Cassava, also called Tapioca or Maniod originated in the American tropics but is now grown widely throughout all tropical countries including parts of South and Central America and West Africa and is an important food crop for humans. In other parts of the tropics the tubers are used as animal feed being very high in starch but low in protein. The tubers are also used to make glucose, the chemical sodium glutamate which is added to many tinned and preserved foods to bring out the flavor of the food and used in the manufacture of high grade starch. The cassava plant grows to a height of 2.5 metres and may be a single stem or a branched stem when it has been growing for about 2 months some of the roots start to swell and form tubers, which may grow straight down into the ground or grow out sideways from the main root, which are easier to harvest without damage or loss of the tubers. The plant manufactures starch, which content can be as high as 30%of the weight of the tuber and is stored in the tubers which are used as a storage organ. Due to the tuber begining to decompose within 48 hours of being harvested, it is necessary to chip them and to allow the chips to dry out in the sun before bagging and storing.
1000 – 1500 Mm per annum is optimum but grows successfully with rainfall of 500 mm per annum and in an area that receives more than 3000mm which is well distributed. It is a valuable crop where rainfall is erratic. Cassava has an optimum temperature range of between 25 – 35oC and can withstand a very light frost but requires a period of 300 days of a frost free period for best results. Growth ceases at temperatures of 10oC and below which adversely effects growth and development.
Cassava can be grown on a wide range of soils that is not susceptible to waterlogging. The best soils are sandy or sandy loam soils. The optimum soil pH is between 5.0 and 8.0. They will also grow well in poor acid soils that have high aluminium content.
There are hundreds of varieties which have been given different local names in the regions where they are grown. They vary in their type of growth, the shape of leaves, size, colour and characteristics of their tubers. Some varieties have high concentrations of toxic substances called hydrocyanic glucosides (prussic acids) which form a very dangerous poison, cyanide, when the tubers are harvested. These can be used for food after they have been prepared otherwise are highly poisonous both to humans and stock. Other varieties are called sweets because their tubers contain very little or no toxic substances and can be eaten when cooked.
There are approximately a dozen varieties with different characteristics of local origin available for planting in Central Africa. All the varieties are unimproved and show great variation between individual plants for factors such as yield, disease and pest resistance. Some varieties appear in all areas where cassava is grown, while others are only found in certain localities. Most of the varieties grown here are the sweet varieties with a low content of cyanide (prussic acid). The main cultivars used are MSaF2 and CMC40.
Cassava is propagated by means of stem cuttings which should be taken from parent plants showing desirable characteristics of growth and yield, free from disease, particularly Mosaic. Farmers normally propagate from their own stock, but there are a number of growers of commercial cassava from whom planting material can be obtained which will be of good quality.
The tall stems of the parent plants are cut into lengths of 20 – 50 cm depending on the amount of material available. Where there are plenty of stems, longer lengths can be used which should have at least 5 nodes with healthy buds and should be undamaged. To ensure a clean cut, the stems should be cut up with a hacksaw or very sharp knife. Any part of the old stem can be used except the green section at the top and can be kept for up to 6 weeks before planting but should be washed in an insecticide solution before drying and storing.
The best time to take cuttings is towards the end of the winter, in June and July and can be planted from August onwards to take advantage of the warmer weather and early rains. Cuttings planted in August must have supplementary watering or irrigation. Cuttings that are planted in June and July will remain dormant until the warmer weather in August and September. Those planted in mid‐ Summer and Autumn do not grow as well as those planted earlier in the season and should be planted at the beginning of the rainy season usually in Spring and early Summer.
The distance between plants and rows and the method of planting will depend on the soil type and the rainfall area where the crop is grown. Usually cuttings are planted in rows 80 – 100 cm apart with similar inter‐row spacing.
Planting on the flat can be done on sandy soils and areas of good rainfall. The cuttings are laid down in furrows horizontally at a depth of about 100 mm below the surface and the furrows are then covered up leaving the ground level.
Planting on sloping soils must be done on ridges which follow the shape of the contours to prevent soil erosion. Planting on heavy soils prevents waterlogging and makes harvesting easier. Cuttings planted on ridges may be placed vertically into holes in the ridge or laid at an angle with the top half or one third of the cuttings sticking out of the ground.
Planting in furrows is done in low rainfall areas to allow water to accumulate in the bottom of the furrow. When the plants have grown to the height of the ridges on either side of the furrows, the ridges are ‘split back’ leaving a furrow between the rows where the water can collect and feed the roots of the growing plants.
When planting cassava cuttings, it is important not to damage the material and make sure the cuttings are planted the right way up with the shoots pointing towards the soil surface. Planting upside down can account for up to 60% of losses in a stand of young plants.
Figure 2: Cassava cutting
If the land has been ploughed and prepared properly for planting weeds should not be a problem during the stage of germination and early growth. The first weeding should take place approximately 3 weeks after planting. Weeding should be done before the crop canopies to be sure the plants are not damaged. Mechanical cultivation either by hand or tractor and cultivator will kill weeds to the time when the growing plants form a complete canopy over the soil and prevent further germination and growth of weeds. Use a pre‐emergent herbicide such as Ametryn at a rate of 6 – 8 litres/ha followed by Paraquat at a rate of 1.25 – 5 litres/ha if chemical weed control is required. Ametryn needs to be applied at planting with 10 – 15 mm of rain or irrigation afterwards to leach the chemical into the rooting zone.
Plant populations of between 10 000 and 20 000 plants per hectare should be satisfactory and can be achieved by planting cuttings from 0.5 ‐ 1 metres apart in rows that are 1 metre apart. When planting cassava as a fodder crop, plant populations can vary from 60 000 – 110 000 plants/ha.
Cassava is a crop suited to areas of low rainfall and poor soils and under these conditions it will produce tubers although the process is rather slow. Under good conditions of rainfall and soil fertility high yields of tubers are produced and the crop will benefit from the application of fertilizers. Fertilizer is required during the early growing period so that the largest possible area of leaf is formed. The green leaves then manufacture the starch stored in the tubers. The period of tuber bulking is from February to July and during this time water is the main requirement.
A 25 t/ha crop will remove 122 kg N, 27 kg P, 145 kg K, 45 kg CA and 20 kg Mg. For high yielding crops growing in good conditions, the general fertilizer recommendations are as follows:
Nitrogen 70 kg/ha
Phosphate 30 kg/ha
Potash 70 kg/ha
Nitrogen 50 kg/ha
Phosphate 30 kg/ha
Potash 70 kg/ha
The top dressing can be applied in one or two dressings up to 3 months before the crop is harvested. Soils with a pH of below 4.5 should be limed with magnesium limestone. Cassava responds well to organic manures and any crop residues and vegetation or farmyard manure should be ploughed in before the cuttings are planted.
Where the crop is grown in areas of low rainfall and poor soils, there is little point in applying large amounts of fertilizer, because the nutrients will not be utilised by the plants. Small quantities of fertilizer may be applied during each growing season and will benefit the plants provided it is washed into the soil by rainfall or watering.
5. PESTS AND DISEASES
Cassava is a host of many insect pests but very few cause great economic loss because of the presence of toxic substances in all parts of the plant. Other pests that can attack cassavas is the white fly which carries the ACMD virus, grasshoppers, spider mites and the cassava mealy bug. Wild pigs, baboons and rodents can damage the tubers while nematodes can cause a reduction in harvestable yield.
The main disease of the crop is African Cassava Mosaic Virus (ACMD) which is spread by aphids which affects the leaves of the plants. The only way to control the disease is to select planting material from resistant parent plants and avoid plant cuttings from infected plants. Infected plants should be removed and burned. Another disease is Cassava bacterial blight.
There are no registered chemicals for the control of diseases and insect pests for the use on cassava.
The harvesting of the crop is carried out mainly by hand, the plants being dug out of the ground and stems cut off to leave the tubers. Where the crop is grown on high ridges, the tubers can be loosened in the soil using a blade behind a tractor but this is liable to cut and damage the tubers. The leaves and stems are not required for planting and should be left on the ground and ploughed into the soil. The leaves can be used as a stock feed or cooked as a vegetable.
The time of harvesting will depend on the area where the crop is growing. Cassava is a perennial plant and its development depends on the available conditions of water and nutrients. Under good conditions the crop will mature and be ready for harvesting 10 months after planting. With poor conditions the crop may be harvested only after 2, 3 or even 4 years in the ground.
The varieties of cassava that contain toxin have to be processed before being used for human or animal feed. In West Africa this is done by placing the tubers in deep ponds where a type of fermentation takes place and neutralises the toxin. The soft tubers are then broken open, grated and the white starchy material dried in the sun.
Fresh tubers may be used for human or stock feed immediately after lifting; however, they have a very short storage life (48 hours) and must be cut into small pieces (chipped) then dried quickly by
|Opaque: not able to be seen through; not transparent.
sun and wind. Commercial growers have a machine for chipping and drying floor where the chips are spread out. The tubers are washed and trimmed of woody pieces, passed through the chipping machine and laid out on the drying floor where they are turned over by a shovel. This is done every 3
– 4 hours during the 2 days that takes the chips to dry. After 2 days the moisture content should be about 14% and the chips bagged up and stored in a dry shed. A normal grain bag will hold 40 kg of chips between 2.5 and 3 tons of fresh tubers will produce 1 ton of dried chips. Under favourable conditions the chips dry to a very white colour.
The dried cassava chips are a valuable source of starch which could be used for many purposes if there were more commercial growers. For human food, cassava tubers can replace potatoes and Cassava meal replaces maize meal and cassava flour can be used for baking. Cassava chips are a good stock feed with high energy value but are very low in protein. Cassava chips can replace maize and sorghum for the brewing
of opaque beer. The starch in cassava could be fermented in the same way as sugar from sugar cane and would produce alcohol and ethanol.
One of the greatest advantages of cassava is that it will produce useful yields of tubers under the poorest conditions of rainfall and fertility. Once the crop is established and the green leaves start manufacturing starch, the tubers will increase in size when conditions are good. During a drought period or Winter, the tubers stop growing but as soon as conditions improve, they will start to grow again. The yields of the plants are determined by the farmer as he can decide whether to leave the crop in the ground until he is satisfied with the yield. This may occur in 1 season or take 3 or more seasons in an arid area. A crop can withstand frosts of ‐ 10° C.
This will depend on when the farmer decides to harvest the crop but yields should be from 20 ‐ 40 tons of fresh tubers per hectare. Under favourable conditions it can yield up to 90 tons/ha. On marginal soils and poor harsh climatic conditions the yield can be reduced to 10 tons/ha.
Figure 3: Cassava plant and Tubers