Cassava (Manihot esculenta Crantz) is a major food and industrial crop in tropical and subtropical Africa, Asia and Latin America. In Africa and most of Latin America cassava is mainly used for human consumption, while in Asia and parts of Latin America it is used for on-farm animal feeding, for production of commercial animal feed, or processing into starch and starch-based products.
When used as human food, cassava can help feed the family; when fed to pigs or sold in nearby markets it can produce income and thus provide for necessities such as clothing, education and health care. Thus, for some, cassava is a staple food, while for others it may serve as a vehicle to increase income and escape poverty.
Because of its tolerance of soil acidity, low soil fertility and drought, the crop is often grown by poor, smallholder farmers in marginal areas with adverse climatic, edaphic and topographic conditions, such as steep slopes and forest margins. Good examples are the dry and infertile areas of northeast Brazil, the seasonally dry areas of southwest Nigeria and east and south of Lake Victoria in Tanzania, as well as the northeast of Thailand or the eastern part of Java. Because of its ease of planting and adaptation to minimum land preparation, the crop is often grown on slopes, both on gently rolling terrain and on steep hillsides. It can be grown with relatively few purchased inputs and often requires only family labor. These favorable attributes make cassava an appropriate crop for resource-poor farmers.
Production in these sensitive areas can lead to environmental degradation. Like any other crop, cassava extracts nutrients from the soil, and its continuous production in already poor soil, without addition of nutrients, will inevitably lead to nutrient depletion and a further decline in soil productivity. In addition, the wide planting distance used, and the relatively slow initial growth of the plants, result in slow closure of the crop canopy which in turn leads to poor protection of the soil surface; this can cause soil erosion when the crop is grown on slopes. Erosion leads to a decrease in rooting depth; a preferential loss of organic matter, clay and nutrients; and a loss of applied fertilizers. Erosion also has off-site effects of sedimentation of waterways and reservoirs, and potential eutrophication of lakes.
Cassava production can lead to new land clearing and loss of biodiversity. This may be especially notable in forested areas. Losses can include wild Manihot species, which may be of future importance for the incorporation of favorable characteristics, such as disease tolerance, in cultivated cassava. Moreover, the tendency to grow high-yielding and high-starch varieties has reduced the number of farmers' landraces, resulting in a narrowed germplasm base; this has increased the danger that new pests or diseases will destroy a large part of a single-variety crop. This problem cannot be solved by individual farmers, but should be considered by planners and policymakers. Research institutions must ensure the collection and safe conservation of cassava germplasm and that of its wild relatives, before they are lost.
Cassava is processed into a wide variety of products. The boiling of peeled roots, the simplest form of processing, is used in much of the Andean zone of Latin America, southern India and in parts of Vietnam. In West Africa, roots are peeled, grated, fermented, dehydrated, and then toasted to produce gari, a product particularly popular in Nigeria. In other countries, like Sierra Leone, the soaked, pounded and fermented mash is boiled with water to a thick paste called "foofoo". Many other processing techniques are used in Africa to produce a variety of either wet or dried products for human consumption. In the Andean zone of Colombia, in southern Brazil, in Java and northern Vietnam, cassava roots are processed into starch in small family-size processing units. Roots are grated, mixed with water and filtered. The starch is allowed to settle in tanks, and is then sun dried.
Extraction of starch from cassava roots requires large amounts of water. After separation of the starch and fiber, the residual water contains small amounts of starch, proteins and hydrocyanic acid. When this water is released directly into streams or rivers, the residual starch can cause rapid growth of bacteria, resulting in a depletion of oxygen and detrimental effects on aquatic life. There are also reports indicating that dissolved hydrocyanic acid has caused the death of fish. Problems arise when there is a high density of small-scale starch factories, which tend to use inefficient starch extraction methods and do not utilize pollution control devices. In larger factories, starch extraction is more complete and water use is more efficient, resulting in lower water use and less pollution. These larger factories are often forced to invest in pollution control devices. Nonetheless, the more efficient technology can also have a potential for negative environmental impact: with new high-speed grating machines, large quantities of cassava roots are mashed up in a minimum of time, resulting in high concentrations of hydrogen cyanide (HCN) in dust and water sprays. Such factory pollution may affect the health of workers.
The objectives of this assessment are to review the current knowledge on the effect of smallholder cassava production on the environment, to assess the degree to which cultivation of this crop is causing land degradation, to determine appropriate management practices to reduce such effects, and to find ways to disseminate this knowledge and enhance adoption of more sustainable production practices.
Similarly, this assessment reviews our present knowledge about the effects of small-scale cassava processing on the environment, suggests ways to mitigate against these problems, to identify constraints to their adoption and suggest possible policy changes that will encourage the use of more environmentally friendly processing techniques.
The study also identifies gaps in our knowledge, suggests areas where additional research is needed, and makes recommendations for planners and policymakers.