IMPACT OF CASSAVA PRODUCTION ON THE ENVIRONMENT

Cassava in Africa, Asia, Latin America and the Caribbean

Table 1 shows the area and dry matter (DM) production of the major food crops worldwide. In terms of dry matter production, cassava is the fourth most important crop in SE Asia, seventh in Asia, fifth in Latin America and the Caribbean (LAC), second in Africa, and the most important crop in Sub-Saharan Africa. Area planted to cassava (FAOSTAT, 1999) is highest in Africa at 10.10 million ha (62% of world total), followed by Asia at 3.48 million ha (21%) and LAC at 2.70 million ha (17%), for a total area of 16.28 million ha worldwide (Figure 1).

Africa accounts for 51%, Asia 29% and LAC 20% of world production. Fresh root yields are highest in Asia at 13.7 t/ha, followed by LAC with 11.9 and Africa with 8.4 t/ha. The higher yields in Asia are partially due to a near absence of diseases and pests (except in India), and relatively intensive crop management, which in a few isolated areas (Tamil Nadu of India) includes irrigation and high rates of fertilizers.

Production environments

Cassava is produced between 30^o north and south latitudes, and near the equator up to an altitude of about 1800 masl. Because of the crop's tolerance to drought and low soil fertility, it is generally produced in marginal areas with poor soils, and/or high risk of drought.

Cassava grows best in areas with a mean temperature of 25-29°C, and a soil temperature of about 30°C; below 10°C the plant stops growing. While the crop grows best in areas with an annual well-distributed rainfall of 1000-1500 mm, it can tolerate semi-arid conditions with rainfall as low as 500 mm, and may have a competitive advantage over other crops under those conditions. Cassava can grow on a wide range of soils, but is best adapted to well-drained, light-textured, deep soils of intermediate fertility. Under high fertility conditions top growth may be stimulated at the expense of root growth. Optimum soil pH is between 4.5 and 6.5. The crop does not grow well in poorly drained soils, gravelly or saline soils, or in soils with a hardpan (Onwueme and Sinha, 1991).

Soils

Table 2 shows the distribution of cassava growing areas with respect to the various soil orders of the US soil classification system. In Asia about 55% of cassava is cultivated on Ultisols, which are characterized by low pH (but not low enough to limit cassava growth) and low nutrient content. Of the major plant nutrients K is usually the most limiting, especially if cassava is grown for many years on the same soil. Another 18% of cassava in Asia is grown on Inceptisols, mainly on Java island of Indonesia. These soils are also low in N, P and K, but which of these is the most limiting depends on the local soil characteristics. Another 11% of cassava in Asia is cultivated on Alfisols, mainly on Java. These soils tend to have a high cation exchange capacity, a relatively high pH and high fertility. Because of high pH and high levels of Ca and Mg, cassava may suffer from micronutrient deficiencies, especially Zn and Fe. Finally, about 9% of cassava is grown on Entisols, again mainly on Java. On these soils cassava production may be limited by an inadequate supply of N and K, or, in very sandy soils, Zn.

Figure 1. Distribution of cassava in the world. Each dot represents 1000 ha.

Source: CIAT, 1995.

Table 1. Area and production (on dry weight basis) of major food crops in various regions of the world in 1997.

¹⁾ On dry matter basis, assuming the grains to contain 86% DM, cassava 38%, sweet potato and potato 20%, and sugarcane 26%.

²⁾ Includes Brunei, Cambodia, Indonesia, Laos, Malaysia, Philippines, Singapore, Thailand and Vietnam.

³⁾ Sub-Saharan Africa includes Angola, Benin, Botswana, Burkina Faso, Burundi, Cameroon, Central African Republic, Chad, Comoros, Dem Rep of Congo, Rep of Congo, Cote d'Ivoire, Dibouti, Equatorial Guinea, Ethiopia, Gabon, Gambia, Ghana, Guinea, Guinea-Bissau, Kenya, Lesotho, Liberia, Madagascar, Malawi, Mali, Mauritania, Mauritius, Mozambique, Namibia, Nigeria, Niger, Rwanda, Reunion, Sao Tome and Principe, Senegal, Sierra Leone, Somalia, Sudan, Swaziland, United Rep. of Tanzania, Togo, Uganda, Zambia and Zimbabwe.

Source: FAOSTAT, 1999.

Table 2. Soils on which cassava is produced in Latin America and Asia and their principal constraints for the crop.

¹⁾ Based on a total cassava area of 2,512,000 ha in Latin America
²⁾ Based on a total cassava area of 3,582,000 ha in Asia
Source:

¹⁾ CIAT, 1983
²⁾ Howeler, 1992.

In sharp contrast to Asia, in Latin America and the Caribbean cassava is grown on almost equal proportions of Ultisols, Alfisols and Oxisols, with lower proportions of Entisols, Inceptisols and Mollisols. Brazil accounts for 70% of the cassava growing area in LAC and most of these soils are found in that country. Unlike in Asia, in LAC cassava is grown extensively on Oxisols, which are generally characterized by low pH (4.2-6.0), high levels of exchangeable Al and very low nutrient contents. In only a few areas, mainly the Eastern Plains of Colombia, will cassava benefit from small applications of lime, principallly as a source of Ca and Mg. The Oxisols also tend to be very low in P and K, the former generally being the most limiting nutrient during the first year of cassava cultivation, while K becomes more important in subsequent plantings on the same soil.

For Africa similar data are not yet available. However, according to the Collaborative Study of Cassava in Africa (COSCA) (Carter et al., 1992), about 54% of the cassava growing areas are constrained by high acidity and low soil fertility. Since "high acidity" is defined as a soil with pH<5.5, it is unlikely that acidity is the real limiting factor, as cassava is known to tolerate pH values as low as 4.0-4.2 and high levels of exchangeable Al (up to 85% saturation) (Howeler, 1991b; Asher et al., 1980). Most likely, cassava production in Africa is constrained by low soil fertility, as most cassava is grown on Oxisols, Ultisols and Entisols. The same COSCA study (Carter et al., 1992) reports that 10% of the cassava area in Africa is constrained by shallow soil depth or texture, and another 4% by poor drainage.

Climate

Tables 3 and 4 show the distribution of cassava-growing areas in the three continents according to climatic zones^[13]. In Latin America almost 30% of cassava is grown under subtropical conditions, sometimes with frost during the winter, which may kill the above-ground growth; in Asia this is about 15% and in Africa only 10%. In both Latin America and Africa about 20% of cassava is grown in the highlands with year-round cool temperatures of <22°C; in Asia almost no cassava is grown at high elevations.

Latin America and Africa are similar in terms of dry-season length, with 40-45% of cassava grown in both the humid and seasonally dry zones, and with 10-15% in the semi-arid zone. According to Table 4 Asia has 26% of the cassava area in the semi-arid zone, but that is partially due to a wider definition of semi-arid climates used in Table 4 than in Table 3. According to the latest agro-ecological zone classification (Peter Jones, personal communication) practically no cassava in Asia is grown under semi-arid conditions, while this same classification used in LAC and Africa shows rather large areas (about 20-30%) in this zone, mainly in NE Brazil and in Mozambique (Appendix 1).

Table 3. Cassava distribution in Latin America and Africa by climatic conditions.

¹⁾ Based on total cassava area of 1,816,000 ha in LA and 7,992,000 ha in Africa
²⁾ Mean growing season temperature: Lowland >22°C; Highland <22°C
³⁾ Length of dry season (<60mm rain): Humid 0-3; Seasonally dry 4-6; Semi-arid 7-9; Arid 10-12 months
⁴⁾ Seasonality (difference between hottest and coldest month): Tropical <10°C; Subtropical >10°C
Source: CIAT, 1989.

Table 4. Cassava distribution in Latin America and the Caribbean (LAC) and in Asia by climatic zones.

¹⁾ Altitude (masl): Lowland < 1000; Highland > 1000
Length of dry season (<60mm rain): Humid: 0-3; Subhumid 4-5; Semi-arid >5 months
Latitude: Tropics: between 20^oN and S

²⁾ Based on 2,781,000 ha in LAC and 3,921,000 ha in Asia
Source: Norel, 1997.

Elevation and topography

About 80% of cassava in Africa is grown in the lowlands and the remaining 20% in the highlands, mainly in tropical highlands in Angola and the Great Lakes Region of Uganda, Rwanda, Burundi and northern Tanzania. In Africa some cassava is also found in subtropical highlands, mainly in Zambia and Madagascar. In these areas cassava may be found on steep slopes of 10-50% (Carter et al., 1992).

In Asia very little cassava is found at elevations above 1000 masl (Table 4). Most is grown on gentle slopes of 0-10%, but in southern China, north Vietnam and on Java the crop can be found on steep slopes of 15-50%. Especially in Indonesia these areas are usually terraced and cassava is often intercropped with other food crops on narrow strips of terraced land. In China the crop is sometimes planted on steep slopes as an intercrop between young trees in areas recently reforested (Henry and Howeler, 1996).

In Latin America about 417,000 ha of cassava are found in the highlands. Cassava grown in the tropical highlands is found mainly in the Andean zone of Colombia, Ecuador and Peru, as well as in Central America; that grown in the subtropical highlands is found mainly in southeast Brazil, eastern Paraguay and northern Argentina. In the highlands, cassava is often cultivated on slopes up to 40%, and occasionally on steeper slopes. In the southern states of Brazil, such as Sao Paulo, Parana and Santa Catarina, cassava is often grown on gentle slopes of <10%, but because of intensive mechanical land preparation serious erosion can occur under those conditions. Where land is prepared by hoe or oxen-drawn plow, land preparation tends to be less intensive and fields are smaller, resulting in less serious erosion.

Cropping systems

In much of the African lowlands cassava is grown in slash-and-burn systems, usually as the last crop in a rotation, after maize, cowpea and upland rice, and before the plot is returned to bush fallow. Cassava is generally the last crop in the rotation because it will still produce a reasonable yield on nutrient-depleted soils. This is most likely a significant part of the reason for very low cassava yields in Africa. In the African highlands cassava is generally grown after short-term fallows.

Cassava is mainly grown intercropped with maize, yam, peanut or cowpea in West Africa, or with banana, maize, cowpea and beans in East Africa (Ezumah and Okigbo 1980); C. Wortman, personal communication). In western Nigeria cassava is grown together with yam, maize and egusi melon on the same mound, while in other areas crops are grown in separate rows (Okeke, 1984). In Sierra Leone cassava is often intercropped with upland rice, planted 4-6 weeks after cassava (Dahniya et al., 1994).

Cassava farmers in Africa seldom apply any chemical fertilizers to cassava (or most other crops) even though trials have shown that cassava is highly responsive to fertilizer applications (Richards, 1979; FAO, 1980). Instead, most farmers rely on natural fallows to restore soil fertility; they may apply small amounts of animal manure or ash, but usually to the intercrop. Soil preparation is mainly done by men, using a short-handled hoe in southern Nigeria, while in the northern part of the country farmers use oxen for plowing. In Tanzania cassava is planted on ridges and in Uganda without ridges. Planting, weeding and harvesting is done mostly by women.

In Central America and the Andean zone of South America cassava is often planted after short (2-4 year) fallows. After slashing and burning of the fallow vegetation, cassava is planted for 2-3 years without fertilizer or manure input, until the soil's fertility has been exhausted and low yields do not permit further cultivation. However, the fallow period is often too short and the native soil fertility too low to allow for adequate regrowth of vegetation to restore soil fertility. If cassava cultivation is also associated with serious erosion, then soil fertility in these areas cannot be restored, and farmers are caught in a downward spiral of productivity decline, leading to ever increasing poverty.

In the mountainous areas of Central America and the Andean zone, cassava is usually grown intercropped with maize, cowpea or beans. The intercrops may receive some fertilizers or manure, but these are seldom applied to cassava, as the low value of the crop does not justify expensive inputs of fertilizers. In Cauca Department of Colombia farmers are now applying rather high amounts of chicken manure, with good results.

Along the eastern coast of Brazil and in extensive areas of the Northeast, cassava is mostly grown by smallholders under very marginal conditions of drought and low and declining fertility. In the southern states of Sao Paulo, Parana and Santa Catarina, climate and soil conditions are more favorable and management is more intensive, leading to higher yields. In Brazil the crop is grown mainly in monoculture or intercropped with maize, beans or cowpea. In many areas in the South, fields are plowed by tractor followed by furrowing. Fertilizers (P and K) are often applied at the bottom of the planting furrow, after which they are covered with a little soil before placing the stakes horizontally in the furrow and filling over with soil.

Table 5 shows the details of the prevailing cassava cropping systems used in the various countries of Asia. Even within countries there may be marked differences among regions. Crop management is mainly determined by the size of farms and the availability of labor. In countries with comparatively larger farms, such as Thailand and Malaysia, and on large plantations in Indonesia and the Philippines, cassava is generally grown in monoculture, and land preparation is by tractor. In areas of small farms, such as Kerala state of India, Java island of Indonesia, northern Vietnam and southern China (Henry and Howeler, 1996), land preparation is usually by hand using a hoe, or by an animal-drawn plow. In many cases cassava is intercropped with peanut or maize, and occasionally with sweetpotato or watermelons (China). In India and Indonesia farmers tend to apply 5-20 tonnes of cattle or chicken manure, while in north Vietnam and China farmers apply 3-7 tonnes of pig manure, often mixed with rice straw. Fertilizers are commonly applied at high rates in Malaysia and Tamil Nadu state of India, but at low rates in Thailand, Indonesia (mainly N), China and Vietnam (Pham Van Bien et al., 1996). Little or no fertilizer is used in Kerala state of India, in the Philippines and in central Vietnam. In Kerala state short-duration (6-7 months) varieties are now often planted after rice in lowland fields, producing very high yields due to adequate soil moisture and better fertility. This, and the very intensive high input systems used in Tamil Nadu, result in very high yields (average of 22 t/ha) of cassava in India (Nayar et al., 1995).