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CONTEXT: Despite recent improvements in living standards, a substantial proportion of farm households in sub-Saharan Africa (SSA) is food insecure, and increasing crop productivity could help address this problem. OBJECTIVE: We estimated the effect of increasing maize yields with mineral fertilizer on household food security and on regional and national maize supply in two East African countries - Uganda and Tanzania. METHODS: We estimated maize yield response to nitrogen (N) fertilization with a machine learning model trained on 15,952 observations of maize responses to fertilizer across SSA. Together with spatial price data, we used this model to quantify the profit-maximizing N fertilizer input for a nationally-representative sample of 4188 agricultural households in the two countries. We computed a food availability indicator for all households. RESULTS AND CONCLUSIONS: The mean profit-maximizing N input was 82 kg/ha in Tanzania, but it was much lower in Uganda (24 kg/ha) mostly because of less favorable prices. The profit-maximizing N input was above the reported N input for 95% of the households in Tanzania and for 43% of the households in Uganda. It was predicted to increase the food availability ratio of food insecure maize growers by 95% in Tanzania, and by 25% in Uganda. The administrative regions where maize supply could increase most were not the same as the regions where the increase in household-level food security was largest. With increased fertilization, food insecure maize growing households (35% in Tanzania and 42% in Uganda) could only contribute about 20% of the overall increase in maize supply, whereas the 20 to 30% food secure households that have a larger area planted with maize could contribute >60%. SIGNIFICANCE: Our study makes two key contributions: i) a substantial increase in national maize supply is more likely to come from already food secure households with relatively large farms, while food insecure households with small farms may nevertheless increase their household-level food security through maize intensification, and ii) high potential areas to increase maize domestic production do not necessarily match with areas where there is immediate scope to improve household-level food security.

Green manure can be a source of nutrients for crops to spare some of the mineral fertilizer required for sustainable intensification of agriculture in smallholder context. We explored with a calibrated model the potential of legume green manure to help spare mineral fertilizer for sustainable intensification of rainfed rice production in Madagascar highlands. This study builds on a two-year rice experiment with rice and green manure as previous crop. The experiment investigated the response of rainfed rice growth to the incorporation into the soil of i) rice residue only (C:N ratio of 44.9) and ii) mucuna + crotalaria green manures (C:N ratio of 18.7). Two levels of fertilizer were investigated: manure only (5 t DM ha−1) (F1) and manure (5 t DM ha−1) in combination with mineral fertilizer (136 kg N ha−1) (F2). The crop model STICS was calibrated against observed soil water and soil mineral N, plant N uptake, aboveground biomass and grain yield to reproduce the impact of residue type and fertilizer input on mineralization of organic N in soil and in crop residue, crop growth and grain yield. The calibrated crop model was then used to explore rice response to mineral N fertilizer, with or without green manure, for the historical climate (1994–2018). Relative Root Mean Square Error (rRMSE) of model simulations fell within the observed relative variation in grain yield due to treatment effect. With green manure, mineral N available for rice increased compared with the situation without green manure, leading to greater agronomic use efficiency of N fertilizer. Green manure helped spare mineral fertilizer to achieve a reference yield set at 75 % of potential yield. This reference yield (3.67 t ha−1 on average across years) was reached with 40 kg N ha−1 with 6 t ha−1 of green manure, and 100 kg N ha−1 without green manure. The analysis of rice yield inter-annual variability indicated that (i) N leaching increased with total seasonal rainfall, which undermined the benefits of green manure on rice productivity in wetter years, and (ii) the increase in rice productivity allowed by the green manure did not fully compensate the rice yield foregone with the cultivation of the green manure. The calibrated model can be a useful tool to explore additional management strategies that will help maximize the benefit of green manure for sustainable intensification of rice-based cropping systems, especially in low-input cropping systems.

Intercropping is a key entry point for sustainable intensification of cropping systems in sub-Saharan Africa where variable rainfall conditions prevail. Crop simulation models can complement field experiments to assess the agronomic and environmental performances of intercropping systems under diverse climatic conditions, including hypothetical future climate. So far, crop models that can handle intercropping, such as STICS, have not often been extensively evaluated for tropical conditions and for species grown by farmers in sub-Saharan Africa. The objective of this study was to evaluate the performance of the calibrated STICS model to simulate sorghum-cowpea intercropping systems in rainfed conditions in West Africa. We used data from field experiments conducted at the N'Tarla Agronomic Station in Mali in 2017 and 2018. Two varieties of sorghum (local and improved) with contrasting photoperiod sensitivities were grown as sole crop and intercropped with cowpea, with additive design. Two sowing dates and two levels of mineral fertilization were also investigated. Model simulations were evaluated with observed data for phenology, leaf area index (LAI), aboveground biomass, grain yield and in-season soil moisture. Large variations in aboveground biomass of sorghum and cowpea was observed in the experiment (i.e. 3.5 – 9.6 t/ha for sorghum and 0.4–2.5 t/ha for cowpea), owing to the treatments (i.e. sole vs intercrop, early vs late sowing, no fertilizer input vs fertilizer input). Such variations were satisfactorily reproduced by the model, with EF of 0.81 in calibration and 0.58 in evaluation (with relative rRMSE of 23 % and 43 %) across crops. Sorghum AGB simulations were more accurate (rRMSE of 21 % and EF of 0.54) than cowpea AGB simulations (rRMSE of 25 % and EF of −0.09). The two main observed features of the intercropping system were well reproduced by the model. Firstly, cowpea and sorghum aboveground biomass decreased with intercropping compared with sole cropping, and the decrease in cowpea biomass was greater than the decrease in sorghum biomass. Secondly, despite a reduction in sorghum and cowpea yield, Land Equivalent Ratio of the intercropping for aboveground biomass was always above one. With regard to grain yield, observed LER was above one only in the non-fertilized treatment. The model failed at reproducing this behavior, probably because of insufficiently accurate calibration of the process leading to grain yield formation: rRMSE for grain yield was 49 % in calibration and 41 % in evaluation. Based on these findings, we discuss avenues to improve model calibration and use the model to explore options for sustainable intensification in land constrained sub-Saharan Africa.

We calibrated the crop model STICS and used it in a simulation experiment to assess the effect of no-tillage cropping systems, with and without millet as cover crop (NTM and NT), on the water-limited yield of maize, its inter-annual variability, water runoff and drainage below the root zone, compared to conventional tillage-based cropping systems (CT). We ran simulations for two contrasting climates in the Brazilian Cerrado, Goiânia with 1600 mm of annual rain and Barreiras with 1160 mm, using a series of historical weather data and considering two soil types with different soil water storage capacities (i.e. different maximum rooting depths of 90 and 180 cm). We identified the best suitable sowing windows based on agronomic (yield and yield variability) and environmental (runoff and drainage) performances of the cropping systems. Compared to CT, NT and NTM systems increased water-limited maize grain yield and its stability. For example, for the simulations with 90 cm set as maximum rooting depth, grain yields for CT, NT and NTM, averaged over all sowing dates, were 3.41, 4.35 and 4.28 t ha−1 in Goiâna, and 7.02, 7.67 and 7.5 t ha−1 in Barreiras. No-tillage systems markedly decreased water runoff and hence erosion risks. For example, simulated runoff was on average 4% of the annual rain for NT and NTM against 13% for CT at both locations. In contrast, no-tillage systems increased water drainage below the rooting zone, hence with risks of increased nutrient leaching. For example, for the 90 cm maximum rooting depth, simulated drainage was 48% and 41% of the annual rain for NT and NTM against 32% for CT in Goiânia, and 29% and 26% against 17% in Barreiras. Reconciling the objective of maximizing yield and its stability with that of minimizing both water runoff and drainage was only achieved under no-tillage with millet as cover crop (NTM) for a very narrow range of sowing dates. Sowing all fields of the farm within a narrow optimal window requires, however, important investment in machinery and preferential allocation of available farm resources for sowing activities at the start of the cropping season. This must be weighed against other farm activities.

International audience; Rainfed crop production predominates in West Africa. Rice is an important staple food, especially in Senegal. The scope for increase in rice production under irrigated conditions is uncertain. Rainfed rice is therefore a key component for regional food security impelling agronomists to assess climate change impact on rainfed rice yield and to design rainfed rice ideotypes suited to future climate scenarios. The DSSAT CSM-CERES-Rice model was thus calibrated and evaluated on 19 agronomic experiments conducted in 2012, 2013, and 2014, in 6 locations, with 21 cultivars and two fertilization levels (20 and 80 kg N ha−1). Simulations were then carried out with the crop model forced with the downscaled projections of seven climate models, with and without considering the impact of an increase in atmospheric [CO2], using an ensemble of global circulation models and two Representative Concentration Pathways (RCP2.6 and RCP8.5). Simulated rice yield was divided by two over the century with RCP8.5 and stagnated with RCP2.6. Elevated [CO2] significantly increased yields, but this effect could not offset the yield decline due to elevated temperatures. Cultivars with longer vegetative phases and greater temperature tolerance were better adapted to climate change than current cultivars. Using these new cultivars with the recommended fertilization rate (80 kg N ha−1) could offset the yield decline due to climate change. For the first time, we bring together a study based on a process-based crop model handling crop response to elevated [CO2], a large set of field experiments and up-to-date climate projections (i) to provide useful insights into plausible impacts of climate change on rainfed rice in Senegal and (ii) to identify cultivar characteristics relevant for adaptation to future possible climates. Our findings will help set priorities for breeding resilient cultivar in the region.

International audience; In the mountainous areas of South-East Asia, family farms have shifted from subsistence to input-intensified and market-oriented maize-based farming systems, resulting in a substantial increase in farm income, but also in new environmental threats: deforestation, biodiversity loss, soil erosion, herbicide leaching and soil fertility degradation. In this typical case study of cash-strapped farms, where the balance between socio-economic and environmental dimensions of sustainability is complex, we used participatory methods (serious games and Q-methodology), combined with agronomic field monitoring, to identify relevant farm and field-level criteria for sustainability assessment.Serious games at farm level showed that short-term socio-economic dimensions prevailed over environmental dimensions in farmers' objectives. However, farmers also greatly valued their capacity to transfer a viable farm to the next generation and avoid herbicide use. Serious games at field level showed that some farmers were willing to preserve soil fertility for future generations. The agronomic field monitoring showed that maize yield deviations from potential water-limited yield were primarily due to weed infestation favoured by low sowing density, due to uncontrolled moto-mechanized crop establishment. This technical failure at the beginning of the maize cycle led to herbicide overuse, poor returns on investment for fertilizer, and increased exposure to soil erosion.Combining the perspectives of scientists and farmers led to the following set of locally-relevant criteria: i) at farm level: farm income, diversity of activities, farmer autonomy, farmer health, workload peaks, soil fertility transfer between agroecological zones in the landscape, rice and forage self-sufficiency; ii) at field level: resource use efficiency, soil fertility, erosion and herbicide risks, susceptibility to pests, weeds and climate variability, biodiversity, land productivity, economic performance, labour productivity and work drudgery. Our approach helped to identify key relevant sustainability criteria and could be useful for designing alternatives to current maize-based cropping systems, and contributed to informing priority-setting for institutional development and agricultural policies in the region.

Relay intercropping of maize with fodder crops is a promising option for sustainable intensification of dairy small-scale farms in the Cerrado of Brazil. Twenty-six intercropping trials were conducted on farmers’ fields with the following experimental treatments: sole maize crop cropping (MS), maize-Brachiaria intercropping (MB) and maize-pigeon pea intercropping (MP). The trials were managed by the farmers, i.e. choice of conventional tillage (CT) versus no-tillage (NT), sowing dates, fertilization and weed control. Maize grain yield varied strongly across the farmer fields, from 100 to 5900 kg ha−1 in the MS treatment, 500 to 6900 kg ha−1 in MP and 300 to 5500 kg ha−1 in MB. Intercropping did not significantly affect maize grain yields under NT, but yields were reduced under CT in one out of two seasons. Maize yields in the intercropped systems were also higher under NT than CT. Total biomass productivity was significantly higher in the maize-fodder than in the sole maize system. An increased interval between sowing of maize and fodder crop significantly reduced the fodder crop biomass. Relay intercropping, especially in combination with NT, is a promising option if crop calendars and fertilization are properly managed by farmers to reduce interspecific competition between the maize and fodder crop.

International audience; Much effort has been spent on formulating guidelines for inorganic fertilizer use in millet crops in Sub-sahelian farms. However, these guidelines do not take into account the diversity of manuring practices. In this study we analyzed over two years (2016–2017) the use efficiency of an inorganic NPK fertilizer as affected by the two most contrasted categories of organic manure strategies (OMS) found in millet fields of central Senegal. 19 farmers’ fields were selected in a village typical of that region, 11 and 8 of which respectively corresponding to categories OMS1 and OMS2 as follows: OMS1, locally referred to as Homefields, were fields continuously cropped with millet over the last 15 years, having received organic manure regularly in the past, and manured again at the onset of the 2016 rainy season. OMS2 fields locally referred to as Outfields, were not manured in 2016 and were rarely manured in the past. Four of them were continuously cropped with millet and the others had followed a triennial millet-peanut-fallow rotation. In 2017, no manure was applied in any of the OMS1 or OMS2 fields. A pairwise treatments with and without the same inorganic fertilizer dose was applied in each field in both 2016 and 2017 cropping seasons. In 2016, the higher the manure application, the higher the use efficiency of the inorganic fertilizer applied. The use efficiency of the inorganic N was most closely related to soil bulk density and P availability. In 2017, with no new manure amendment, millet yield in OMS1 was about three times higher than in 2016. It was close to the water-limited yield, suggesting that the residual effect of the manure applied in 2016 was high. The use efficiency of the inorganic N was generally low under these conditions. In OMS2, millet yield and use efficiency of inorganic fertilizer remained low in both years. The crop rotation with peanuts did not enrich the soil compared to the millet returning every year, but it reduced Striga hermontica infestation and increased the millet 1000-grain weight. The methodological approach developed here may help in formulating guidelines to deal with the diversity of farming practices in Sub-sahelian villages.

Family farms in the tropics mainly rely on rainfed agriculture. Water availability is limited to rainfall and is one of the main constraints to crop productivity. Conservation agriculture (CA) is promoted as an alternative that, among other functions, enhances water infiltration and limits evaporation from the soil thanks to a mulch of crop residues left on the soil surface. These functions are assumed to reduce the water availability constraint by limiting water stress during crop growth. But the variability of rainfall distribution combined with the wide range of agroecological conditions and the variety of crop husbandries in the tropics makes it difficult to evaluate the efficiency of mulching. The aim of this study was to capture the variability through a simple modeling approach using the crop growth model PYE-CA, which requires a limited set of parameters and a virtual experiment (VE). We applied our approach to a case study of upland rice in the Lake Alaotra region in Madagascar, where rainfall distribution is highly variable. The VE used a 17-year series of weather data with a range of soil water conditions, sowing dates, and growth and yield limitations that cover the variability of agroecological conditions and management systems in the study area. The VE revealed that variable successions of wet and dry episodes during the rainy season resulted in both water stress and an increase in deep drainage. In the majority of conditions simulated, enhancing water infiltration through CA mainly increased water loss through drainage. However, better water infiltration may also reduce the production risks involved in early sowing or crop intensification, thereby offering new opportunities to farmers. As an alternative to time consuming and labor intensive experimentation, we propose a suitable modeling approach to identify the main drivers of rainfall × crop interactions that could be extrapolated to other regions in the tropics.

International audience; While crop yields in Sub-Saharan Africa are low compared to most other parts of the world, weather-index insurance is often presented as a promising tool, which could help resource-poor farmers in developing countries to invest and adopt yield-enhancing technologies. Here, we test this hypothesis on two contrasting areas (in terms of rainfall scarcity) of the Senegalese groundnut basin through the use of a bio-economic farm model, coupling the crop growth model CELSIUS with the economic model ANDERS, both specifically designed for this purpose. We introduce a weather-index insurance whose index is currently being used for pilot projects in Senegal and West Africa. Results show that insurance leads to a welfare gain only for those farmers located in the driest area. These farmers respond to insurance mostly by increasing the amount of cow fattening, which leads to higher crop yields thanks to the larger production of manure. We also find that subsidizing insurance is not the best possible use of public funds: for a given level of public funding, reducing credit rates, subsidizing fertilizers, or just transferring cash as a lump-sum generally brings a higher expected utility to farmers and leads to a higher increase in grain production levels.

International audience; Direct seeding mulch-based cropping (DMC) systems are often considered as an efficient way of combining ecological sustainability and economic viability while maintaining or increasing agricultural productivity. This paper describes a modelling analysis of the functioning of family farms in rural settlements of the agrarian reform in the Cerrados of Brazil. The aim was to assess the impact of the introduction of DMC systems with and without cover crops on crop-livestock management and net household income. A bio-economic farm model based on optimization of a utility function under multiple constraints was developed, capturing the interactions between livestock activities and the introduction of DMC systems. The model was run for six farms representing three farm types in the study area: 1) subsistence-oriented mixed crop-livestock farms; 2) market-oriented dairy farms; and 3) mixed crop-livestock farms, oriented to meat marketing. The following maize-based DMC systems were evaluated: a DMC system with mulch from residues of the previous maize crop and no cover crop, and two DMC systems with a fodder species, Brachiaria brizanta or Cajanus cajan, as cover crop. The simulated adoptability of DMC systems by farmers of the assentamentos depended to a large extent on the yields and feed quality of the fodder species. The modelling results suggested that DMC with C cajan as cover crop was the best suitable option for all simulated farm types. This was mainly explained by its high feed value (expressed in crude protein content). Furthermore, in the process of intensification and specialization in dairy production, farmers were likely to shift from using maize for pig husbandry to feeding it to the dairy cows. The introduction of cover crops in the farm systems was a source of additional animal feed during the dry season that was cheaper than the purchase of feed concentrates, when the size of the dairy herd did not exceed a certain threshold.

Agroforestry is pointed out by the Intergovernmental Panel on Climate Change report as a key option to respond to climate change and land degradation while simultaneously improving global food security ( IPCC, 2019 ). Faidherbia albida parklands are widespread in Sub-Saharan Africa and provide several ecosystem services to populations, notably an increase in crop productivity. While remote sensing has been proven useful for crop yield assessment in smallholder farming system, it has so far ignored the woody component. We propose an original approach combining remote sensing, landscape ecology and statistical modelling to i) improve the accuracy of millet yield prediction in parklands and ii) identify the main drivers of millet yield spatial variation. The parkland of Central Senegal was chosen as a case study. Firstly, we calibrated a remote sensing-based linear model that accounted for vegetation productivity and tree density to predict millet yield. Integrating parkland structure improved the accuracy of yield estimation. The best model based on a combination of Green Difference Vegetation Index and number of trees in the field explained 70% of observed yield variability (relative Root Mean Squared Error (RRMSE) of 28%). The best model based solely on vegetation productivity (no information on parkland structure) explained only 46% of the observed variability (RRMSE = 34%). Secondly we investigated the drivers of the spatial variability in estimated yield using Gradient Boosting Machine algorithm (GBM) and biophysical and management factors derived from geospatial data. The GBM model explained 81% of yield spatial variability. Predominant drivers were soil nutrient availability (i.e. soil total nitrogen and total phosphorous) and woody cover in the surrounding landscape of fields. Our results show that millet yield increases with woody cover in the surrounding landscape of fields up to a woody cover of 35%. These findings have to be strengthened by testing the approach in more diversified and/or denser parklands. Our study illustrates that recent advances in earth observations open up new avenues to improve the monitoring of parkland systems in smallholder context.

SUMMARYIn mountainous areas of Continental South East Asia, double cropping in the irrigable valleys and terraces is often promoted as a way to increase farmers’ income while alleviating the pressure of agriculture on fragile slopes. However, cold temperature and low global radiation may constrain this strategy. Lethally cold events may occur, and increases in crop cycle length may jeopardize the correct timing of the main rice crop, taking place in summer. The model PYE (potential yield estimator) simulating the impact of temperature and radiation on the development and yield of annual crops was adapted to account for the range of temperature occurring in the area under study. It was calibrated against experimental data for the three crops that are most often considered as spring crops in the irrigable land of the northern mountains of Vietnam: rice, soybean and maize. Then a virtual experiment was designed in order to simulate various scenarios combining crop species and sowing date with climatic data accounting for variability of climate across years, location and elevation. Completed with a sensitivity analysis, it allowed to define favourable ‘sowing windows’ for non-water limited environments, based on the three following criteria: high average yield, low incidence over years of lethally cold events and low incidence over years of delays in the maturity of the spring crop. The length of this sowing window varied greatly across the scenarios tested. The widest was obtained for the case of soybean whatever the location and elevation, which makes it the less risky of the tested options. The approach followed proved effective to identify favourable and unfavourable environments in order to help better targeting the policy in support to the introduction of a spring crop in the mountains of Vietnam.

Given the scarcity of manure and the limited land available for fallowing, cotton cultivation with its input credit schemes is often the main entry point for nutrients in cropping systems of West Africa. In an experiment carried out during 25 years in southern Mali, the crop and soil responses to organic fertilizer (=OF), inorganic fertilizer (=IF), and a combination of both (=OIF) were quantified and compared to a control treatment for a typical cotton-sorghum-groundnut rotation. From 1965 to 1979 (15 years, period 1), fertilizers were only applied on cotton and the control treatment was not fertilized. From 1980 to 1989 (10 years, period 2), the amount of manure applied was split between cotton and sorghum, and inorganic fertilizers were applied to the three crops. Inorganic fertilizers were also applied to plots with cotton and sorghum that were previously unfertilized control treatments. In favorable rainfall seasons maximum yields of fertilized treatments reached ca. 3.5 t ha −1 in the case of cotton and groundnuts, and ca. 2 t ha −1 in the case of sorghum. During period 1, cotton yields were steady (ca. 1 t ha −1 ) when no fertilizers were added. Cotton yields were 20% higher in the OF and OIF treatments than in the IF treatment. Sorghum and groundnut benefited from residual effects of fertilizer application on cotton leading to a 200% and 50% yield increase respectively compared to the control treatment. During period 2, yields of the three crops were similar across fertilized treatments. Groundnut yields in the OF treatment, and cotton yields in the OF and IF treatments were respectively 45%, 30% and 20% significantly higher than those in the respective control treatments. No added benefit on crop yields was observed from the combined use of inorganic and organic fertilizer. Soil nutrient contents (SOC, N, P, K) did not significantly change in any of the treatments after 25 years. Soil pH decreased in treatments receiving inorganic fertilizer. Despite low level of soil organic matter, crops responded to organic or inorganic fertilization and crop productivity over time was mostly influenced by the interaction between fertilization and rainfall variability. Our results highlight the role of cotton in West African landscapes as an entry point of nutrients via fertilization, which impacts positively on the productivity of the other crops in the rotation. Credit schemes by the cotton company for farmers to purchase fertilizer to which they would otherwise not have access are thus crucial for sustained crop productivity.

International audience; AbstractConservation agriculture, which is based on minimum tillage, permanent soil cover and crop rotations, has widely been promoted as a practice to maintain or improve soil quality and enhance crop productivity. To a large extent, the beneficial effects of conservation agriculture are expected to be provided by permanent soil cover with crop residues. Surface crop residues play an important role for crop growth through their benefits on soil-related structural components and processes in the agro-ecosystem, referred to in this study as agro-ecological functions. Through a meta-analysis of the literature, we have studied the relative effects of surface crop residue levels on the performance of a set of agro-ecological functions compared with a no-till bare soil, i.e., without surface residues. The selected agro-ecological functions were soil water evaporation control, soil water infiltration, soil water runoff control, soil loss control, soil nutrient availability, soil organic carbon (SOC) stocks and gains, weed control and soil meso- and macrofauna abundance. The potential effects of crop residue cover were quantified using boundary line models. Our main findings were (1) 8 t ha−1 of residues were needed to decrease soil water evaporation by about 30% compared to no-till bare soil. (2) To achieve the maximum effect on soil water infiltration, water runoff and soil loss control, residue amounts of at least 2 t ha−1 were required. (3) The effect of increasing the amounts of surface crop residues on soil nutrient supply (N, P and K) was relatively low; the boundary line models were not significant. (4) The average annual SOC gain increased with increasing amounts of residues, with a mean of 0.38 t C ha−1 year−1 with 4 to 5 t ha−1 of residues. (5) Weed emergence and biomass can be reduced by 50% compared to a no-till bare soil with residue amounts of 1 t ha−1 or more. (6) There was a weak response in soil meso- and macrofauna abundance to increasing amounts of surface crop residues. The maximum effect corresponded to an increase of 45% compared to a no-till bare soil and was reached from 10 t ha−1 of residues. Our findings suggest that optimal amounts of surface residues in the practice of conservation agriculture will largely depend on the type of constraints to crop production which can be addressed with mulching.

International audience; Controversy surrounds the promotion of conservation agriculture (CA) in smallholder farming systems in sub-Saharan Africa. The introduction of CA is a profound change in farm management. Benefits in reduced erosion and stabilized crop production may be obtained, but technical performance at field level is but one of the determinants of adoption. For various reasons, all of the CA principles are not always fully implemented by farmers and results not as favourable as expected. As with other approaches to increasing agricultural productivity, the production constraints, farmers' objectives, and the expected benefits and costs of implementing CA are important aspects that influence adoption. At farm and village levels, trade-offs in the allocation of resources become important in determining how CA may fit into a given farming system. At a regional level, factors such as the market conditions, interactions among stakeholders and other institutional and political dimensions become important. At each level, opportunities or difficulties emerge that enhance or impede development, adaptation and adoption of CA. The ex-ante identification of situations for where CA (and which form of CA) is appropriate demands research from a multi-stakeholder, multi-level, and interdisciplinary perspective. Recommendations are made where research is required to address key knowledge gaps.

Conservation agriculture (CA) is considered a possible option for reducing the environmental impact of tilled maize on sloping land in two mountainous districts of northern Vietnam. Within this context, our study aimed at building a regional farm typology that can support the design of CA cropping systems, suiting different farm types. 411 maize-producing farms were surveyed. Using multivariate analysis, we identified 5 farm types with contrasting resources and sustainability performances. We discuss the range of possible CA systems to be designed for each type, according to its specific constraints and opportunities. We especially consider diverse intensification levels and kinds of cover plants. We propose the development of farm models to further explore the economic attractiveness of the options selected for each type., Dans deux districts montagneux du nord du Vietnam, l’agriculture de conservation (AC) est envisagée pour réduire l’impact environnemental de la culture de maïs sur pente. Dans ce contexte, notre étude vise l’élaboration d’une typologie régionale d’exploitations agricoles qui facilite la conception de systèmes de culture en AC adaptés a` chaque type. Au total, 411 exploitations maïsicoles ont été étudiées. Cinq types d’exploitations aux caractéristiques et aux performances de durabilité contrastées ont été identifiés par analyse multivariée. Nous discutons des systèmes de culture en AC a priori adaptés aux contraintes et possibilités de chaque type. Les prototypes envisagés diffèrent par le niveau d’intensification visé et la nature des couvertures végétales ciblées. Sur cette base, nous proposons le développement de modèles d’exploitations pour évaluer ex ante l’attractivité économique des options retenues pour chaque type d’exploitation

A key principle of direct seeding mulch-based cropping systems is the retention of crop residues on the soil surface to preserve soil water for crop growth. In this study the impact of surface crop residue on water use and production risk associated with rainfall variability is analysed for two contrasting tropical sites. The two sites are La Tinaja in semi-arid Mexico and Planaltina in humid Brazil. The crop growth model STICS, version 3.0 was updated with a simple empirical module, incorporating the following effects of surface residue on soil water balance: (1) rainfall interception and subsequent mulch evaporation; (2) radiation interception with associated reduction of soil evaporation and (3) reduction of surface water runoff. The results of the model simulations showed that the effect of radiation interception at both sites was much more important than the effect of intercepting rain. The high sensitivity of model yield predictions to surface water runoff, especially in La Tinaja, highlighted the importance of a careful parameterisation of the surface water runoff formalism. The model results suggested that even small amounts of surface residue are effective at reducing water loss and increasing yield. In La Tinaja, grain yield was increased at least twofold with retention of 1 Mg ha-1 surface residue. In Planaltina, the advantage of water conservation by surface residue is partly offset by increased drainage losses. As a consequence, the impact of crop residue mulching on grain yield was lower at that site.

In many developing countries, yield increases obtained through intensification of agriculture seems necessary to face the high population growth rate. In the case of rainfed agriculture of semiarid regions the effect on yield of some intensive farming practices is highly variable, however, due to interactions with rainfall distribution and amount. This paper presents a model (SARRA) to perform regional comparisons of the risk associated with intensive practices in semiarid environments. The model, built for this specific and restricted objective, is suitable for crops and regions where few data are available. The intensification level of a given crop situation is assumed to be the potential yield that would be obtained without any water constraint. The model was calibrated and validated for millet in Senegal and then run for three locations in that country, using long-term sequences of climatic data from wet and dry periods. Three potential yield levels and two runoff loss levels were tested for each location and period. At one site, the study revealed that using management to increase potential yield is highly risky. In a location only 200 km south of the former, the risk associated with intensive practices is much less, provided that all rainfall infiltrates. Considering 25% rainfall loss by runoff, which was reported to be common in this region, however, the risk associated with high yield potential increases. This case study highlighted the extent to which the risk faced by farmers in semiarid regions can vary spatially and with management practices. The approach proposed in this paper appears suitable for use in designing agricultural development strategies in semiarid regions.

Yield gaps of major food crops are wide under rainfed family agriculture in the tropics. Their magnitude and causes vary substantially across agro-ecological, demographic and market situations. Methods to assess yield gaps should cope with spatio-temporal variability of bio-physical conditions, management practices, and data scarcity under smallholder conditions. Particularly challenging is to determine the most relevant methods for estimating potential (Yp) and water-limited (Yw) yields against which actual yields (Ya) are compared. We assessed yield gaps of main staple rainfed crops across contrasting family farming systems in Senegal (millet, subsistence oriented systems), central Brazil (maize, market oriented systems) and Vietnam (maize, market oriented systems and upland rice, subsistence oriented systems). In each region, actual aboveground biomass, Ya and yield components were measured over 2–3 agricultural seasons in a network of farmers’ fields, covering the diversity of soils and farmers’ management practices. Yp and Yw were calculated using a simple ad hoc crop simulation model (potential yield estimator, PYE) that was calibrated for each situation with observed and secondary data. Maize yields measured on farmers’ fields were on average relatively high in market oriented systems, but extremely variable (4.14 ± 1.72 Mg ha−1). In contrast yields of crops of subsistence oriented systems were very low (0.80 ± 0.54 Mg ha−1 and 0.80 ± 0.47 Mg ha−1 for millet and upland rice, respectively). Ya − Yp was 0.15 for millet in Senegal, 0.33 for upland rice in Vietnam, 0.26 for maize in Vietnam, and 0.46 for maize in Brazil. In Vietnam, there was little difference between Yw and Yp suggesting a low incidence of water constraints. The gap between Ya and Yw was equal to (millet in Senegal) or twice (maize in Vietnam and Brazil) the difference between Yw and Yp, indicating that yield gaps depend strongly on factors other than global radiation, temperature, rainfall and soil water holding capacity. Previous studies in the case study areas showed that the main causes of yield gaps were poor soil fertility and weed infestation related to the inability of farmers to access chemical inputs. Simple methods to estimate Yw and Yp, such as the values at the 90th percentile of Ya, or a bilinear boundary function fitted between seasonal rainfall and the best farmers’ yield both led to strongly underestimated yield gaps. Yw and Yp estimated with a crop simulation model appeared to be more accurate, even in situations of relative scarcity of field data to calibrate cultivar-specific model parameters.

Cotton (Gossypium hirsutum L.) is sensitive to water shortage during its establishment phase. In the Sudan–Sahelian zone, a significant part of rainfall events can be lost through runoff, particularly as the rainy season begins. Using residues from previous crops as soil cover is expected to reduce runoff and thus improve water availability to young cotton plants. To test this hypothesis, we set up a trial in Sikasso, Southern Mali, with a two-year rotation of cotton and sorghum (Sorghum bicolor L.). The conventional cropping system (CS) was compared with two no-tillage, mulch-based cropping systems, where cotton was grown on straw residues of a sorghum crop (DMC1) or sorghum/brachiaria (Brachiaria ruziziensis) intercropping (DMC2). Soils in this region are classified as ferruginous types with hydromorphic characteristics at depths below 0.7 m (Plinthic Haplustalfs). Measurements included runoff, soil water content, biomass, cotton yield, and soil surface status (i.e. covered, open with visible macropores, or closed with no visible macropores). On DMC systems, the amounts of residues from the preceding crop kept on the soil surface at the beginning of the cotton cycle ranged from 4.4 to 9.3 × 103 kg ha−1 of dry matter, depending on the year and preceding crop. The soil surface status was more open under DMC1 and DMC2 (38 and 62%, respectively) than under CS (30%). Crop residues reduced runoff at the beginning of the rainy season in both years (−26 and −56 mm between CS and DMC2 in 2006 and 2007, respectively) and increased soil water availability for cotton (+26 mm and +50 mm, respectively). This gain in soil water did not result in higher cotton yields, probably because 2006 and 2007 rainfall amounts were unusually high, with a low gap between water supply and cotton requirements. Row ridges made perpendicular to the slope and after sowing in CS, created an effective obstacle to runoff and increased soil water content. The effects of crop residues on rainwater infiltration accumulated over the entire growing season were minimal. In conclusion, during wetter periods, the potential benefit of DMC systems on water balance and cotton yield in these types of soil may be low when compared with conventional systems if ridging done perpendicular to the slope is added.

In the context of conservation agriculture on small scale farms of the Brazilian Cerrado, we hypothesized that planting a cover crop in relay with a commercial crop improves the efficiency of use of available natural resources, increasing biomass for use as fodder without reducing the grain yield of the main crop. The objective of this study was to measure the performance of two intercropped systems in terms of total above-ground biomass production and maize (Zea mays) grain yield: pigeon pea (Cajanus cajan) and Brachiaria (Brachiaria ruziziensis) sown as cover crops in established maize under a no-tillage management. The cover crops were sown at two different dates and a comparison was made with the three crops sown as a sole crop at the early sowing date. The experiment was conducted during the 2007-2008 and 2008-2009 growing seasons. Maize grain yield was not reduced by the presence of the relay cover crops in comparison with maize as the sole crop, even when the cover crop was sown soon after maize emergence. In contrast, the production of above-ground biomass by the cover crop was significantly lower when grown with maize than it was when grown as a sole crop. In the intercropped systems, when sown early, the cover crop produced higher total biomass than when sown late. Total above-ground biomass production of maize intercropped with a cover crop was much higher than that of any of the crops sown alone: the total biomass (average of the two growing seasons) produced by maize and pigeon pea was more than double that of maize grown alone. The land equivalent ratio (LER) of maize grain yield and biomass production was higher than one whatever the intercropped system used. It was particularly high when maize was intercropped with early sown pigeon pea; grain yield LER and biomass LER reaching, respectively, 1.72 and 1.73 in 2007-2008 and 2.02 and 2.03 in 2008-2009. These high LER values provide evidence for the complementary and the high efficiency of use of available resources by the intercropped plants and thus the advantage of such systems to produce both maize grain and cover crop forage under the conditions of our study.

Correspondance: affholder@cirad.fr; International audience; Substantial initiatives are under way in the tropical world to develop and promote direct-seeding mulch-based cropping systems (DMC) in order to reduce soil erosion and improve crop nutrient and water balances. DMC have been adopted by large-scale mechanized farmers, especially in America and Australia, but seldom by resource-poor farmers in the developing world. This study was conducted in Vietnam with the aim of evaluating the feasibility of farmers’ implementing DMC in a mountainous area. The method involved simulation of rational households maximizing their income subject to food security constraints and availability of resources. It generated insight into why farmers of a small region were reluctant to adopt DMC due to the extra labor and input required to implement these techniques during the first years, which hampers their economic performance. In another region, under different biophysical and economic environmental conditions, the study showed that DMC were more likely to be adopted provided that possible constraints at the community level are overcome. The method also allowed us to discuss the types of technical improvements that would make DMC more attractive to farmers. For most farm types, labor required by mulch establishment would have to be reduced by more than 30%. This would mean spreading much less biomass than the 7 t ha−1 currently necessary, compromising the weed-control function of mulch. This would be technically feasible only by using herbicides but this would not be economically sound since it would increase cash requirements. The study showed that subsidies of 50 to more than 200 USD ha−1 were necessary to enable the conversion of all conventionally managed sloping land into DMC in the simulations. These amounts are high relatively to gross margins (250–750 USD ha−1) under conventional management

National audience; Significant efforts are being pursued in the tropical world to develop and promote cropping systems based on mulch or cover crops (SCV, système de culture sous couverture végétale, cover crop systems), in order to reduce soil erosion and improve both nutrients and water balances. However, adoption of SCV by smallholders is often limited due to the important change induced by these techniques in the management of farm’s resources. The objective of this study conducted in Vietnam was to assess the feasibility of SCV that had been previously elaborated at field scale for several farm types. The method was based on the simulation of rational households optimizing the use of their resources for food-security and income. We showed that the proposed SCV were not economically attractive because of the extra labour and inputs that these techniques require during the first years of implementation. We also showed that improving the attractiveness of those SCV would require both technical adjustments, such as the use of herbicides combined with the reduction of the amount of biomass in mulches, and subsidies. More research is needed in order to better quantify the environmental services that could legitimate such subsidies. It is also necessary to better understand the causes of spatial and temporal variations of the agronomic and economic performances of the SCV and of the currently practised cropping systems. Lastly, SCV should be compared to other soil conserving techniques, especially cultivation on dry terraces that is tends tending to spread out in the region.; Cahier de l'agriculture 17, n° 3, 289-296 D’importants efforts sont menés dans le monde tropical pour mettre au point et promouvoir des systèmes de culture sous couverture végétale (SCV) en vue de réduire l’érosion des sols et d’améliorer les bilans minéraux et hydriques. En agriculture familiale, l’adoption des SCV reste cependant limitée par les profonds changements que ces systèmes induisent dans la gestion des ressources de l’exploitation. L’objectif de cette étude menée au Vietnam est d’évaluer la faisabilité, pour différents types d’exploitations, de SCV préalablement mis au point à l’échelle de la parcelle. La méthode repose sur la simulation de ménages rationnels optimisant l’usage des ressources en vue d’objectifs de sécurité alimentaire et de revenu. Elle révèle la faible attractivité économique des SCV proposés en raison du surcroît de travail et d’intrants que ces nouveaux systèmes requièrent la première année de mise en oeuvre. L’étude montre aussi que pour rendre les SCV plus attractifs, il serait nécessaire de combiner des adaptations de la technique, telles que la diminution des quantités de biomasse dans les paillis associée à un recours aux herbicides, avec des subventions. Des recherches complémentaires sont nécessaires pour mieux quantifier les services environnementaux qui justifieraient de telles subven- tions. Il est également nécessaire de comprendre les causes de variations spatiales et temporelles des performances agronomiques et économiques des SCV et des systèmes actuellement pratiqués. Enfin, il faudrait comparer les SCV à d’autres techniques de conservation des sols, dont en particulier la culture en terrasse qui se répand actuellement dans la région.

O objetivo deste trabalho foi avaliar o efeito das palhadas de milho, milheto e soja na capacidade de interceptação e de armazenamento de água, velocidade de dessecamento, porcentagem de cobertura do solo, interceptação da radiação solar e escoamento superficial, assim como incorporar estes efeitos em um modelo de crescimento de plantas. As palhadas de milheto e de milho apresentaram capacidade maior de armazenar água do que a de soja: 3,26, 3,24 e 2,62 g de água por grama de palhada, respectivamente. A quantidade de água perdida pelos resíduos foi proporcional à evapotranspiração potencial. As taxas de cobertura foram equivalentes nos três tipos de material. Os três tipos de palhada foram similares na interceptação das radiações fotossinteticamente ativa e infravermelha. Os resíduos do milheto foram eficientes no controle do escoamento superficial: do total de 843,5 mm de água precipitada, apenas 45,5 mm foram perdidos pelo escoamento superficial no sistema de plantio direto enquanto no convencional as perdas de água foram de 222,5 mm. Os modelos linear e exponencial expressam de forma significativa a maior parte das relações estudadas. The objective of this work was to evaluate the effects of crop residue mulches from maize, millet and soybean on water storage capacity, water evaporation, soil cover, solar radiation interception and surface water run-off as well as to incorporate these effects in a crop growth model. The mulch of millet and maize presented higher capacity for water storage than soybean mulch: 3.26, 3.24 and 2.62 g of water per gram of dry matter, respectively. Water losses from wet mulches were related to the potential evapotranspiration. The soil cover levels were similar among the three types of material. The three types of mulch intercepted similar quantities of photosynthetically active radiation and infrared radiation. The mulch of maize straw was slightly more efficient in intercepting radiation than that from millet or soybean. Mulching with millet residues was efficient in the control of surface water run-off: only 45.5 mm of water (out of 843.5 mm rainfall) was lost through runoff under the no-till system with millet as cover crop, whereas 222.5 mm of water was lost in the conventional system with tillage. Most of the relations derived in this work could be described by exponential models.

In Senegal, rainfed millet production is influenced by the combination of scarce and unpredictable rains and falling soil fertility. In order to analyse the interaction between the water and fertility factors under conditions of low rainfall, the water use by a millet crop ( Pennisetum glaucum L.) was studied under different organic matter input situations. Water balance was evaluated using a model and in situ soil moisture measurements. Application of manure raised the crop water requirements without substantially increasing the water supply of the soil. At the end of the crop cycle, the soil dried faster where manure was applied than without manure. The consequences of this on growth and production were studied: changes in the pattern of evolution of the soil water content induced by manuring limited the depth of root penetration, and promoted post-flowering water constraint that can cancel the positive effect of manure on growth and production. As a result, the use of manure increases the risks taken by the farmer and should therefore be taken into consideration for the analysis of existing cropping systems or the definition of possible new ones.