Small-scale farming diversity and bioeconomic environment variability : a modelling approach. Sub theme : 1.2. Integration of micro-strategies with macro-economic factors

Modelling is seen by many scientists as a powerful tool to assess the interaction between variability in the bioeconomic environment of small-scale farmers and their resource management strategies. Two case studies on small-scale farming in Mexican and Brazilian savannas are presented. On one hand, the effect of climate variability on crop yields is highly dependent of the managing techniques used. On the other hand, the variability of prices, resulting in Mexico's case from the establishment of NAFTA and, in the Brazilian case, from the "-plano Real," dramatically increases the uncertainty of the economic results linked to technical choices. Modelling the real conditions of such farmer decisions requires integrating of both biophysical and economical knowledge. This work analyses the characteristics required for such models and ways to integrate them. It appears that the building and scaling up of the models should be driven by results of a study on farming system diversity

L'eau et l'activité agricole : diagnostic et modélisation du fonctionnement de quelques hydrosystèmes agricoles tropicaux

L'objet d'étude est l'hydrosystème agricole, ensemble des composantes économiques et sociales, techniques, biologiques et physiques de l'écosystème agricole qui déterminnent les flux hydriques. La méthode d'étude appliquée par les auteurs aux composantes biophysiques et techniques de ce système est une démarche empirique dans laquelle on fait converger progressivement avec la réalité l'image virtuelle fournie par un modèle. Les résultats obtenus au Brésil, au Mexique et au Sénégal indiquent les facteurs de productivité et de durabilité de ces hydrosystèmes agricoles et l'impact de nouvelles contraintes ou techniques. Les avantages de cette approche et les lacunes du niveau socioéconomique sont discutés dans la perspective d'élaborer des outils d'aide à la décision des agriculteurs et de négociation entre acteurs du développemen

Yield gap and the shares of climate and crop management in yield and yield variability of staple crops in West Africa. [O-3330b-01]

" Yield gap " (Yg) is a key concept of agricultural science for identifying the room for improvement of yields through better management of the agroecosystem. in rainfed agriculture Yg is the difference between actual yield (Ya) and the theoretical water limited yield (Yw) that would be achieved if solar radiation, temperature and precipitations were the only factor limiting the crop's growth and yield. Changes in Yw over regions and years are due to climate-soil interactions that are not easily modified by crop management, whereas changes in Yg are due to limiting factors that are typically within the scope of crop management such as nutrient availability, weeds, and pests. We provide an example of yield gap estimates in semi-arid a frica, using yield and other agronomic data collected in famers' fields of Senegal in 1990 and 1991 and from 2006 to 2012. i t illustrates how contrarily to what most people would expect climate is not, on average, what most limits yields in that region: yet, actual yields are on average a quarter of water limited yield, and this is due to constraints whose reduction is technically possible albeit subject to the economic and environmental relevance of doing so. Most studies dealing with the impact of climate change on agriculture in West a frica compare Yw under present and future climate as predicted by climate models. t he magnitude of those predicted long term changes in Yw by 2050 is down to –20% in the worst scenario combining a +6°C change with a -20% rainfall change. s uch changes in water limited yields are certainly concerning, but they are remarkably small compared to the potential +390% increase that would result from closing the current yield gap. When considering yield variations observed across plots and years, and not anymore regional averages over a few years, what strikes is the stability of observed yields compared to variations of Yw. We used crop model simulations with historical series of 20 years of weather data to compare yield distributions over years of a crop grown using 3 contrasted levels of fertilisation and no incidence of weeds, pests or diseases. For each fertilisation level, the simulated yield reached a maximum value the 'best year' of the series. t he three fertilisation levels were chosen so that the maximum simulated yield reached 0.25 Yw, 0.5 Yw, and 0.75 Yw respectively. t he resulting simulated yield distributions show that even if management allows increasing the median yield, in many years the climate is the main limiting factor and fertilising has no or a slight impact only. i n other words, the way the current climate limits crop production in this region is by making uncertain the output of investing for high yields. Buying fertilizers or working hard for manure collection, transport and distribution do not translate, a certain number of years, into more production. For farmers struggling for the daily subsistence of their family, that kind of risk may not be justified while alternative use of family resources in cash and labour force provide less risky ways to produce subsistence means. Until recently, in many farming systems of West africa, the growth in food needs due to population growth in rural areas was matched thanks to increases in cultivated or pasted areas rather than increases in crop yields or livestock pressure on land (i.e extension rather than intensification of crop or livestock activities). When rural families reached the limits of this strategy, migrations of many kinds of distance and duration became the adjustment variable to the gap between resources available from farming and population needs. T his suggests that for many, it is less risky to leave home than to intensify cropping or livestock systems. Anyway, as job opportunities for migrants from the rural zones are currently low in West african cities and elsewhere, there are legitimate concerns about the way this strategy may soon reach its limit as well. i n terms of climate change, the worst scenario for farmers of that region would be if crop intensification became even more risky under future climate than at present. t here is thus an urgent need for joint agronomic and climate research to go beyond the prediction of Yw or of yield under unchanged crop management and determine whether or not the future climate will increase the yield risks associated with crop intensification in that region. But this should not divert from designing and implementing policies incentive to such intensification under present climate, as this might be much easier now than later. (Texte intégral