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).
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.
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.
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.
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
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
(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
(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
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
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.
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
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)
1 Bitter varieties usually average about 30 percent starch content.
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.
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. - AVERAGE YIELDS OF TROPICAL STARCHY STAPLES, 1948-52
|Brazil||Java||India||China (prov. Of Taiwan)|
|Millions of calories per hectare|
|Yams and sweet potato||7.5||5.4||5.6||8.6|