How to intensify crop production in a context of drought and in a sustainable way? It is a multidimensional scientific and societal challenge to ensure future food security.
Europe is experiencing an exceptional heat wave and a devastating lack of water for its agriculture, just like Southern Europe). In the Horn of Africa, four deficit seasons in a row are already threatening more than 16 million starvation people in Somalia, Ethiopia and Kenya. These drought episodes are not ready to stop with climate change, which increases their intensity and frequency.
Agriculture, which is mainly rain-fed, is very vulnerable to these fluctuations in precipitation. And where farmers benefit from irrigation technologies, reserves are quickly depleted, accentuating the water crisis. Thus, California, paradoxically one of the main rice-growing regions of the United States in extremely arid conditions, has been in a drought situation for the past 15 years; in Morocco, the water crisis risks making agriculture simply impossible in the near future as the case of Agadir shows.
The lack of water has become one of the major constraints of food production in the XNUMXste century, even though we need to increase agricultural production up to 70% in the next 20-30 years, especially on the African continent. The agricultural sector, the primary user of water resources since it consumes 70 to 80% of them, must therefore radically increase its water use efficiency in the face of the depletion of water resources and the demand from other sectors, also increasingly pressing – domestic water/urbanization, industrialization, tourism and ecosystem balances.
Producing more with less water has in this context become an interdisciplinary challenge. How to intensify crop production in a context of drought and in a sustainable way? It is a multidimensional scientific and societal challenge to ensure future food security.
The varietal selection is an essential lever for farmers' resilience to drought. Research is continually making progress to understand how and how plants can adapt to lack of water. genetic diversity within cultivated species can be mobilized to meet production challenges under water stress conditions. In food crops, a plant's ability to produce more grain with less water is a combination of three phenomena :
On the one hand, the ability to capture water from the soil at the root level: Within cultivated species, there is considerable genetic diversity in the architecture and functioning of the root system, representing an essential source for the selection of suitable varieties. to different drought situations.
Then, how the plant converts this water into biomass by the physiological mechanisms of photosynthesis without losing too much water via the evapotranspiration mechanism. The leaves have indeed tiny orifices on their surface: the stomata. These stomata open and close to capture CO2 used by the plant to produce, through photosynthesis and under the effect of light energy, the carbohydrates necessary for its growth. But when they open, the stomata let water vapor escape from the plant to the atmosphere. By regulating the mechanisms of stomatal opening in response to light, certain varietal types can save up to 25% water per unit of carbon assimilated.
Finally, by maximizing the transformation of this biomass into grain (efficient use of water or water use efficiency). An innovative lysimetric device revealed that the transpiration efficiency of plants such as sorghum varied greatly between varieties, without necessarily affecting yields, offering interesting opportunities for varietal selection.
Diversification of cropping systems: intercrops and varietal mixtures
Different forms of diversification of cropping systems also offer agroecological and sustainable solutions for adaptation to drought, such as intercropping or varietal mixtures. millet (pennisetum glaucum) associated with cowpea under water stress thus achieves a yield equivalent to that in monoculture but with an additional harvest of protein-rich legumes. Combinations of cereals and legumes indeed optimize the use of soil resources, particularly water, due to the beneficial effect of the legume on the fixation of resources, and a limitation of the evaporation of water from the soil. through denser plant cover. These associations also contribute more to food security by benefiting long-term soil health and fertility and by producing more vegetable protein per unit of cultivated area.
Another lever in response to increasing aridity consists in maximizing the stock of "green" water in the soil, thanks to preservation, or water management in the plot to force the plant to explore soil water more efficiently, especially at depth, or different landscaping techniques (terraces, contour stops, etc.), among others.
Thus, in the face of drought, the farmer's vulnerability will depend on his choice of crops, varieties, cultural practices including the sowing date – while predicting the next rains has become a real headache – these choices will define his vulnerability not only at the scale of a growing season but also over the longer term through their effects on soil health and fertility and their ability to retain water longer.
All of these choices depend on the specific environment of each farm, where agronomic and climatic parameters fluctuate and change rapidly. The resulting uncertainty and variability of agroclimatic situations means that traditional agronomic trials are no longer sufficient to explore adaptation solutions and respond to them urgently.
To address this complexity, some recent technological advances are promising:
- increasingly affordable sensors accurately measure soil moisture in real time and the water status of the plant ;
- high-throughput phenotyping (or observation of plant behavior) platforms allow the evaluation of hundreds or even thousands of varieties directly in the experimental field or greenhouse under different irrigation situations to measure indicators keys to adaptation to drought such as leaf surface or temperature, or root depth, contributing for example to selection of sorghum that remains green even under intense water stress ;
- remote sensing, which allows record foliage temperature as an indicator of plant water stress combined with the means of analysis of artificial intelligence, offers agronomic research a wealth of more precise data to shed light on this problem of the optimization of crop production to the lack of water from new angles.
But how do you sort through this complexity? How to predict solutions for the agricultures of tomorrow? Plant and crop growth simulation tools combined with statistical models allow for the creation of endless virtual trials to evaluate different scenarios combining varieties, cultural practices in current or future pedoclimatic situations.
This type of approach has recently made it possible to list, at much less cost than a purely experimental approach, the level of adaptation of 150 wheat varieties to 13 different drought environments according to dry, wet or intermediate years in the North- East Australian.
These approaches based on the analysis of massive data sets (“big data”) and on “in silico” experiments represent new opportunities for decision support, towards innovative solutions for adaptation and risk management. related to drought; this, whether at the scale of the plant (variety), the field (cultivation practice), the farm or the territory (for example water management).
Farmer or political decision-maker, you have to deal with a complex balance between risks, uncertainties and benefits. Create more suitable drought insurance products for small-scale agriculture in the South can help transform a region's agriculture, by better managing risk and planning from year to year.
New methods and old species
At the country level, in a context of pressing food crisis and therefore the resurgence of questions of food sovereignty, some African countries such as Senegal will have to invest to produce more grain per drop of water. This will eventually require the radical transformation of their type of agriculture.
In Nepal, in the Terai plain or in Punjab in India, farmers no longer have enough water to cultivate rice in continuous immersion, as has been done for centuries. New cultivation methods where the rice is alternately dry or submerged (AWD ou alternate wetting and drying in English) reduce water requirements by up to a third (15-30%) while reducing greenhouse gas emissions.
Some rice-growing regions could also switch to rainfed rice. Sub-Saharan Africa could produce durum wheat, as shown this experiment from Senegal and Mauritania, to fill a growing trade deficit (4 billion dollars per year of imported wheat for the local industry).
An alternative is to make better use of traditional species such as cowpea legume, Or funio, already well adapted to local arid conditions. Some of these national choices could have consequences on the vulnerability of their agriculture to future droughts.
The conference Inter drought which takes place in 2022 for the first time in Africa will be a key moment to imagine at the scale of the plant, the field, the farm and our societies, the resilient, sustainable and water-efficient agriculture that we urgently need need.
Delphine Luquet, Ecophysiologist, CIRAD; Ndjido Ardo Kane, Plant geneticist, molecular biologist and director of the Senegalese Institute of Agricultural Research ISRA-CERAAS, Senegalese Institute for Agricultural Research (ISRA), and Vincent Vadez, Principal researcher in ecophysiology, agronomy and modelling, Research Institute for Development (IRD)