only the cork layer.
Agricultural practices
PREPARATION OF THE LAND
When cassava is grown as the first
crop in forest land. no further preparation is required than the clearing of
the forest growth. When cassava is grown after other crops. it often can be
planted without further preparation of the soil, once the preceding crop has
been harvested or the soil has been ploughed two or three times until free from
grass and other plants.
Clearing of forest land is done to
let in more sunlight to the ground and to remove weeds and undergrowth which
might otherwise compete with economic plants. The practice in tropical
southeast Asia is to clear the forest soil completely, including the removal of
all roots and other obstructions beneath the soil, by cutting and burning the
forest cover; the land is then deeply ploughed. African practice is to burn the
land cover only. Burning removes only small branches and underbush but does not
consume all of the trunks and branches. It also destroys soil parasites, and
the layer of ashes increases the amount of potassium salts available to the
growing plants. However, some reports have indicated that complete clearing of
the soil in certain parts of Africa caused deterioration due to the leaching
out of nutrients.
PLANTING
Cassava culture varies with the
purposes for which it is grown.
Cassava is either planted as a
single crop or intercropped with maize, legumes, vegetables, rubber, oil palm
or other plants. Mixed planting reduces the danger of loss caused by
unfavourable weather and pests by spreading the risk over plants with different
susceptibilities.
For agricultural purposes, cassava
is propagated exclusively from cuttings. It is raised from seed only for the
purpose of selection Seeds produce plants with fewer and smaller roots than
those of the parents and as many as half of the seeds may fail to germinate. On
the other hand, cuttings taken from the stalks of the plant take root rapidly
and easily, producing plants identical in character with the parent plants.
Cuttings are obtained from the stems
of plants at least ten months old and 2.53.5 cm thick. After harvesting, these
stems are stored in a dry place until the next planting (Fig. 5). Cuttings
about 25 cm long should be taken from the lower 75150 cm of the stem after the
first 20 em have been discarded. Cuttings from the upper part of the stem will
grow faster, but their final yield is less. The best practice is to saw a
bundle of stalks supported by a girder and then to point the cuttings thus
obtained at the lower end (Fig. 6), taking care not to bruise the buds or
otherwise damage the stem.
Experiments in the Philippines on
the relation between the age of cuttings and yields showed that cuttings taken 75
cm or more below the apex of the stem gave the best starch yields. Other
experiments concluded that older wood from the basal areas to the midpoint of
the stems outyielded apical propagating material.
Key to Figure 4:
Left upper quadrant. after staining
with iodine:
1. Peel
1a. Outer cork layer
1b. Inner layer
1b. Inner layer
2. Cambium
3. Centre
4. Pith and primary xylem
3. Centre
4. Pith and primary xylem
(Both peel and pith contain
comparatively little starch.)
Right upper quadrant, showing
structural elements of the root:
5. Cork
6. Sclerenchymatous fibres
7. Latex vessels
8. Cambium
9. Xylem vessels
6. Sclerenchymatous fibres
7. Latex vessels
8. Cambium
9. Xylem vessels
Inset A - Enlarged cross section of
peel:
10. Cork tissue
11. Sclerenchymatous fibres
12. Starch (small grains)
13. Parenchyma cell
11. Sclerenchymatous fibres
12. Starch (small grains)
13. Parenchyma cell
Inset B - Enlarged cross section of
centre:
14. Cell wall (larger cells than in
peel)
15. Starch ( big grains)
15. Starch ( big grains)
Cuttings are planted by hand or by
planting machines. Hand planting is done in one of three ways: vertical, flat
below the soil surface or tilted 45" from ground level. Under low rainfall
conditions. vertical planting may result in the desiccation of the cuttings,
while in areas of higher rainfall, flat-planted cuttings may rot. In general,
flat planting 5-10 cm below the soil surface is recommended in dry climates and
when mechanical planting is used. Germination seems to be higher; tubers tend
to originate from a great number of points and grow closer to the surface of
the soil, making better use of fertilizers applied on the surface and also
making harvesting easier. On the other hand' vertical planting is used in rainy
areas and tilted planting in semi-rainy areas.
The cuttings are planted on flat
soil or on ridges or hills. Some experiments have shown ridging to produce
somewhat lower yields than flat cultivation; but the work of weeding and
harvesting is greatly reduced by ridge planting. As machine planting would be
impossible with furrows or on ridges, flat fields are the most desirable.
Spacing between rows is about 80100 cm, and the plants are spaced along the
rows according to local conditions. The number of plants per hectare varies in
different regions between 10000 and 15000.
Time of planting is influenced by
weather conditions and the availability of planting material. Cassava is
usually planted at the beginning of the rainy season. In order to reduce risk
and to distribute the hard work of cultivation more evenly, planting is
sometimes divided between the two rainy seasons. It is usually carried out
throughout the year in regions with year-round rainfall. If the stalks are
saved for a long time after harvest, they are not apt to root and grow well. It
is therefore desirable to plant and harvest at the same time.
Experience has shown that, from the
standpoint of starch production' the development of the cassava plant is most
profitable when planting takes place at the beginning of a humid period (i.e.,
in tropical regions at the beginning of the monsoon).
CULTIVATION
Cassava is frequently cultivated as
a temporary shade plant in young plantations of cocoa, coffee, rubber or oil
palm. In Thailand, however, it is grown mostly as a sole crop and the farmer
may for ten years or more grow cassava on the same land. If the price of
cassava roots drops, the farmer may shift to another crop (e.g., sugarcane,
maize or sorghum) until cassava again becomes the more profitable crop.
Water is essential until the plant
is well established. In moist soil, sprouting takes place within the first week
after planting. Generally about 5 percent of the cuttings will not come to
development, so a corresponding surplus has to be provided for. Within a month
of the beginning of planting, the substitution of new cuttings is still
possible.
When cultivated as a temporary shade
plant, no special attention is given to the cassava plant. When grown alone,
the plants require little maintenance after planting. Irrigation may be
required if there is no rain, and hoeing of the earth helps preserve the
subsoil humidity, especially in dry sandy soils. The chief problem is weed
control. It may be desirable to weed the crop two or three times until the
plants are well developed and their shade prevents the growth of weeds.
CLIMATE
Cassava is a typical tropical plant.
The approximate boundaries for its culture may be accepted as from 30ºN to 30ºS
latitudes; however, most cassava growing is located between 20ºN and 20°S. In
general, the crop requires a warm humid climate. Temperature is important, as
all growth stops at about 10ºC. Typically' the crop is grown in areas that are
frost free the year round. The highest root production can be expected in the
tropical lowlands, below 150 m altitude, where temperatures average 25-27°C,
but some varieties grow at altitudes of up to 1 500 m.
The plant produces best when
rainfall is fairly abundant, but it can be grown where annual rainfall is as
low as 500 mm or where it is as high as 5 000 mm. The plant can stand prolonged
periods of drought in which most other food crops would perish. This makes it
valuable in regions where annual rainfall is low or where seasonal distribution
is irregular. In tropical climates the dry season has about the same effect on
Cassava as low temperature has on deciduous perennials in other parts of the
world. The period of dormancy lasts two to three months and growth resumes when
the rains begin again.
As a tropical crop, cassava is a
short-day plant. Experiments conducted in hothouses show that the optimum light
period is about 12 hours and that longer light periods inhibit starch storage.
SOIL
Cassava grows best on light sandy
loams or on loamy sands which are moist, fertile and deep, but it also does
well on soils ranging in texture from the sands to the clays and on soils of
relatively low fertility. In practice, it is grown on a wide range of soils,
provided the soil texture is friable enough to allow the development of the
tubers.
Cassava can produce an economic crop
on soils so depleted by repeated cultivation that they have become unsuitable
for other crops. On very rich soils the plant may produce stems and leaves at
the expense of roots. In some parts of Africa freshly cleared forest soils are
regarded as highly suitable after they have borne a cereal crop.
FERTILIZATION
No fertilization is required when
the land is freshly cleared or when there is enough land to enable the
cultivator to substitute new land for old when yields fall. Like all rapidly
growing plants yielding carbohydrates, cassava has high nutrient requirements
and exhausts the soil very rapidly. When cassava is grown on the land for a
number of years in succession or in rotation. the soil store of certain
nutrients will be reduced and must therefore be returned to the soil by
fertilization.
Various experiments in Brazil, India
and many regions of Africa and the Far East showed significant increases in
yield, of roots as well as starch content, obtained by the application of
fertilizers. Potassium salts favour the formation of starch, and nitrogen and
phosphorus are essential for growth. However, if the soil contains large
quantities of assimilated nitrogen, the result will be heavy development of
vegetative growth without a corresponding increase in root production.
Generally speaking, fertilization is
practiced at present in most parts of Africa and South America only on
commercial plantations. In Thailand, only a few farmers apply artificial
fertilizers, as they are usually too costly for the small farmer. Most farmers
use different kinds of organic manures, such as cattle or duck manure or garbage.
The kinds and quantities of
fertilizers required by a cassava crop depend on the nature of the soil.
DISEASES AND PESTS
In many regions, the cassava plant
is not normally affected by diseases or pests. However, in others it may be
attacked by the following:
(a) Virus diseases. Mosaic, the
brown streak and leaf curl of tobacco may attack leaves, stems and branches.
Many parts of Africa harbour these diseases and attempts are being made to
select resistant varieties.
(b) Bacterial disease. Bacteria such
as Phytomonas manihotis (in Brazil), Bacterium cassava (in Africa) and
Bacterium solanacearum (in Indonesia) may attack roots, stems or leaves of
cassava plants.
(c) Mycoses. There are kinds which
attack roots, stems, or leaves of cassava plants and cause various diseases.
(d) Insects. Some insects affect the
plant directly (locusts, beetles and ants); others affect the plant indirectly
by the transfer of virus (aphids).
(e) Animals. Rats, goats and wild
pigs are probably the most troublesome; they feed on the roots, especially in
areas adjacent to forests.
TOXICITY
The toxic principle in cassava is
hydrocyanic, or prussic, acid, found in the roots, branches and leaves of the
plant in both free and chemically bound forms. The plant contains a cyanogenetic
glucoside called phaseolunatin begins to break down upon harvest into
hydrocyanic acid, acetone and glucose by the action of the enzyme linase. The
presence of hydrocyanic acid is easily recognized by a bitter taste. At the
harvest of cassava roots, the amount of the acid in the plant varies from
harmless to lethal - from a few milligrams to 250 milligrams or more per
kilogram of fresh root. Investigations show that the glucoside content in the
cassava plant is markedly increased by drought and by potassium defciency.
Hydrolysis of the glucoside by the
enzyme can be accelerated by soaking the roots in water, by crushing or cutting
them or by heating. It was found that the hydrocyanic acid content varied
little in different tubers of one plant but varied considerably in tubers
obtained from different locations. The distribution of the acid in roots varied
in different varieties. In sweet varieties, the major part of the acid is
located in the skin and in the exterior cortical layer, while in bitter varieties
the acid is uniformly distributed in all parts of the roots.
In choosing a strain, the
hydrocyanic acid content should be taken into account. Highly poisonous strains
are preferred for plantings with the object of starch manufacture, thereby
minimizing thefts by both animals and men.
VARIETIES
Although cassava is an established
commercial crop in many tropical countries and hundreds of varieties are in
existence, little is generally known of the nomenclature and identification of
varieties. Various varieties are usually differentiated from one another by
their morphological characteristics such as colour of stems, petioles, leaves
and tubers. Moreover, in many instances the same variety is known in various
places by a number of names.
The numerous varieties of cassava
are usually grouped in two main categories: Manihot palmata and Manihot aipi,
or bitter and sweet cassava. This grouping is a matter of economic convenience,
as it is difficult to distinguish the two groups by botanical characteristics.
However, the distinction between them rests upon the content of hydrocyanic
acid, which causes toxicity in the roots. This toxicity is not a variety
constant but varies from place to place; all cassavas are now regarded as
varieties of Manihot utilissima, and in certain circumstances a
"bitter" variety may become "sweet" and vice versa.
Hydrocyanic acid content tends to be higher on poor soils and in dry
conditions. According to the recognized classification, sweet. or nontoxic.
roots contain less than 50 milligrams of hydrocyanic acid per kilogram of fresh
matter.
At one time it was thought that the
toxicity of a cassava root was associated with species or variety, but the
hydrocyanic acid content was found to vary markedly with growing conditions,
soil, moisture, temperature and age of the plant. Certain varieties in Africa,
for instance, were found to be innocuous in Dahomey and poisonous when grown in
forest soils in Nigeria; the so-called bitter type from Jamaica failed to
produce the toxic substance when grown in Costa Rica.
The chemical composition of cassava
roots differs considerably. Studies of 30 varieties in Mexico gave the
following results: the dry-matter content of the roots varied between 24 and 52
percent, with a medium of 35 percent; protein content varied between I and 6
percent, with a medium of 3.5 percent.
Table 1 is based on an analysis made
in Madagascar comparing the cassava root with the potato
For industrial development, many
efforts are being made to organize research and experiments in various
geographical regions for the selection of new varieties with high yields of
roots and higher starch content. For purposes of nutritional improvement.
strains with a high protein content are being sought.
TABLE 1. - AVERAGE COMPOSITION OF
THE CASSAVA ROOT AND THE POTATO (COMMON VARIETIES AT HARVEST TIME)
Cassava
|
Potato
|
|
Percent
|
||
Moisture
|
70.25
|
75.80
|
Starch
|
121.45
|
19.90
|
Sugars
|
5.13
|
0.40
|
Protein
|
1.12
|
2.80
|
Fats
|
0.41
|
0.20
|
Fibre
|
1.11
|
1.10
|
Ash
|
0.54
|
0.92
|
1 Bitter varieties usually average about 30 percent starch
content.
HARVESTING
Harvesting of cassava can be done
throughout the year when the roots reach maturity. In regions with seasonal
rains, like Madagascar, harvesting is usually done in the dry season, during
the dormant period of the plant; where rain prevails all year round, as in
Malaysia, cassava is harvested throughout the year.
Maturity differs from one variety to
another, but for food the tubers can be harvested at almost any age below 12 months.
From the standpoint of starch
production, cassava should be considered ripe when the yield of starch per
hectare is highest. An optimum age of 18-20 months was found in experiments
with certain strains of the variety "São Pedro Preto" in a tropical
climate (Java). The graph in Figure 7 shows the influence of the age at
harvesting on the starch yield as the percentage loss of yield in relation to
the yield at the optimum age as found in an experiment with a definite strain.
It is seen that both root and starch
production increase rapidly to their maximum value, after which root production
decreases slowly and starch production much more rapidly on account of the
declining starch content of the tubers.
If the roots are left in the ground,
starch content increases with age until, at a certain point, lignification
takes place, causing the roots to become tough and woody, so that they are
harder to prepare for consumption and other uses.
Once the roots are harvested, they
begin to deteriorate within about 48 hours, initially owing to enzymatic
changes in the roots and then to rot and decay. The roots may be kept
refrigerated for up to a week. They may be stored in the ground for longer
periods if they are not detached from the plant.
Harvesting is still generally a
manual operation, although equipment to facilitate this operation is being
considered. The day before harvest, the plants are "topped" - the
stalks being cut off 40-60 cm above ground by hand, machete or machine and
piled at the side of the field. This length of stalk is left as a handle for
pulling. Material required for the next planting is selected and the rest is
burned. In light soils the roots are slowly drawn from the soil simply by
pulling the stems or with the help of a kind of crowbar and the tubers are cut
off the stock. In heavier soils a hoe may be required to dig up the roots
before the plant is pulled out. It must be noted that once the plants have been
topped, lifting of the roots must not be delayed, as sprouting and a drastic
fall in the starch content of the tubers will result.
YIELD
Cassava is not usually grown on
soils where it would be most productive - that is, the light sandy loams,
fertile and deep, which are reserved for other crops less tolerant of poor
soils. When cassava is grown by traditional tropical methods, yields lie
between 5 and 20 tons per hectare, varying with the region, the variety, the
soil and other factors. However, when the crop is given more attention, yields
of 30 40 tons per hectare are obtained. It has been reported that it is normal
for some varieties, under appropriate cultivation methods, to yield over 60
tons per hectare.
The high yields frequently achieved
at agricultural experiment stations and occasionally by some active farmers
show what might be accomplished with improved varieties and better cultural
practices.
Nevertheless, cassava yields in
total calories per hectare compare very favourably with those of other starchy
staples, as shown in Table 2.
TABLE 2. - AVERAGE YIELDS OF
TROPICAL STARCHY STAPLES, 1948-52
Brazil
|
Java
|
India
|
China
(prov. Of Taiwan)
|
|
Millions of calories per hectare
|
||||
Maize
|
4.4
|
2.4
|
2.3
|
5.0
|
Rice
|
3.9
|
3.9
|
2.8
|
5.5
|
Yams and sweet potato
|
7.5
|
5.4
|
5.6
|
8.6
|
Cassava
|
14.2
|
7.1
|
5.8
|
11.6
|
SOURCE: FAO Yearbook of Food and
Agricultural Statistics (1955).
Mechanization
In most of the tropical world
cassava is grown on small plots; however, in some countries (e.g., Mexico.
Brazil and Nigeria). large plantations have been started and interest in
mechanization is growing. The degree of mechanization depends on the amount of
land, available labour in the area and general policy regarding the use of
manual labour.
The use of machinery for land
preparation is preferable to manual labour to ensure the best possible seed bed
for tuber development. Subsequent operations of planting. weeding, topping and
harvesting can be done by hand as well as by machinery.
Labour input for the production of a
hectare of cassava varies widely in different parts of the tropics. It has been
estimated that in Zaire 778-830 manhours are required per hectare of cassava,
including preparation of cuttings, planting, weeding and harvesting. as
compared with 1 868-2206 man-hours in Uganda. In South America and the
Caribbean a maximum of 494 man-hours has been estimated for the same operations.
A possible reason for the higher requirements in Africa is the growing of
cassava as an intercrop, with more timeconsuming operations.
The peculiar nature of the cassava
crop presents a number of problems as regards mechanization, hut it has been successfully
mechanized to a degree in some countries.
The following is an outline of the
present use of machinery in cassava cultivation:
(a) The hoe remains the principal
implement for cultivating, weeding and harvesting.
(b) Basic operations. such as ploughing
and harrowing, may be done by tractor.
(c) A mechanical planter made in
Brazil is in use there and in Mexico. It is a tworow planter using a tractor
driver and two men on the machine to feed cuttings from the reserve bins into
the rotating planting turntable. In operation, the cuttings fall in succession
through a hole into a furrow opened by a simple furrower. A pair of disks throw
dirt into the furrow and floats pulled by chains pack the soil over the
cuttings. The planter is able to cover about 5 hectares per day.
(d) A unit for ridging and planting
has been developed by modifying a ridger to work as a ridger-cum-planter and a
cultivator to work as a six-row planter after the area has been ridged.
(e) A simple machine used in Mexico
is a gasoline-powered table saw to prepare the cuttings for planting. The
machine has the advantage of speed and regularity of produced cuttings. As
compared with manual work the time saving is 3:1.
(f) It has been found to be
virtually impossible to carry out the first weeding operation between the
ridges with a cultivator. Mechanical weeding of the top of the ridges presents
a number of difficulties.
(g) A topping machine consisting of
a heavy screen mounted on the front of a tractor has been developed to push
down the tops: then a rotary mower on the back of the same tractor can cut the
downed top to make harvesting by hand possible. The height at which the tops
are cut back can be easily regulated with any rotary mower.
(h) Cassava is not a crop that lends
itself readily to mechanical harvesting because of the way the tubers grow.
They may spread over I m and penetrate 50 60 cm. Careless use of machinery for
harvesting can damage tubers, resulting in a darkening due to oxidation that
will lower the value of the flour. However, in Mexico and Thailand, mould-board
ploughs have been used to make hand harvesting less tedious. Stalks can be cut
successfully by a mid-mounted mower or a topping machine, and the roots are
lifted mechanically with a mid-mounted disk terracer. In Ghana, about 2000 m2
could be harvested in 21/2 hours by a tractor' whereas ordinarily 5 man-days
were required A modified beet or potato harvester has been suggested for use
behind the tractor, with a pulling mechanism in place of the digging shares to raise
the tubers by pulling at the cut stems left after topping.