Lessons learnt

7.2.1. The confounded effect of the low agricultural potential of the area and the closeness to urban market motivates the adoption of cassava production intensification options in West Africa

Poor soil fertility and erratic rainfall were perceived by farmers as major constraints to cassava production within the study zones (Chapter 2). However, the agricultural potential of the area for cassava production was a major determinant of farmers’ use of intensification options. The Plateaux Zone in Togo was more suitable for cassava production from an agro-ecological perspective than the Maritime Zone that had lower rainfall, poorer soils and generally lower yields. Within the zones, there was no significant difference between resource endowment groups in soil fertility management, neither in cassava yields. However, the closeness of the Maritime study sites to urban markets (Vogan and Afagnan markets within 9 km) has likely motivated, irrespective of the resource endowment group, i) the diversification of crop and livestock production, and ii) the application of crop production intensification options with the use of mineral fertilizers and organic resources including manure and household wastes, and the cultivation of legumes as intercrops in cassava fields. Although crop intensification options were mostly directed to maize, cassava can benefit from these as an intercrop. Improving nutrient management in maize can be an entry point towards enhancing fertilizer use for cassava production in maize-cassava systems in southern Togo. Despite this tendency to intensify crop production in the Maritime zone, yields were still low. This implies that either the current amounts of fertilizer applied were not enough, or other factors are limiting. Moreover, our study revealed that all households live below the poverty line of US$ 1.25 per day in the

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Maritime zone. Our study areas in the Maritime were also identified as zones vulnerable to poverty by the Government of Togo for the implementation of an IFAD funded project (PADAT: “Projet d’Appui au Développement Agricole du Togo”) for improving agricultural development within the area (FIDA 2010).

7.2.2. Indigenous soil nutrient supplies and plant needs for a given target yield explain the response of cassava to mineral fertilizers

The calibrated QUEFTS model for cassava helped to assess nutrient uptake, physiological use efficiency and yield responses of cassava to fertilizer applications in the study zones of Ghana and Togo (Chapter 3). We found that cassava response to fertilizer depended on the indigenous fertility of the soil. If the indigenous supply of a given nutrient was below the plant’s need to achieve a given target yield within a location, the uptake of that nutrient and the yield of cassava increased in response to fertilizer application. However, the assessment of the indigenous soil nutrient supply did not yield any strong relationship between soil properties and plant uptakes. Conducting more multi-locational nutrient omission trials, analysing soil samples in each location, and measuring plant uptakes and yields in these locations will help improve QUEFTS equations relating total nutrient supply to soil physical and chemical properties. It is noteworthy that QUEFTS modelling assumes good management practices, resulting in an overestimation of yields on control plots in farmers’ fields (Chapter 3). This implies that farmers could considerably improve cassava yield through enhanced management practices, even without fertilizer application. The model provides realistic estimates of yields in good rainy years but overestimates yield under severe drought conditions, like in the case of Nyankpala (Chapter 3). This is because the model is static and does not simulate water dynamics. Coupling QUEFTS with a dynamic model that simulates water-limited yields may adjust target yields to seasonal variabilities within a given location for enhanced fertilizer recommendations. 7.2.3. Balanced nutrition increases cassava yields and benefit-cost ratio of

fertilizer use

The balanced nutrition approach provided higher nutrient use efficiency and benefit- cost ratio compared to blanket fertilizer applications (Chapter 4). Site-specific fertilizer recommendation based on the balanced nutrition approach is crucial to optimize nutrient use and achieve high yields and profits in cassava production systems in West Africa. However, due to the generally high costs of single fertilizers and the difficulty to blend site-specific fertilizer mixes for cassava for each location of West Africa, major soil type or agro-ecology specific recommendations will be a step forwards towards improving fertilizer use efficiencies in West Africa. With major soil types or agro-ecology specific recommendations, which expand across countries, fertilizer

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companies will have larger markets, and this can contribute to reducing the fertilizer price for the farmer’s benefit. A fertilizer blend with larger K content than the existing NPK complexes found in West Africa (N-P2O5-K2O: 15-15-15, 20-10-10, etc.) will be a good start, given the strong demand of cassava for K. The key challenge is to increase the use of fertilizers by farmers in cassava production systems. We found a greater intensity of fertilizer use in the Maritime compared to the Plateaux Zone irrespective of the farmer resource endowment groups. The former zone has a high need of intensification due to its closeness to urban markets as well as its lower agricultural potential for cassava production compared to the latter. The intensity of fertilizer use could have been limited to resource endowed households only in the Maritime Zone in case of lack of access to close urban markets since resource- constrained farmers are generally less inclined to use fertilizers due to financial limitations (Franke et al. 2014). Beyond the use of fertilizer, emphasis should be placed on the promotion of site-specific balanced fertilizer rates, which should go along with the promotion of good management practices (recommended planting density, planting periods, weeding, disease control, etc.) and improved cultivar cultivation in order to optimize returns on investment in fertilizer use.

7.2.4. Potassium increases water and radiation use efficiencies of cassava

We found that the effect of potassium (K) on cassava productivity was mainly explained by the impact of K on radiation use efficiency and water use efficiency (Chapter 5). Potassium showed a leading role in improving radiation use efficiency and water use efficiency, whereas nitrogen (N) was the leading nutrient affecting light interception and water transpiration in cassava. These findings show that K and N play complementary roles in improving cassava productivity. The improvement of water- use efficiency by K supply is of paramount importance in climate-smart agriculture in the context of rain-fed cassava production systems, and indicates the need for enhanced K management in such systems to achieve high yields. The use of computer models like QUEFTS can help to estimate K need within a given location when water- limited yields of this location are known.

7.2.5. Inappropriate planting periods can cause considerable drought-related yield losses in cassava production systems in Southern Togo

Development of the cassava model has been a challenging experience. Unlike other crops like cereals, cassava is a perennial crop cultivated as an annual crop, which still remains longer in the field beyond 6-10 months after planting. This has consequence on the duration and costs of field experiments and data collection. Moreover, there is no distinct separation between the vegetative stage and the production of storage roots as main commercial product (which is not a fruit), unlike cereals for instance, for

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which the grain is the main commercial product. These characteristics added to other specific attributes of the crop including its tolerance to drought versus its response to water dynamics make the development of the cassava model quite complex.

The newly developed version of LINTUL-Cassava estimates water-limited yields of cassava, assesses the impacts of drought stress on cassava yields and simulates dormancy and the effect of recovery from dormancy on yields. The comparison of simulated water-limited yields and potential yields shows the prevalence of drought stress impacts on yield formation within the study sites with 9-60% yield reduction. The largest yield loss due to drought was obtained when drought occurred early in the crop life cycle, as reported by Alves (2002). The model showed that drought related yield losses can be reduced and storage roots yields could be increased with planting earlier than April. Although planting before April may be practically challenging in the current cropping systems which lack proper mechanization like planters, irrigation systems and harvesters, this result calls for attention to the possibility to improve cassava storage roots yields by adjusting planting dates. However, confirmation of this result and further improvement of the model are required. The lack of data to estimate some soil and crop parameters constrained further advances of this model. Among the key soil parameters, soil hydraulic properties such as water content at field capacity and wilting point are major determinants of the model’s behaviour. Their accurate estimate in experimental fields is required for proper model development. Field experiments for model improvement should be established near a synoptic weather station to obtain daily weather inputs. After the improvement of the water-limited production version of LINTUL-Cassava, a nutrient limited production version can be developed. Further studies to develop the nutrient limited production of this model requires a strong consideration of the effects of nutrient availability, especially K, on stomata regulation. Conducting research on the physiological impacts of K deficiency on water use efficiency of cassava may unveil the main mechanisms involved in this process relatively to K effects on stomata regulation.