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Cassava (Manihot spp)

Cassava is a very important tuber crop, which is widely used as a source of food in Rivers State, and other parts of Nigeria and in many parts of the world. According to available statistics, of 1980 from the Federal Ministry of Agriculture, Rivers State produced 5.7’/0 of the total outputs of cassava in that particular year. This low percentage of outputs of the tuber crop has now been increased due to the high level of farming activities carried out in the state.

Environmental requirement
Cassava is a crop that is grown in the tropics, that is the region that lies around the equator where the temperature is high. It grows best in a warm moist climate with an average temperature range from 25-29 degree C. It grows poorly under cold climate and at temperatures below 10 degree C, growth in the plant does not take place. Cassava grows best when rainfall is between 1000-1500mm per year and is well spread throughout the year. Tuber formation in cassava depends on the amount of sunlight received per day in a particular area. Hence, it is described as a photoperiodic crop. The best soil for cassava cultivation is a light sandy-loam soil that is fertile. Good drainage is also important, as cassava growth is poor on clay or poorly drained soils because of the inability of air to enter into the pores of such soils. The degree of acidity and alkalinity, referred to as pH of a soil must normally have a range of 5.5 – 6.0. This means that the tuber crop grows in a slightly acidic soil.

imageCassava Plant

Pre-planting operation

(i) Land preparation: Cassava normally needs good soil preparation. The first step in land preparation is the clearing of the land, which usually involves cutting the grasses and shrubs, removal of stumps and burning. This tuber crop is commonly grown on mounds that range from 30-60cm high. These mounds are normally lower than those made for yam but have broader bases. This is simply because cassava tuber tends to spread more widely and grow. Their roots grow less deeply than yam tubers. With the use of mechanisation, plough and harrow are used for the land preparation. Ploughing may be to a depth of 25cm, cassava is planted on flat land, ridges, or in furrows in modern agriculture.

(ii) Planting: The planting of cassava is done using fresh stem cuttings from mature plants as an ideal planting material that may be planted upright or at an angle in the soil or horizontally below the soil surface. Cuttings planted in the vertical position normally sprout and grow leaves faster than those at an angle or horizontal in soil.

(iii) Time of planting: The choice of time of planting is to make sure that high yields of the tuber crop by the proper use of the rain during the growing season, is attained. Cassava is planted as early as possible after the beginning of the rains, or just before the main rain begins. In Nigeria, the most ideal time for planting cassava is between April-May. However, cassava can be planted at any time of the year provided the soil moisture is enough.

(iv) Depth of planting: The depth of planting of cassava depends on the type of environmental conditions prevailing in different areas. A basic idea is that where the soil is a dry sandy soil, cassava cutting should be planted fairly deep, and where the soil is moist and heavy, the planting should be fairly shallow.

(v) Spacing: The distance between planted cassava plants largely depends on the soil type and climatic factors, cassava varieties, soil fertility, cultural practices and the level of tuber use. In general, the distance between cassava plants in a field range from 1 -4m, where cassava is grown as a second crop along with the main crops such as yarns, maize, banana and melon. The rows and spacing on a field where cassava plants are grown as the only crop, are between 80-100cm from one plant stand to another. In most cases, under crop rotation system, it is used as a test crop to determine the fertility of soil.

Post-planting operation

(i) Weeding: Cassava plant is usually prone to weeds attack in a field during the first 3 to 4 months after planting, due to the slow initial growth of leaves sprouts from the cassava stem cuttings, and this leads to increased weed growth in the field along with the cassava planted in the soil. Weeding in cassava is carried out by hand or hoeing in small plots. Weeding should be done at 3, 8 and 12 weeks after planting, first is done 25-30 days after planting, the second and third weedings are carried out, 60 days and at 90 days from planting, respectively. Chemicals are now largely used to control weed in cassava production. Pre- emergence herbicides, for cassava such as Duiron, Atrazine, are applied to cassava before the seeding weeds that attack the newly planted stem cuttings in the field appear. Where cassava planting or weed control is not done in time, that is until seedling weed appears in the field, post-emergence herbicide such as glyforsinate ammonium (Basta) should be used. Integrated weed control (I. W. C) in cassava involves the proper use of other weed control methods such as cultural, chemical, biological and mechanical methods.

(ii) Nutrient requirements: Cassava is a root tuber and dicotyledonous plant that has a high requirement for potassium. Enough potassium (k) level is important for its proper growth, in order to meet up with the nitrogen level in the soil. A high level of Nitrogen, (N) in cassava plant makes the plant to grow too many leaves with little tubers. It is due to the high need of potassium by cassava that has made the tuber crop well adapted to the harsh traditional agriculture where bush burning is largely practised.
Generally, cassava plants need a fertiliser containing the nutrients nitrogen, phosphorus and potassium in the ratio 12:12:18 NPK fertiliser. For instance, a fertiliser containing the total content of 1600kg applied to cassava plants in a field on the ratio of 12: 12: 18 NPK, means that particular fertiliser supplied 600kg Nitrogen (N) 600kg phosphorus (P), and 900kg potassium to the cassava plants based on the ratio of the NPK in the fertiliser. It is best to apply fertiliser as bands on one or both sides, of each plant. Application of fertiliser by broadcast, where the fertiliser is scattered on the plant by simply throwing it from a point is not good because it tends to make weed grow between the cassava plants. This may cause the death of the plants due to the falling of the fertiliser on the leaves.

(iii) Harvesting: The exact time of harvesting a cassava crop depends on many factors: amount of rainfall, the cassava type, soil conditions and temperature of the region. However, it is best to harvest cassava at a time when the tubers are mature enough to have stored high amount of starch. Where cassava that normally matures late are planted, harvesting is carried out after 12 months from the period the cassava cuttings were planted in the field, while some early maturing varieties are harvested after 7 months from the time it is planted. In practice, cassava tubers may be rarely kept in good condition for more than two days. Therefore, farmers harvest the amount of tubers they need for immediate use, leaving the remaining tubers unharvested until when needed. Cassava is harvested by manual method using hand to pull up the tubers from the ground and shaking the soil particles from the root tubers. Mechanical harvesting of cassava has been carried out in many parts of the world where the technology has been developed.

(iv) Processing: Cassava consists of 60-70% water, and processing it into a more needed form tends to reduce the water content in its raw state. Processing of cassava depends on the form of the products desired. Generally, processing of cassava begins with the first step of removing the peels from the tubers followed by washing the cassava tubers. It could then be processed into cassava chips, flour, fermented cassava dough (foo-foo), garri and cassava starch. In processing cassava into garri, the cassava tuber is grated using the grating machine found around different parts of the state, as well as in other parts of the country, to make the fibrous materials contained in the cassava to be broken into small particles, which are then placed in a strong sack that is pressed locally using manual effort, to remove the water content in it. The dried cassava is filtered using local filter to give the dry smooth cassava which is mixed with oil before it is fried in locally made pot, to the desired garri product.
Cassava chips are made from cassava by first boiling the peeled cassava tubers and then slicing them after cooling followed by soaking in water to allow the hydrogen cyanide, HCN level in the cassava chips to be reduced before it is finally ready for use as food.

(v) Storage: Cassava is not stored as fresh tuber after it has been harvested. The reason for avoiding this practice is that cassava tuber contains a high amount of prussic acid or hydrocyanic acid. It is this acid that makes the cassava tuber to give out an offensive odour when not consumed few days after harvesting. Some cassava products can be stored in a dry place after they have been processed for a period of a month, under good hygienic conditions. The cassava products that are good for storage include cassava flour, and garri, other products such as cassava chips, fermented cassava dough (foo-foo), and cassava starch may not be properly stored for a long period due to the high hydrocyanic acid content. It is this acid that causes cassava tuber and its product to have poor quality after few days, when not consumed.

Economic importance of cassava
Cassava is a highly valued tuber crop and has become important as a result of the following:

(i) It withstands drought (i.e. severe dryness of water in soil).
(ii) It grows with low nutrients supply.
(iii) It has high resistance to diseases and .pests.
(iv) It is good for peasant production.
(v) It has high energy.
(vi) It is easy to produce.
(vii) It has relative ease of breeding.

Problems of cassava

(i) Low protein content.
(ii) The high hydrocyanic acid problem.
(iii) Storage problem.
(iv) Mechanisation of harvesting.
(v) Diseases and pests.

Cassava diseases

(a) Mosaic disease
Causative agent: Virus.
Transmissive agent (vector): White flies.

(i) Appearance of deformed leaves.
(ii) Curly leaves.
(iii) Yellowing of leaves.
(iv) Stunted growth.

(i) Planting resistant variety
(ii) Uprooting and burning of infected plants promptly.

(b) Leaf spot
Causative agent: Fungus -Cercospora spp.
Transmission: Spores are airborne and spread by deposit on leaves.

(i) Appearance of pale brown spots which turn dark brown.
(ii) Deadening of cells in the infected (i.e. Necrosis) area.

(i) By spraying with fungicides.
(ii) By planting resistant varieties.
(iii) By practise of crop rotation.

Pests of Cassava:
Grasshopper, Spider, Mite, Mealy bug, rodents.

Through biological, chemical, cultural and integrated pest control methods.

Cassava is a root tuber which grows best in light sandy-loam soil with less acid content. It is planted using stem cuttings. Its products, are not stored for a long period due to its high hydrocyanic acid content.