ISFM for banana systems

Piet van Asten,, Severine Delstanche, Lydia Wairegi, Tony Muliele, Syldie Bizimana, Godfrey Taulia, Ken Giller, Peter Leffelaar, Laurence Jassogne, Philippe Baret, and Charles Bielders

Banana systems in Rwanda. Photo by Piet van Asten
Banana systems in Rwanda. Photo by Piet van Asten

Banana is the primary food crop in the Great Lakes Region, providing food and income for over 85% of the population. Unfortunately, current banana yields of 5–30 t/ha/year are low compared to potential yields of over 70 t/ha/year. Although between 25% and 70% of this yield gap can be explained by low soil fertility (Fig. 1), the use of external inputs such as fertilizers is virtually nonexistent and soil fertility is mostly managed by recycling local organic residues.

A study done by Severine Delstanche at the Catholic University of Louvain-la-Neuve (UCL) showed that very little nutrients were released from the soil through weathering of the soil minerals (Fig. 2). Hence, soil fertility depended almost entirely on the soil’s organic matter content. In banana-based farming systems, nutrient recycling is very important, as the harvest index is relatively small (<30%). This helps maintain relatively high organic matter content in the soil.

Figure 1. Primary crop constraints identified by use of boundary line approach.
Figure 1. Primary crop constraints identified by use of boundary line approach.

Furthermore, the large and perennial canopy and root system of banana help protect the soil from erosion. Banana therefore plays an important role in protecting the environment in this hilly landscape.

In addition to being an important component in sustaining soil fertility, banana plays an important socioeconomic buffer role for the smallholders. The crop provides food security as bunches are harvested throughout the year and any surplus can be sold to generate a continuous cash flow.

Banana systems particularly occur in areas with high population pressure and small (<2 ha) farm sizes. A study by Lydia Wairegi at Makerere University (Uganda), showed that fertilizer use was very profitable in the peri-urban area close to Kampala with marginal rates of return sometimes exceeding 500%. However, in areas far from the market (>150 km), the intensification process seemed less promising, but banana continues to play an important buffer role to maintain food security and protect the environment. In these remote locations, it seems wiser to invest in improved use of local (nutrient) resources, than to purchase mineral fertilizers.

Figure 2. The proportion of annual K fluxes in banana systems originating from soil mineral weathering (2%), crop<br /> residues (77%), and the crop harvest (21%).
Figure 2. The proportion of annual K fluxes in banana systems originating from soil mineral weathering.

To maintain its buffer role, banana can be integrated with other crops such as coffee and beans. The Ph.D. studies of Tony Muliele and Syldie Bizimana (both UCL) showed that intercropping beans with banana could be improved. Traditionally, farmers in Rwanda, Burundi, and DR Congo would till the banana field at the beginning of the wet season to suppress weeds and prepare the land for bean intercropping. Unfortunately, this practice damages a large proportion of the superficial root system of banana plants. Based on practices observed in Southwest Uganda, a technology of zero-tillage and mulching was tested. Beans are planted in the mulch. The planting holes for the beans were made using a stick. The use of external mulch greatly improved banana performance in eight trials across the region. However, beans did suffer some setback in some instances when the improved banana growth would lead to a more dense canopy, outshading the understory beans. In collaboration with Bioversity International and the Tropical Soil Biology and Fertility Institute of CIAT (TSBF-CIAT), a series of trials was conducted to reduce the banana canopy through leaf pruning. The results are almost ready, but they provide farmers with advice on how to best manage the trade-offs between banana and the understory legumes.

To improve fertilizer use efficiency and profitability, it will be important for farmers to apply the right nutrients at appropriate rates. To enable the identification of the most deficient nutrients (see photo) that need primary attention when applying fertilizers, compositional nutrient diagnosis (CND norms) were developed by Lydia Wairegi in Uganda and by Severine Delstanche in Rwanda. The CND norms are based on foliar analysis and allow a quick assessment of nutrient deficiencies observed within the plant. Contrary to critical norms for single nutrients, the CND allows for an integrated assessment of nutrient imbalances within the plant.

Farmers can use pictures to diagnose nutrient deficiencies for nitrogen, phosphorus, potassium, and magnesium.
Farmers can use pictures to diagnose nutrient deficiencies for nitrogen, phosphorus, potassium, and magnesium.

Besides developing fertilizer recommendations based on foliar analysis, IITA conducted a series of large nutrient omission trials in central and southwest Uganda. Based on the quantification of nutrient uptake, soil nutrient supply, and crop response, a QUEFTS model was developed to predict fertilizer requirements in collaboration with Wageningen University (WUR). This work was led by Ph.D. student Kenneth Nyombi and is currently being carried forward by Ph.D. student Godfrey Taulya. He observed that potassium nutrition was particularly important for banana to alleviate drought stress. The result from the ongoing research effort clearly shows that strong synergies can be achieved when integrating soil fertility, agronomic, and economic research approaches at the plot, farm, and regional levels.

NRM in cassava and yam production systems

Stefan Hauser,

Cassava has the potential to produce roots even under poor soil conditions. Photo by IITA
Cassava has the potential to produce roots even under poor soil conditions. Photo by IITA

Why are yields of cassava in Thailand and India three times higher than in Africa and production costs in Brazil only one-third of those here? Although Africa suffered from the Cassava Mosaic Disease pandemic and currently faces the threat of Cassava Brown Streak Disease, breeding tolerant and resistant germplasm has contributed to yield gains over the last three decades. Thailand, India, and Brazil have been successful in commercial cassava production with yields between 25 and 40 t/ha. The question arises: how can African farmers realize more of the >80 t/ha yield potential of cassava?

Natural resource management (NRM), agronomy, and crop husbandry have hardly ever been credited with “breakthrough” solutions to hunger and poverty. However, when more than 50-75% of the cassava yield potential is not being realized, major improvements are clearly possible through NRM, agronomy, and appropriate crop husbandry.

Agronomy and crop husbandry
For West Africa there is still a dearth of agronomic information on cassava. Currently a density of 10,000 plants/ha is the standard, while further increases are being recommended without concrete data on the yield responses to increased density by different growth types. Cassava varieties vary widely in their branching height and level of ramification, leading to different levels of ground cover by single plants and of the start and intensity of intra-specific competition. Cassava yield distribution within the same variety is highly biased (Fig. 1), raising questions on the optimum plant density and issues such as genetic uniformity and crop responses to edaphic (soil) factors.

Figure 1. Relative contribution to yield of individual plants of cassava variety Zizila in DRC.
Figure 1. Relative contribution to yield of individual plants of cassava variety Zizila in DRC.

One future effort will be to determine optimum plant densities for monocrops by major cassava growth types.

Intercropping cassava with maize or grain legumes is still widely practiced and needs to be improved. The short-term intercrops are cleared from the field, leaving space unused that can be invaded by weeds, thus there needs to be a follow-up either with weed control or a second crop to occupy the open space. Various crops will be tested for their capacity to perform between developed cassava plants and their contribution to total system productivity.

Weed control remains a problem as there are no postemergence herbicides that cassava would tolerate. Combinations of preemergence herbicides with appropriate planting techniques have the potential to reduce weed competition and labor. For instance, a preemergence herbicide can be applied before planting cassava. The herbicide kills germinating seeds as they break through the herbicide-sealed soil surface. The cassava stakes need to be planted in a vertical position and the orientation needs to be correct so that no emerging cassava leaves touch the soil surface. Such technologies combined with the follow-up use of postemergence herbicides with shields can drastically reduce labor and increase productivity as weeding can be done at the most efficient time.

Fertilizer use is low in Africa yet it appears certain that fertilizer or other forms of soil nutrient replenishment can contribute to yield increases, higher farm incomes, possibly to lower consumer prices, and thus to better livelihoods. Using average nutrient uptake into all cassava plant parts (dry matter basis) of 6.2 kg N/t, 1 kg P/t, and 5.3 kg K/t, a total supply of 165–25–145 kg N-P-K/ha is required to attain 50% of the current potential yield (45 t/ha fresh roots). Such amounts are unlikely to be supplied by the soil and thus nutrient supply is a crucial factor in achieving higher cassava yields.

There are no recent fertilizer response curves for cassava and yam in West Africa, hence, farmers do not know the composition and amounts of fertilizer to apply. The nutrient(s) most limiting to cassava production have not been quantitatively determined. The replenishment of any most limiting nutrient would lead to substantial yield increases. Depending on the limiting nutrient, productivity and profitability increases may be possible at a very low cost and risk. IITA uses a stepwise approach, first determining the most limiting nutrient(s) followed by elaborating the optimum quantity required and the construction of recommendations for optimal nutrient composition and quantities. IITA is currently working with the International Fertilizer Development Center on testing special fertilizer blends for cassava, addressing the augmentation of neglected nutrients such as sulfur, magnesium, zinc, and boron.

Use of other nutrient sources
Compost, manure, mulch, and rock phosphate have all been proposed as means to improve soil nutrient status and crop production. However, none of these sources has had a major impact as farmers need land to produce biomass or else infrastructure is required to mine, process, and distribute rock phosphate. Although the biological sources are important, constraints in biomass production need to be overcome first.

Figure 2. Crop yield response to planted herbaceous fallow in West and Central Africa.
Figure 2. Crop yield response to planted herbaceous fallow in West and Central Africa.

Mineral fertilizers alone cannot sustain crop production on degraded land. Soil organic matter and  soil micro-, meso-, and macro-fauna are important in maintaining soil quality and health. Traditionally, fallow phases between crops were replenishing the soils’ production capacity. With increased population densities, fallow phases have been shortened or no longer exist. Thus, soils do not recover but continue to lose their production potential. Farmers do not seem to invest in soil fertility but look for ways of coping with ever less fertile soils, thereby degrading them to a stage where cropping becomes unprofitable.

Such situations have been encountered in southern Bénin. Soil fertility and quality management techniques, such as cover crops, manure application, or any other form of organic matter and nutrient recycling have not been adopted at larger scales. In retrospect, there have been constraints to the adoption that were not considered in the process of technological development. Today, with more options available and a stronger and earlier involvement of farmers in research for development, such approaches are worth reconsidering. One such technology, using leguminous cover crops, had little if any success in cassava (Fig. 2).

Controlling the cover crop was a major problem. Consequently IITA works today on efficient and effective control methods. Pueraria phaseoloides was introduced to smallholders in southern Cameroon but it was not readily accepted as farmers immediately identified it as an aggressive weed, able to destroy crops. However, two years into the use of Pueraria, fallow farmers noticed that the weeds most difficult to control had disappeared and that it was easier to clear Pueraria than the natural fallow. Some farmers burned the Pueraria only to find the land ready to crop without major labor input. Yields of cassava, maize, and groundnut were generally higher after Pueraria, whereby the labor-saving burning produced the highest yields (Fig. 3).

Figure 3. Cassava fresh root yield in burned and mulched maize-cassava and burned maize-cassava-groundnut intercrop.
Figure 3. Cassava fresh root yield in burned and mulched maize-cassava and burned maize-cassava-groundnut intercrop.

Considering farmers’ needs
Pueraria was introduced for soil fertility replenishment but was adopted for its labor-saving effects. Soil fertility was not perceived as a problem and thus positive effects on the soil could be compromised (by burning) without compromising yields. Effects such as weed suppression and the reduction of soil-borne pests and diseases may contribute to the yield increases after Pueraria.

Livestock integration and the search for synergies
Few farmers adopted the use of green manures for soil fertility improvement because they have no direct benefits from it. Herbaceous legumes have rarely been used to feed livestock, although there is (anecdotal) evidence that livestock feed on them and that they are beneficial to growth and reproduction. In the IITA-led CRP on Humidtropics, livestock integration will be a major aspect. It will add value to green manure species when these are used to feed livestock that will also benefit from the canopies of root and tuber crops (cassava leaves) remaining at root harvest. Thus, there will be an increase in returns of animal manure to fields, and to crop yields through the combined use of green and animal manures for improved food security and farm incomes.

Efficient combinations of agronomic practices, nutrient supply, and soil management practices will be developed to increase the productivity of cassava and yam while improving the status of the natural resource base. Synergistic effects between these measures and the integration of livestock or fish farming will increase resource use efficiency and income generation as well as the quality of the farm food supplies. Due consideration of social and gender aspects in farm household operations will identify the entry point best suited for IITA’s interventions. Farmers’ feedback and innovations will be integrated into approaches on sustainable intensification to increase food production and improve rural livelihoods while enhancing the capacity of the agroecosystems to deliver essential services.

Hauser, S. and C. Nolte. 2002. Biomass production and N fixation of five Mucuna pruriens varieties and their effect on maize yields in the forest zone of Cameroon. Journal of Plant Nutrition and Soil Science 165: 101–109.
Hauser, S., C. Nolte, and R.J. Carsky. 2006. What role can planted fallows play in humid and sub-humid West Africa? Nutrient Cycling in Agroecosystems 76: 297–318.

Boosting the productivity of cassava-based systems in DR Congo

Bernard Vanlauwe,, Patrick Mutuo, Nzola Mahungu, and Pieter Pypers

The intensification of African agriculture is an essential response to the increasing demands for more food without endangering important natural resources, including the forests in the Congo basin.

Because of its potential to produce some roots, cassava is often considered a crop that “likes” poor soils but, as all other crops, it responds to and requires a sufficient amount of nutrients to produce attainable yields. The transformation of cassava from a food security to a cash crop in many countries in Africa further stresses the need for nutrient replenishment strategies in cassava-based systems since the transport of cassava roots to processing plants can accelerate the amount of nutrients exported from the farm.

In recent years, integrated soil fertility management (ISFM) (see article by Vanlauwe et al. this publication) has been accepted by many organizations as the underlying technical framework for the sustainable intensification of smallholder systems in Africa. This article highlights the progress of work on the application of ISFM principles in the context of cassava-based systems in DR Congo.

In DR Congo application of 2 bags of NPK fertilizer per hectare results in root yield increases from 30% to 160%.
In DR Congo application of 2 bags of NPK fertilizer per hectare results in root yield increases from 30% to 160%.

Current situation
In the highlands of Sud-Kivu province, cassava and common beans are among the main food crops traditionally cultivated in mixed cropping systems. Cassava monocropping is done only in marginal fields where other crops fail to yield. Farmers generally allocate about 0.2–0.3 ha (30–45% of their farm area) to cassava–legume intercropping and obtain average yields of 400–800 kg/ha of legume grains and 10–15 t/ha of cassava fresh roots. Pressure on land is very heavy due to high population densities and justifies agricultural intensification and investment in soil productivity.

In Bas-Congo province, on the other hand, farmers practice slash-and-burn agriculture. Cassava is grown for 1 or 2 years, followed by fallow periods of 2 to 4 years. In the past two decennia, the population has grown by more than 50% leading to more demands for food, especially from nearby Kinshasa. Improved and sustainable, fire-free production systems are urgently needed. In DR Congo, most farmers have no access to improved varieties and have very limited options to improve soil fertility.

Fertilizer and germplasm
First, the use of improved, pest-and disease-resistant varieties in combination with appropriate rates of NPK fertilizer was observed to result in a 30–160% increase in cassava root yields in eastern DR Congo. A visible increase was observed in yields of the stems, important for the production of planting material (see photo). In western DR Congo, cassava yields doubled from 12 to 25 t/ha with the application of moderate rates of NPK and reached over 40 t/ha with higher rates. Several initiatives are taking place to ensure that large quantities of planting materials reach smallholder farmers with specific attention being given to those varieties resistant to cassava mosaic disease and the brown streak virus.

Figure 1. Cassava storage root yields as affected by application of compound fertilizer (17 nitrogen: 17 phosphorus: 17 potassium) at rates of 0 (F0), 40 (F1), 120 (F2), and 200 (F3) kg K/ha, and green manure (TI = <em/>Tithonia sp.; CH = <i>Chromolaena </i>sp.) at 2.5 t dry matter/ha alone or together with compound fertilizer at 40 kg K/ha in two trial locations in Bas-Congo, DRC. SED: standard error of difference at P<0.05. Source: Adapted from Pypers et al. 2012
Figure 1. Cassava storage root yields as affected by application of compound fertilizer (17 nitrogen: 17 phosphorus: 17 potassium) at rates of 0 (F0), 40 (F1), 120 (F2), and 200 (F3) kg K/ha, and green manure (TI = Tithonia sp.; CH = Chromolaena sp.) at 2.5 t dry matter/ha alone or together with compound fertilizer at 40 kg K/ha in two trial locations in Bas-Congo, DRC. SED: standard error of difference at P<0.05. Source: Adapted from Pypers et al. 2012

Combined application of fertilizer and organic inputs
Fertilizer response and the effect of combining inorganic and organic nutrient resources were also evaluated on cassava systems. The most common fertilizer, NPK 17:17:17, was applied in western DR Congo with or without green manure made from Tithonia sp. or Chromolaena sp., and the effects on storage root yield were evaluated in two locations with a differing soil fertility status (Fig. 1). Both plants are commonly found in fallow vegetation in western DR Congo. Control yields were similar at the two sites (12 t/ha), but the response to fertilizer differed between the sites: storage root yields reached 40 t/ha at Kiduma but only 20 t/ha at Mbuela with the addition of 200 kg K/ha. A much larger response to Tithonia sp. green manure was also observed at Kiduma, which was likely to have been related to the higher quality and nutrient contents of the green manure grown at that site.

Combining organic and inorganic nutrient resources did not result in positive interactions. No significant differences were observed between the yields after the sole application of either fertilizer or green manure to the control and those obtained with the combined application of both nutrient sources (Pypers et al. 2012). In maize-based systems, positive interactions between organic and inorganic fertilizers often arise from better synchrony in N release and N uptake by the crop. In cassava systems, where K is more often the most limiting nutrient, such a mechanism is likely to be less relevant.

In the highlands of eastern DRC alternative spacing of cassava  (2 × 0.5 m instead of 1 × 1 m) allows integration of four lines of legumes during one season and two lines of legumes during the second season without affecting cassava densities and yields.
In the highlands of eastern DRC alternative spacing of cassava (2 × 0.5 m instead of 1 × 1 m) allows integration of four lines of legumes during one season and two lines of legumes during the second season without affecting cassava densities and yields.

In eastern DR Congo, the use of improved germplasm did not result in yield increases without the simultaneous implementation of other ISFM components. Modifying the crop arrangement by planting cassava at 2 m between rows and 0.5 m within the row, intercropped with four legume lines, increased bean yields during the first season and permitted a second bean intercrop. This can also increase the total legume production by up to 1 t/ha and resulted in additional revenues of almost $1000/ha (see photo). The crop arrangement or a second legume intercrop did not affect cassava root yields. Fertilizer application increased both legume and cassava yields, and net revenue by $400–700/ha with a marginal rate of return of 1.6–2.7. Replacing the common bean intercrop by groundnut increased net revenue by $200–400/ha, partly because of the higher market value of the grains, but mostly due to a positive effect on yield of cassava storage roots. Soybean affected cassava yields negatively because of its high biomass production and long maturity period; modifications are needed to integrate a soybean intercrop into the system.

Local adaptation
Due to the high variability in soil fertility status, the varying landscape features, and the variation in access to inputs, local adaptation is required to ensure that the investments made by cassava-producing households  result in the highest returns, in line with the resources (e.g., cash, land, labor) that these households have (see photo). Such adaptation efforts are best led by extension and development partners that have the required skills and capacity to implement such efforts at scale. In eastern DR Congo, erosion is the most urgent issue to be tackled to enhance the sustainability of cassava-based systems. Results from Southeast Asia with the use of live hedges should be explored for these environments.

Cassava land preparation varies widely in DRC partly affected by slope and water status of the land.
Cassava land preparation varies widely in DRC partly affected by slope and water status of the land.

These findings demonstrate the large potential of ISFM to increase productivity in cassava–legume systems in DR Congo. This is crucial in view of the fact that cassava is changing from being almost only a food security crop to one for which there is high demand in local and urban markets. The intensification of production is thus a prerequisite for sustaining cassava-producing households and ISFM can assist in achieving such benefits. However, these benefits were not observed in all study sites. In poor soils, productivity increases were variable or absent, and soil amendments are required. Better targeting and local adaptation of the technologies are possible with a better understanding of the conditions under which positive effects occur.

Pypers, P., J.M. Sanginga, K. Bishikwabo, M. Walangululu, and B. Vanlauwe. 2011. Increased productivity through integrated soil fertility management in cassava–legume intercropping systems in the highlands of Sud-Kivu, DR Congo. Field Crops Research 120: 76–85.
Pypers, P., W. Bimponda, J.P. Lodi-Lama, B. Lele, R. Mulumba, C. Kachaka, P. Boeckx, R. Merckx, and B. Vanlauwe. 2012. Combining mineral fertilizer and green manure for increased, profitable cassava production. Agronomy Journal, in press.

Edition 4, March 2010

Biodiversity and NRM
Biodiversity conservation is key
Insect biodiversity for NRM
Why manage noncrop biodiversity
A research park for Africa
Unlocking the diversity of yam
Cassava: improver of soils
Participatory yam conservation strategies
Smart NRM approaches
DNA barcodes for pathogens
A new food security crop?

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Made to measure: smart natural resources management approaches

Coffee-banana intercropping. Photo by P. van Asten, IITA.
Coffee-banana intercropping. Photo by P. van Asten, IITA.

Coffee and banana yields in the East African highlands are often only 10 to 30% of those achieved in commercial farms in Latin America and Asia. This is the result of a mixture of biotic stresses on the crops such as pests, diseases, and weeds, and abiotic constraints such as poor soil quality and drought.

Poor crop management practices that do not sufficiently address these constraints prevent farmers from reaping maximum benefits from their efforts.

However, the importance of these yield-limiting factors differs from region to region. The natural resources management (NRM) approach therefore starts with identifying the gap between the actual, attainable, and potential yields for each location.

Diagnostic surveys and analytical tools such as the boundary line analysis are used to rank and quantify the causes of low yields. This then guides the development of tailor-made measures and actions for farmers.

Smart use of mineral fertilizer and organic matter
Poor soils are one major cause of low yields in the East African highlands. Much of Africa’s soils are old and poor, situated on very old continental plates. Only a few places have soils that still have substantial nutrient stocks, such as those derived from young volcanic material and metamorphic rocks.

Years and years of soil erosion and poor farming methods that mine minerals have worsened the situation.

IITA is working with farmers to combine organic manure and mineral fertilizer to replenish soil nutrients to meet the needs of banana and coffee.

Piet van Asten, IITA systems agronomist based in Uganda, says the approach stresses the judicious use of mineral fertilizer that is moderate in quantity, applied at the right time and in the right way, and combined with locally available organic matter.

“The combination of fertilizers and organic matter provides much-needed additional nutrients that are efficiently used up by the crops. The organic matter helps to retain mineral fertilizers applied in the topsoil and reduces losses from leaching,” he says. “It also improves the soil physical properties which help to retain soil humidity and control the temperature. Plants thrive in such humid and temperate environments as the roots are better able to take up nutrients.”

Poster on banana fertilizer recommendations for Uganda. Courtesy of P. van Asten, IITA.
Poster on banana fertilizer recommendations for Uganda. Courtesy of P. van Asten, IITA.

Sources of local organic matter are mulch, urine, manure, and compost.

Research has shown that adding mineral fertilizers and mulch to both coffee and banana nearly doubles their yields. However, the fertilizer type and dose have to supply the nutrients that are lacking.

Through mapping soil and plant nutrient status, IITA identified the missing nutrients in each region. Subsequently, it developed region-specific recommendations for using fertilizer and organic mulch in parts of Uganda.

Training materials were also developed to teach farmers how to identify nutrient deficiencies in their own farms by observing plant leaves. This should ultimately help them to localize their fertilizer needs down to the farm level.

Halting and preventing soil erosion by placing contour bunds stabilized by forage/mulch grasses and leguminous plants are also important to conserve and improve soil quality.

Smart intercropping systems
IITA has been working on promoting the intercropping of banana/plantain and coffee as research has clearly shown that intercropping works better than monocropping either crop.

Coffee, a shade-loving plant, performs well when grown under banana/plantain. Research findings showed that creating space for the banana/plantain does not reduce the yield of coffee but instead, the farmer gets bonus income from the banana.

Such intercropping systems, says van Asten, spread the socioeconomic risks of farmers as they become less vulnerable to the price fluctuations of a single crop.

“The two intercrops provide farmers with permanent piecemeal harvests from banana and annual or biannual cash booms from coffee,” he said.

Intercropping has other benefits. It leads to sharing of inputs, such as fertilizers purchased through the cash crop system, such as coffee farmers’ cooperatives. It also improves fertilizer-use efficiency, as fertilizer applied to the cash crop also benefits the food crop.

Coffee plants perform better when grown under the shade of banana plants. Photo by P. van Asten, IITA.
Coffee plants perform better when grown under the shade of banana plants. Photo by P. van Asten, IITA.

Intercropping improves the biophysical efficiency of the systems by providing better and more permanent canopy and soil cover that reduce erosion. It improves soil organic carbon stocks (carbon sequestration) through the biomass produced.

Another benefit, says van Asten, is that intercropping can sometimes increase the quality of some crops. For instance, under suboptimal growing conditions, shade-grown coffee is often of better quality and thus could fetch more money on the market.

Linking to input and output markets
In a study of the factors that limit farmers’ usage of mineral fertilizers for their banana plants, Uganda farmers cited lack of access as one constraint. Moreover, they said it was not available in smaller packaging and more affordable sizes. IITA is working to encourage farmer cooperatives that are organized around postharvest handling, sorting, and bulking to organize the supply of inputs such as fertilizer for their members.

According to van Asten, cooperatives have better access to input/output markets and improved powers of negotiation. They have improved access to market information, bulking and storage facilities, savings and credit schemes through collaboration, and agreements with input/output dealers. They can also facilitate the exploration of niche markets through the certification of products in terms of quality, production, and techniques.

Smart extension services
To meet the information needs of farmers, IITA and partners are exploring options to make location-specific information accessible. This includes the use of extension publications, videos, and mobile phone services.

Farmer detrashing banana intercropped with coffee. Photo by P. van Asten, IITA.
Farmer detrashing banana intercropped with coffee. Photo by P. van Asten, IITA.

Together with the Grameen Foundation, IITA is exploring how information can be tailored to the location of the farmer through a (decision-tree) series of questions. The more information a farmer can provide, the more precise the recommendations will be.

The NRM work on coffee and banana shows that there are practical, readily available measures that farmers can use to increase yield and contribute towards the fight against poverty and hunger. However, they have to be region- and crop-specific for maximum impact.

“For all these measures to be successful, they must start with using clean and resistant planting materials. Investing in fertilizers for use on diseased plants is a futile exercise,” concludes van Asten.

Cassava: improving sustainability of farming systems

Anneke Fermont,

Throughout Africa populations are growing fast and pressure on land is steadily increasing. To maintain productivity, farmers are constantly adapting their management of natural resources. Farming systems are thus changing from ”slash and burn systems” to ”natural fallow” systems into ”continuous cropping” systems without external inputs and ultimately into more ”intensive” systems using agricultural inputs.

Pauline Auma of Busia district, western Kenya, proudly shows her cassava harvest. Photo by  A. Fermont, IITA.
Pauline Auma of Busia district, western Kenya, proudly shows her cassava harvest. Photo by A. Fermont, IITA.

Cassava-maize systems in East Africa
A principal crop in Africa’s farming systems is cassava, with a total production that has quadrupled in the last five decades to about 118 million t/year. Cassava is a major crop in East Africa, where it is often produced together with maize by smallholder farmers. Such cassava–maize-based systems are found around Lake Victoria and in Burundi, Rwanda, and eastern DR Congo. Apart from being dominated by cassava and maize (on average one-third of cropped land is planted with cassava and one-quarter with maize) these systems have a high self-sufficiency in food. Sixty percent of all households sell cassava and maize; each crop generates an average of US$90 per year.

Due to its widely varying levels of land pressure, this region allows an interesting study of natural resource management and opportunities to improve both the productivity and the sustainability of cassava-based farming systems.

Cassava is widely grown in East Africa today, but this is a recent development. Only three decades ago cassava production was limited to the odd corner in farms as enforcement of its production during colonial times had given the crop a very bad image. The remarkable change in the importance of cassava has been driven by sharply increasing land pressure. No longer having the land available to restore soil fertility through natural fallows, farmers replaced fallows with cassava.

Does cassava improve soil fertility?
Jacinta Ouma, a farmer in Teso district, western Kenya, explains: “Cassava drops its leaves on the soil while it grows. This improves the soil, so if I plant maize after cassava it grows better.” Jacinta is not alone in this belief. A similar practice, known as jachère manioc or ‘cassava fallow’, exists in West Africa.

Almost 90% of farmers interviewed in Uganda and Kenya had the same opinion. Farm surveys in Uganda and Kenya showed that farmers plant cassava on all soil types to maintain soil fertility. If land pressure increases and soils consequently become more acidic (pH <5.8) and deficient in phosphorus (P) (available P <4–5 mg/kg), farmers increasingly plant cassava in the poorest fields in their farm. In Siaya district, western Kenya, with nearly 400 people/km2, farmers planted nearly twice as much cassava on infertile soils than on fertile soils.

Women in Teso district, western Kenya, peel cassava for eating. Photo by A. Fermont, IITA.
Women in Teso district, western Kenya, peel cassava for eating. Photo by A. Fermont, IITA.

Modeling to substantiate farmer claims
To understand farmers’ observations, we used a modeling approach. Our results suggest that planting maize on an infertile soil will result in slowly declining levels of soil organic matter, while planting cassava will slowly increase soil organic matter over time. The difference is explained by the fact that cassava grows much better than maize on infertile soils. The large amounts of easily available nitrogen (N) in its crop residues likely give cassava its reputation as a soil improver.

The model estimated that cassava returns about four times more N to the soil than maize. Through its deep rooting system and its association with mycorrhizae, cassava can pump up nutrients from the subsoil and absorb nutrients from less easily accessible pools. Nutrients from its N-rich litterfall are then redistributed to more labile pools in the topsoil.

But all is not sunshine and roses. Continuous cropping systems without external nutrient inputs deplete the soil’s nutrient pool. On the highly weathered soils found in large parts of Africa, this will unavoidably result in nutrient limitation and declining crop yields. In East Africa, N and P limitations for cereal crops are widely documented. A series of field trials with over 100 farmers demonstrated that cassava production is often limited by N and P, and commonly by potassium (K).

Cassava grows better on good soils
Cassava is known for its ability to produce fair yields where other crops fail. This has led many to believe that soil fertility is not important in cassava production. Our field trials show that this is a misconception. On the contrary, using improved varieties but no fertilizer, low soil fertility was the principal constraint to production and caused farmers an average loss of 6.7 t/ ha with respect to an attainable yield of 27 t/ha. Drought caused a loss of 5.4 t/ha and poor weed control 5.0 t/ha, whereas pests and diseases caused an average loss of 3.8 t/ha.

The farm surveys showed that Kenyan and Ugandan farmers harvested on average between 7 and 10 t/ha using farmer practices. This is far below the maximum yield of 35 t/ha that was observed during the two-year on-farm fertilizer trials and clearly shows the potential for improving yields.

The field of Nikirima Arajabu in Iganga district, Uganda, shows a very strong response to NPK fertilizer. Photo by A. Fermont, IITA.
The field of Nikirima Arajabu in Iganga district, Uganda, shows a very strong response to NPK fertilizer. Photo by A. Fermont, IITA.

Using an integrated management package that consisted of an improved genotype, recommended planting practices and NPK fertilizer, average yields in farmers’ fields more than doubled from 8.6 to 20.8 t/ha. About 30% of the yield increase was due to the use of improved genotypes, while a whopping 60% was the result of fertilizer use. These findings reinforce the idea that soil fertility/nutrient availability is a principal production constraint for cassava.

Options to improve system sustainability
Though fertilizer use may be the easiest way to improve cassava productivity and improve system sustainability, high prices limit the adoption of fertilizers, unless strong markets develop. Farmers have, however, other options to improve cassava productivity, increase nutrient availability, and reduce nutrient losses within their farming system. These include: (1) better weed control and drought avoidance strategies; (2) improving cassava’s efficiency as a soil fertility improver; (3) returning cassava stems to the field after harvest to reduce nutrient losses; and (4) planting cassava in rotation/intercrop with (cash) crops that receive manure/fertilizer.

Dealing with the challenges from increasing land pressure and related sustainability issues while substantially improving crop yields requires R4D teams with a strong interdisciplinary character. African farmers have shown great resourcefulness in maintaining system productivity by introducing cassava as a soil fertility improver. Now, IITA and its partners have the challenge to come up with innovative strategies to maintain or further improve system sustainability and crop productivity in increasingly stressed farming systems.

A tough puzzle: Biodiversity and NRM

Peter Neuenschwander,


In the past, natural resources management covered approximately half of all activities and funds of IITA and similar institutes in the Consultative Group on International Agricultural Research (CGIAR). Most often, it did not include the conservation of wild biodiversity. The other half of funds and personnel were allotted to crop plant biodiversity, mainly the varieties available worldwide in genebanks. Increasingly, however, farmers’ varieties and wild relatives of crop plants became important and the biodiversity of pathogens and witchweed were investigated in view of their use for resistance breeding.

Thanks to new technologies, breeding barriers between species could be overcome and foreign genetic material was incorporated into so-called “genetically modified organisms” (GMOs). These are being tested at a relatively small scale in some African countries. They are the source of real worries and polemical distortions, while countries such as the USA, China, Argentina, Brazil, and India have chosen to grow some GMOs on vast areas. Today, GMOs are at the center of a heated debate in an unnecessarily antagonistic manner, pitting the ideals of biodiversity conservation against the need to feed the world.

Since the end of the 1980s, the importance of biodiversity in general for a sustainable future of Planet Earth has been increasingly publicized. At the Rio Conference in 1990, global warming and the loss of biodiversity were singled out as the two most important issues facing mankind. The climate conference in Copenhagen last year was supposed to reach goals on halting and mitigating climate change. The conference is generally considered to have been a failure; nevertheless, great efforts to avoid a climatic disaster are being taken by many governments, even without the wished-for strict regulations.

And here we are in 2010, the “International Year of Biodiversity”. International nongovernmental organizations such as the International Union for Conservation of Nature (IUCN), BirdLife International, and many others are highly active in conservation and their efforts are showing successes. Most countries have subscribed to their ethics, signed the international treaties, and established focal points for the Convention of Biodiversity. For the CGIAR, though, biodiversity conservation mostly remains germplasm conservation. It is the world leader in the conservation of genetic material of crop plants and their wild relatives (for instance, yam and cowpea, of particular interest in West Africa). It is instrumental in the development of rules and regulations about the ownership of germplasm under the umbrella of the Food and Agriculture Organization.

Meristem excision under aseptic conditions (laminar flow workstation) using stereomicroscope, IITA genebank. Photo by IITA.
Meristem excision under aseptic conditions (laminar flow workstation) using stereomicroscope, IITA genebank. Photo by IITA.

IITA is also co-developing best practices and tool kits for collecting germplasm and houses important pathogen collections. Generally though, conservation of other forms of biodiversity is treated rather timidly. The general antagonism between agriculture and nature conservation thus persists. Yet, it probably need not be so: In 2001, IUCN and Future Harvest came together to publish a policy paper outlining ‘The common ground and common future, how eco-agriculture can help feed the world and save wild biodiversity’. While some of the claims might be overwrought, enough is known to allow progress towards the twin goals of saving the bulk of biodiversity while feeding the human population.

Insects are the majority of all described species. On a worldwide level, BioNET INTERNATIONAL organizes and stimulates the coordination of taxonomic research (of all taxa, but with special emphasis on insects). The IITA biodiversity collection of insects, housed in IITA-Bénin, serves as the network center for West and Central Africa. This collection, the largest in the CGIAR, is instrumental in providing basic information about the biodiversity of natural enemies used in all types of biological control.

In addition, the insectary at IITA-Bénin houses numerous live beneficial insects and mites. IITA-Bénin can respond to the changing situations of ever more invading insects and mites. Thus, in the last few years and in West Africa alone, we have seen the invasion and sometimes the control of spiraling whitefly, a new invading fruit fly (Sri Lanka fruit fly), and very recently the papaya mealybug. Last year, when the cassava mealybug invaded Thailand, IITA was able to provide effective parasitoids without delay.

Many more natural enemies are out there in the wild, suppressing their hosts or their prey. Most concern agricultural pests, but increasingly, conservation biological control is becoming important to save natural habitats from invaders. IITA is participating in these international efforts through its biological control of floating water weeds across Africa.

To assess the elusive so-called “ecosystem services”, sophisticated biodiversity studies are required. IITA’s historic classical biological control projects were against cassava and mango mealybug and cassava green mite, three formidable agricultural pests. The first two were not even known to science before they appeared in new habitats. These examples from South America and India illustrate how the ‘ecosystem services’ provided by pests’ natural enemies in the home environments remain hidden until harmful insects and mites get dissociated from their predators. Important services are also provided by microbials and pollinators, but these become visible to farmers and policymakers only when their function is impaired. Examples are lack of conservation because of wanton destruction or by bad agricultural practices, such as those that lead to the depletion of nutrients in soils or the destruction of suppressive soils.

Researcher monitoring cowpea seeds kept in cold storage room in the IITA genebank. Photo by J. Oliver, IITA.
Researcher monitoring cowpea seeds kept in cold storage room in the IITA genebank. Photo by J. Oliver, IITA.

The contribution to sustainable agriculture and conservation that IITA can make is by improving the tools (GIS, sociological, etc.) and by significant advances in research and its application to real world needs. We can thus establish an intellectual agenda for discussion and change within IITA, collaborating organizations, and society at large. Comparing this claim for action with the actual situation at IITA, we find that traditional biodiversity conservation in the form of crop plant germplasm is rather well implemented; but the conservation of nonplant biodiversity is weakly institutionalized and would need better support. Natural resources management offers the intellectual platform to integrate the different disciplines in a sustainable manner. Unfortunately, the inclusion of all biodiversity activities in a holistic natural resources management remains a dream.

Within the period of 20 years, biodiversity conservation has moved from being a specialized field to becoming an urgent task to be carried out before it is too late and extinction takes away the organisms we might one day have to rely on for survival. Even where we do not completely understand the benefits of biodiversity in providing stability to ecosystems, conservation should be implemented for the good of future generations. Apart from research, this also takes the form of providing refuges for biodiversity for future studies, as is the case with the IITA-Ibadan forest or the rehabilitated forest at Drabo Gbo in Bénin. Our national partners have many more examples; they might cherish our leadership in this matter.