NRM in cassava and yam production systems

Stefan Hauser, s.hauser@cgiar.org

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
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.

Outlook
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.

References
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.