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.
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.
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)
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 met