Roots and Tubers
Cassava (Manihot esculenta) is a commonly produced tuber crop in Africa. It can be used as food, as a cash crop, as feed for animals and as a source of industrial raw material. In sub-Saharan Africa, cassava is mostly used for human consumption in various forms ranging from boiling the fresh tuber to processing it into cassava flour. Cassava tubers are an important source of carbohydrates, while the leaves, eaten as a vegetable, are a good source of protein and vitamins.
Challenges facing cassava production in Africa
Cassava remains easy to produce, adaptable to many environments, with minimal labour requirements and less susceptible to pests and diseases. However, there is need to address increased productivity, marketing opportunities and profitability of cassava production. The following organic practices can contribute to achieving these goals.
Establishment of the cassava garden
In organic farming, crop management begins by giving the plants good growing conditions through improving soil fertility, and healthy planting material. This allows the crop to grow healthier, and produce higher yields.
Suitable varieties for organic production
Cassava varieties differ with regard to yield potential, flesh colour (white or yellow-fleshed), diameter and length of the tubers, disease and pest resistance levels, time from planting to harvest, cooking quality and taste. Some cultivars require 18 months or more from planting to harvest, while others are ready to harvest in 9 months. Most cultivars have been selected by farmers under their growing conditions based on yields and cultural tendencies. Each growing region has its own special cultivars with farmers, often growing several different cultivars in the field at the same time.
Recommendations to farmers for selecting suitable cultivars
Selection of an appropriate planting site
Cassava is drought tolerant, can grow on most soils, and gives some yields even on poor soils where most other crops fail. However, high yields are obtained in areas with well-drained, loamy soils, well-distributed annual rainfall of 1,000 to 1,500mm, and warm and moist climatic conditions only. The best site for planting cassava is flat or gently sloping land. Steep slopes are susceptible to erosion and are, therefore, not very suitable areas for growing cassava. Valleys and depression areas are also not recommended because they are prone to water logging. Cassava is sensitive to water logging and heavy soils do not allow the crop’s roots to proliferate and develop.
Land and seedbed preparation
In cassava cultivation, it is important to till the land to loosen up the soil, improve soil drainage and make it easy for roots to develop. The level of tillage required for the cassava field mainly depends on the soil type and the drainage at the selected site. In places with shallow soils or poorly drained clayey soils, it is important to make mounds or ridges onto which the cassava is planted, as it encourages better root development and yields. In sandy soils, only minimum tillage is necessary and the cassava can be planted flat into the soil, as the soil is sufficiently loose to allow root development.
Preparing good quality planting material
Cassava is propagated by planting pieces of the stem (stem cuttings). The development of cassava and amount of yields depends on the quality of stem cuttings. There are several cassava pests and diseases, which are stem-borne. Selecting healthy stem cuttings reduces the spread and damage caused by pests and diseases.
Recommendations to farmers in selecting good cassava stem cuttings
To get the best sprouting and growth from cassava stem cuttings, the following considerations are recommended:
Due to the fact that cassava has a slow initial development, intercropping during early crop development is feasible, and helps reduce soil erosion. However, farmers should consider that cassava is a poor competitor and can easily be shaded out by tall intercrops like maize. For this reason, it is important to consider the branching habit of both the cassava and the other crops in the intercropping system and make sure there is enough space for both crops. Furthermore, cassava can suffer from nutrient and/or water competition from intercrops. Therefore, attention must be given to the intercropping species that have different root systems and nutrient requirements.
Farmers usually intercrop cassava in simple or complex mixed cropping systems with vegetables such as amaranth and okra, plantation crops such as coconut, coffee, maize or legumes, and pulses such as cowpea and groundnuts. The intercropping pattern depends on the environmental conditions, food preferences and market conditions of the region.
Simple mixtures consist of the intercropping of only two crops, in which farmers select arable crops on the basis of differences in growth habit and time of maturity. For example, cassava, which is a long-duration crop with 9 to 18 months to maturity, is often intercropped with short-duration crops with 2 to 5 months of maturity process, such as maize, cowpeas, groundnut, okra and melon. These crops mature when the cassava is just attaining its maximum leaf area development and thus is able to expand its root tubers without competition. In complex mixtures consisting of three or four crops, good yields have been obtained with the following combinations:
Complex mixtures improve weed suppression, reduce soil temperature, retain soil moisture in the topsoil, and produce more organic matter than single cropping or simple mixtures. Nutrient loss from erosion in complex mixtures is less than in single cropping.
The continuous planting of cassava in the same field year after year leads to increased disease and pest levels, reduced yields and crop failure. To avoid such development, organic farmers should wait for at least 2 years before planting cassava on the same field again and develop a crop rotation system. A rotation system generally improves soil fertility, reduces soil erosion and helps to control diseases and pests. The suitable crop rotation depends on several factors such as the climatic conditions, the market requirements and the skills and objectives of the farmer. However, within a pattern of crop rotation, cassava is often grown in sub-Saharan Africa at the end of the sequence, as it can still produce relatively well at lower fertility levels, where other crops would not grow well.
This practice leads to lower cassava yields. It is important to establish a balanced crop rotation, which maintains or improves soil fertility, and to give cassava a place in the rotation that corresponds to farmer expectations. Cassava is a good crop to follow such crops as pumpkin, squashes, maize, sorghum or improved fallow. A 3-season rotation example that can be used in organic cassava production is maize-beans / cassava / groundnuts.
Reducing postharvest losses
Post-harvest handling of organic cassava aims at maximizing tuber quality by minimizing any damage or cuts on the tubers during harvesting and transportation of the tubers. Young leaves and shoots of cassava are also harvested to be consumed as vegetables and may be as important as tubers for generating income. However, excessive harvesting of the leaves can have a negative effect on the yield of tubers.
Early-maturing cassava varieties are ready for harvesting at 7 months, while late-maturing varieties are ready 12 months after planting. The proper stage for harvesting is when the leaves turn yellow and fall down and the roots are mature. It is advisable to harvest cassava once it is mature. If the tubers are left in the ground over long periods, they lose quality and become woody due to hydrolysis of starch to sugars. Care should be taken to avoid damage to the tubers during harvesting. Damaged roots are highly susceptible to fungal attacks and decay.
Harvesting cassava tubers is labour-intensive and done by hand. It is easy if the soil is sandy or during the rainy season, but in heavier soils or during the dry season, harvesting usually requires digging around the tubers to free them of the covering soil and then lifting/pulling the plant. The day before harvest, the plants are normally ‘topped’; the stalks are cut off 40 to 60cm above ground and piled at the side of the field. From this material, the stalks for the next planting are selected. Excess soil is then scraped off from the tubers by hand. This should be done carefully so as not to peel or damage the outer protective skin of the tubers.
The first thing to be done after the harvest is to transport the tubers from the production and harvest field to the processing and utilization site. This is because fresh cassava is highly perishable (within 2 to 3 days after harvesting). Transportation of cassava tubers should also be done carefully to avoid bruising and dehydrating the cassava tubers, especially if it is meant for fresh consumption.
Since cassava roots can remain in the soil for up to 18 months after reaching maturity, the simplest preservation technique is to delay the harvest until the crop is needed. However, this method has the following disadvantages:
Freshly harvested tubers can be preserved by the following methods:
Marketing and organic certification
Much of the cassava production is used at household food consumption level. It is also increasingly becoming a raw material for industrial production, especially for starch production. Organic certification of cassava production is only reasonable as a market requirement if there is a market that demands it. In such a case, interested farmers should be willing to adopt general organic production requirements, like not using synthetic pesticides and fertilisers, and applying other sustainable production methods.
Other considerations include:
Specific national or international organic standards may define additional requirements for production and postharvest handling of cassava. Farmers should consult the national organic movement or organic certification body operating within the region or country.