Dr. Joe Manu Aduani

Dr. Joe Manu Aduani, breeder CSIR Crops Research Institute Ghana, demonstrates Ampong cassava, an improved mosaic tolerant variety yielding up to 60 t/ha in the humid forest zone. Photo source: YouTube.

Back in April, a recruiter with ACDI/VOCA in Washington, D.C., pitched an upcoming assignment in Ghana aimed at improving cassava root production. The project would be funded through USAID’s Farmer to Farmer program, which provides short-term (two to four week) voluntary technical assistance to farmers, farm groups, and agribusinesses in developing and transitional countries to promote sustainable improvements in food security and rural industry. Funding for the program was authorized by the U.S. Congress in the 1985 Farm Bill and continues to operate to this day under that vehicle.

Initially I declined the assignment because of fieldwork through the period and, in spite of some prior experience, I did not consider myself a cassava “expert” at all. Saying that, there are probably very few cassava experts in the continental United States, where this long-season tropical root crop is off the agronomic radar. Real cassava experts like Reinhardt H. Howeler (CIAT), Luis Fernando Cadavid (CIAT), B.T. Kang (IITA), and others have contributed tremendously to improving cassava production in Asia, Africa, and Latin America. My experience, in turn, is limited to a couple long-ago African stints where I tackled, among other things, propagating and disseminating improved, mosaic virus tolerant cassava.

On second thought, the prospect of returning to West Africa beckoned, and it didn’t take too long for me to rationalize the gig in my mind on that basis alone.  Plus, it was a welcome excuse to get re-acquainted with a plant that, above all, stirs deep memories of Africa, my artisan Mende cutlass and grub hoe (hand forged from salvaged truck suspension spring metal), rubber tire sandals (“Ho-Chi-Minhs”), cassava leaf stew, and the country life.

So the die was cast that I should awaken on August 22, in Accra, Ghana, to rendezvous with Caltech Ventures Limited, a Ghanaian agribusiness wanting to expand cassava production for food, starch, and ethanol in the country’s Volta region.

Let us now praise the virtues of cassava (Manihot esculenta Crantz, family Euphorbiaceae), a drought-tolerant, soil agnostic, under-appreciated root crop, presently the third most important source of calories in the tropics after rice and corn. Cassava is a source of food, livestock feed, and income generation for some 800 million people, for whom it also holds strategic potential as a famine reserve crop. Cassava roots and leaves, which constitute about 50% and 6% of the mature plant, respectively, are both edible. Lately, cassava has attracted interest as a bioenergy crop with potential as a cash crop for smallholders. Cassava is popular with African farmers because it is amenable to mixed cropping, with processing and infrastructure technology firmly embedded in many local and regional food systems. Pulling off the Cassava Transformation does not, however, imply injecting anything foreign into the culture. In truth, cassava has spread across sub-Saharan Africa spontaneously under no fiat, borne on the rims of human lading.

cassava sticks for planting

Figure 1. Cassava sticks ready for planting. Each stick is a clone of the mother plant from which it was taken. Photo: Hauser et al./ASHC

Growing cassava couldn’t be easier. In a nutshell: the plant is propagated by cuttings or “sticks” taken from a mature plant (Figure 1). Sticks should be cut from the middle or lower portion of the stem, and have 5-7 nodes. Each cassava stick is a “clone”, i.e. it is identical to the mother plant from which it was taken. The offspring are therefore genetically stable, sharing exactly the same characteristics as the mother plant. The sticks are pushed into the ground about two-thirds of the way. Make sure the nodes are facing up! After about one week the cutting develops fibrous roots from the underground nodes and around callus tissue that forms on the buried end. During this period the cassava stick uses energy stored in the stem to survive. If the stem cutting is too green it will not have enough carbohydrate and mineral reserves to nourish the nodal sprouts. After about one month the plant begins depositing starch in the fibrous roots. After about 12 months, the cassava is ready to harvest. The plant must be dug out of the ground in a manner similar to sweet potato (Figure 2). But before you do this, cut off the above-ground stems. These can be used for cuttings, fodder or recycled for their nutrients.

cassava roots Nigeria

Figure 2. Let us now praise famous roots. “Mother” cassava, food and income giver to millions. This root was dug up on a farm in Osogbo, Osun State, Nigeria. Photo: Pius Utomi Ekpei/AFP/Getty Images.

Starch is the primary product exploited from cassava. The numerous large, swollen secondary fibrous roots contain 32-35% carbohydrates (fresh weight basis), of which about 80% is starch, mainly amylopectin (83%) and amylose (17%). The starch is processed into diverse value-added products like paper, textiles, noodles, salad dressing, glue, among others. In Africa products like homebrew cassava beer compete with regular brews that use imported barley. Barley is ill-suited to tropical climates, which has stimulated local brewers to exploit indigenous crops like cassava (bananas are also swill-worthy, but I refrain). Cassava reputedly churns out more carbohydrates than just about any other crop except sugar cane. Maximum daily energy production has been reported as 250 kcal/ha, compared with 200 for corn, 180 for sweet potato, 156 for rice, 114 for sorghum, and 110 for wheat (de Vries et al., 1967). I don’t know if these figures still hold true, but they point up that cassava is a gifted carb factory. The plant’s secret may be related to elevated activities of PEP carboxylase, an enzyme in the leaf associated with efficient carbon “C4” metabolism in some species (El-Sharkawy, 2006).

On the other side, the protein content of cassava roots is low at 0.4% – 1.5% (fresh weight basis). Cassava root protein does not have a well-balanced amino acid profile and has lower nutritional value due to low protein quality. Cassava leaf protein content is 5x to 10x higher than that of the root but it, too, is deficient in certain amino acids. Further, the roots are high in some vitamins and minerals (vitamin C, calcium) but deficient in the B-vitamin complex (thiamine, riboflavin, niacin). So cassava is far from the “ideal” food.

Another cassava sin is that all parts of the plant contain quantities of two glucosides linamarin (93%) and lotaustralin (7%) stored in the vacuoles of the cassava cells. When cassava cells are damaged during processing for food these glycosides mingle with the enzyme linamarase, which converts the harmless linamarin to acetone cyanohydrin. The latter chemical breaks down spontaneously to produce unwelcome, free poisonous hydrogen cyanide (HCN).

Fortunately not all cassava is created equal. There are two kinds: low glucoside “sweet” cassava, and high glucoside “bitter” cassava. The sweet varieties are favored for human consumption, whereas bitter varieties are exploited for non-food industrial products like ethanol. Cassava-eating Indians in South America have known about the toxic properties of the root for centuries. This knowledge has led to the development of processing techniques for efficient HCN removal. Nonetheless, cases of cyanide poisoning still occur from eating under-processed roots, and heavy, long-term, consumption of cassava coupled with poor diet, has been linked to onset tropical ataxic neuropathy.

Ironically, the tale of cassava’s ascent from the crop of last resort for the world’s pestilence- and war-afflicted unfortunates, begins with the Green Revolution (GR) in the 1960s. The GR harnessed genetic improvements for wheat and rice by breeding for dwarf and semi-dwarf plant types that partition more of their dry matter into the grain. This avoided massive starvation in parts of Asia, Africa, and Latin America where rapid population growth was outstripping domestic production. The GR depended on fertile land, improved production technology (fertilizer, pest, and water management), and infrastructure, and improved varieties able to exploit the improved technology. Further expansion of the GR was constrained by the scarcity of new high-quality lands and water resources, particularly in Africa. Thus expansion of the production area happened to spread to degraded land where high population growth resulted in shorter fallow cycles under shifting cultivation, resulting in growing soil acidity and nutrient exhaustion. Where the GR succeeded, its beneficiaries have largely been the privileged sectors of the population, whereas resource-limited farmers saw far fewer, if any, benefits. Post-colonial expansion of cassava in sub-Saharan Africa has mainly been driven by population growth, but this has also been the case in Thailand, southern India, Bangladesh, and Indonesia.

Since the heady GR days, high energy costs have accelerated the search for crops and farming systems with lower energy overhead and ecological footprint. Cassava’s agronomic traits make it a good fit for this modern perspective. Once established, cassava has no critical period in its indeterminate growth when drought stress would drastically affect yield, although sensitivity is greater in the early growth stage but decreases over time. Another virtue is cassava’s relative indifference to acid, infertile mineral soils high in active aluminum (Al+3). High Al+3 levels are toxic to the major grain species, but cassava is remarkably tolerant of soil Al+3 and low pH. Its ability to produce up to 10 metric tons per hectare (t/ha) of fresh roots in acid, nutrient exhausted soil has been documented (Berkhout et. al. 1985).

Precise cassava production data are impossible because a large amount of the crop is grown by smallholders and consumed locally. Official figures published by the commodity tracking arm of the Food and Agriculture Organization (FAOSTAT) estimated 277 million tons of cassava were produced in 2013 (Figure 3).

world cassava production statistics

Figure 3. World cassava shares by region (left) and top six producing countries (right). Data source: FAOSTAT; charts by R. Walters

Africa was the leading producer (57%) followed by Asia (31.9%), Americas (11%), and Oceania (0.1%).  In fact, three of the top six producers are African countries, which is noteworthy given the continent’s lagging grain production figures. But production statistics alone do not provide a holistic view of the situation. It is production per unit area that counts, and productivity is where Africa has fallen behind its peers in the global food race. How does African cassava measure up against its rivals on that score?

Thanks to FAOSTAT, global cassava statistics reaching back to 1961 are available for download. From this database, three categories were queried: production, yield, and area under cultivation. Then, I calculated the average annual growth rate by decade, up to 2013 (the most recent year for which there is data).  Here is the look of things after plotting the data:

global cassava production trends

Panel “a” shows cassava production by region: Africa, Asia, and the Americas (I did not bother with Oceania, no offense, since its 0.1% share is of little consequence, globally). Indeed, Africa has performed exceptionally well in this category, registering positive growth rates 2% or more, for each decade since 1960.  Asia is also on the up-and-up, with impressive growth tracking above 5% in two decades and only one negative from 1990-2000. Trailing behind the pack is the Americas (someone has to pull up the rear…).

Ah, but witness Panel “b”: yield per unit area.

global cassava root yield trends

Here, we see that Africa is behind the yield curve from the very beginning and never catches up. Asia zooms ahead, finishing 2013 with the highest regional yield averaging 21.1 t/ha (fresh weight). In contrast, Africa finished last, with 11.5 t/ha, not far behind the lackluster Americas 12.4 t/ha (Oceania logged 12.3 t/ha in 2013, but they are off the hook, so to speak). Among the top six producers, Indonesia and Thailand scored 22.5 and 21.8 t/ha, respectively. Ghana also scored impressively with 18.3 t/ha, and Nigeria with 13.9 t/ha. So there definitely are bright spots in Africa, but not in the DR Congo where 8.1 t/ha was all she wrote. And poor Brazil making only 14.1 t/ha, what can I say? Brazil may be a top producer, but is clearly not putting much effort into improving yields.

Panel “c” area under cultivation, brings the true picture into focus:

global cassava production by area

Here, we discover the true reason for Africa’s impressive production lead in Panel “a” has largely come from the expansion of cassava area under cultivation. Where the extra hectares materialized from is not known, but I believe a large proportion is impoverished land since cassava thrives there (or at least produces some return on labor). Cassava hectares in Asia have risen slightly since 1960, but production there has come largely from improvements in yield, not area under cultivation. And in the Americas, cassava is going nowhere in all production categories.

What do we conclude?

First, Asia has advanced not only with new technology, but also with technology transfer to farmer’s fields. There, cassava yield per hectare has increased over 2.5-fold since 1961 and is rising. This suggests that Asian countries have stronger agriculture research infrastructure, and stronger research-extension linkages, compared to Africa. It also suggests that Asian farmers have better access to inputs, and are willing to use them on cassava. But what’s wrong with the Americas?

Cassava in the Americas, in my opinion, has been eclipsed by other crops, perhaps corn and soybean are the biggest culprits. Rising income usually goes hand-in-hand with more animal protein in the diet. Livestock depend on high-quality feed, and corn and protein-rich soybean have a comparative advantage over cassava. Brazil also substituted sugar cane for ethanol production, as cane is more productive and cost-effective feedstock compared to cassava. But the failure of cassava research-extension linkages can not be ignored, either.

What will it take to boost the cassava yield curve in Africa? The technology ingredients in my opinion are these: improved cassava varieties, strong, disease-free planting stock, timely weed management, fertilizer, and finding more efficient ways to use fertilizer. Today, Africa’s 11.5 t/ha average cassava yield is about equal to the nutrient-limited (unfertilized) yield reported by Berkhout et. al. back in 1985. Research conducted by Centro Internacional de Agricultura Tropical (CIAT) and the International Institute of Tropical Agriculture (IITA) over the last four decades has indisputably shown that cassava responds positively to fertilizer, especially nitrogen and potassium. Many farmers, however, are reluctant to “waste” precious fertilizer on cassava. Long-term fertilizer trials on agricultural experiment stations in Africa would go a long way demonstrating to farmers the importance of well-balanced fertilizer use. In addition simple, farmer-managed field trials with help from researchers and extensionists showing farmers that correct fertilizer application is essential for maintaining high cassava yields and can be highly economic even when fertilizers are relatively expensive, should be vigorously promoted.

Interested in the ACDI/VOCA Farmer-to-Farmer cassava gig? The full debriefing report with field shots is available here.

Further Diggings

Berkhout, J.A.A., Van Deipen, C.A., Faber, D.C. and Janssen, B.H. 1985. Potential food production increases from fertilizer aid: a case study of Burkina Faso, Ghana, and Kenya. Center for World Food Studies, Wageningen Vol. 1, 48 pp.

Centro Internacional de Agricultura Tropical (CIAT). 2011. The Cassava Handbook. A Reference Manual based on the Asian Regional Cassava Training Course, held in Thailand. Available at: http://ciat-library.ciat.cgiar.org/Articulos_Ciat/biblioteca/The%20Cassava%20Handbook%202011.pdf (last access: 15 August 2015).

El-Sharkawy, M.A. 2006. International research on cassava photosynthesis, productivity, eco-physiology, and responses to environmental stresses in the tropics. Photosynthetica 44: 481-512.

de Vries, C.A., Ferwerda, J.D. and Flach, M. 1967. Choice of food crop in relation to actual and potential production in the tropics. Netherlands Journal of Agricultural Science 19: 241–248.

Howeler, R.H. and Aye, T.M. 2014. Sustainable management of cassava in Asia-from research to practice. Centro Internacional de Agricultura Tropical (CIAT) Cali, Colombia. Available at: ciat-library.ciat.cgiar.org/Articulos_ciat/SUSTAINABLE_MANAGEMENT_CASSAVA_ASIA_From_Research_Practice.pdf
(last access: 15 August 2015).

Hauser, S., Wairegi, L., Asadu, C.L., Asawalam, D.O., Jokthan, G., and Ugbe, U. 2014. Cassava system cropping guide. Africa Soil Health Consortium, Nairobi, Kenya. Available at: https://africasoilhealth.cabi.org/materials/cassava-production-booklet/  (last access: 10 February 2017).

Montagnac, J.A., Davis, C.R., and Tanumihardjo, S.A. 2009. Nutritional value of cassava for use as a staple food and recent advances for improvement. Comprehensive Reviews in Food Science and Food Safety 8: 181-194

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