To increase cereal yields in Africa, the production of green manures through the cultivation of legumes is promising. But some obstacles remain.
Ending hunger by 2030 is one of the UN's Sustainable Development Goals. A major challenge in sub-Saharan Africa, where a third of households are still in food insecurity situation. To meet the demand of Africa's growing population, the need a significant increase in cereal yield.) is widely accepted.
The latter are severely limited in the region due to the depletion of soil nutrient stocks after decades of continuous cultivation under insufficient fertilization, itself resulting from the low use of mineral fertilizers and the low biomasses available for fertilization. organic.
Currently, the amount of nitrogen applied should be multiplied by 15 to reach a level of cereal yield food safe compatible.
Valuable nutrients in green manures
Mineral fertilizers are one of the means to increase the availability of nitrogen for crops. While they are crucial for raising grain yields, they can harm the environment if overused and their price is often too high for farmers, as the disruption of world markets linked to the war in Ukraine has aggravated the constraint.
Nitrogen is abundant in the atmosphere and legumes have the unique ability to fix this atmospheric nitrogen in their plant tissues through symbiosis with bacteria. Cultivating them before cereals by burying their residues before sowing could provide the soil with substantial quantities of nitrogen and optimize the use of mineral fertilizers necessary to obtain a better yield of cereals.
A well-known effect in temperate and tropical systems, but which can vary greatly from year to year. With a rise in temperature, the legume residues will decompose faster, while in the event of intense rains, the nitrogen obtained by the decomposition of the residues could be leached and lost to the crop.
Fertilizer, climate and rice growth in Madagascar
For more than 20 years, Fofifa and Cirad (two research institutes from Madagascar and France respectively) have been exploring various avenues aimed at sustainably intensifying the production of Malagasy agriculture and increasing household income.
In this recent study, we questioned the advisability of replacing part of the mineral fertilizers used to increase yields, by the use of green manures for growing upland rice in Madagascar. The general policy of the State in Madagascar plans to increase rice production to ensure the country's food self-sufficiency.
However, the saturation of low-lying floodplains where irrigated rice is grown, and the recent development and dissemination of altitude-tolerant varieties have led to a rapid expansion of upland rice cultivation in the Highlands region: in 2011, 71% of farmers grew rainfed rice.
But they are currently getting a average yield of 1,6 t/ha, well below the maximum yield of around 4 to 6 t/ha that can be obtained in research stations under experimental conditions, i.e. when nutrients are not limiting and bioaggressors – pests, diseases, weeds – are mastered.
Mucuna and Crotalaria
In our experience, the green manure grown was a combination of the legumes 'Mucuna' and 'Crotalaria'. Thanks to complementary growth and architecture of the aerial parts, these two plants can produce a large quantity of plant biomass and thus fix atmospheric nitrogen. This green manure was grown during the rainy season of the first year of a two-year rotation based on upland rice, the residues of which were then incorporated into the soil, before tillage and sowing of the rice in the next growing season.
The rotation was repeated twice for the purposes of the experiment and compared to a rice monoculture. We carried out an extensive follow-up on this experiment, measuring the amount of nitrogen in the soil and in the plants, the growth dynamics of the rice, the biomass produced and the grain yield. The data collected made it possible to adapt a crop simulation model to the case of upland rice in Madagascar, in order to reproduce the impact of the incorporation of green manure and its decomposition on rice growth.
A promising model
The model was then used to explore the possible benefits of green manure on rice growth, for 24 hypothetical rice growing seasons, constructed based on climate measurements corresponding to our study region, for a period from 1994 to 2018. In this virtual experiment, we set the amount of green manure incorporated into the soil before rice cultivation at 6t/ha (i.e. 140 kg of nitrogen) based on what was obtained in the field.
According to our findings, the cultivation and integration of green manure saved part of the mineral fertilizer required to improve rice yield. We set the target yield at 3,7 t/ha, i.e. 75% of the maximum yield obtained in on-station experiments. This reference yield was reached with 40 kg/ha of nitrogen provided by the mineral fertilizer when the green manure was added to the soil. By comparison, more than double that amount, or 100 kg of nitrogen per hectare, was needed without the use of green manure.
Compromises to be made
However, this promising practice requires significant adjustments to allow its implementation within Malagasy family farms. Three major aspects should be considered.
First, in years with high rainfall, the benefits of green manure on rice productivity were not seen: the additional nitrogen provided by the decomposition of the green manure was leached out and was no longer available for rice. This therefore requires a dynamic management of residues and fertilizers, and a need for technical advice to adapt the offer to the needs of the plant.
Second, the increase in rice productivity enabled by green manure did not fully compensate for the loss in rice yield due to green manure cultivation. If farmers want to grow green manure, they must dedicate part of their land to this crop – this part would otherwise be used to grow rice for household food self-sufficiency. Incentive mechanisms could be implemented to compensate for this loss because the cultivation of these fertilizers in rotation can render various non-negligible services (better management of pests, nematodes and weeds, improvement of soil fertility over time, maintenance of biodiversity.
Finally, crop residues are generally left on the plot and grazed by livestock in the region. This therefore requires having alternative solutions for feeding livestock, developing the appropriate sectors to guarantee a supply for these farmers who are often far from the infrastructure, but also ensuring that mechanisms exist, here too, to compensate for this additional expenditure for the exploitation of which the majority remains with less than €540 per year per agricultural worker.
Dual Purpose Pulses
The rainfall constraint can be overcome by adjusting the date of green manure incorporation and the date of rice sowing to better synchronize green manure decomposition and crop growth. The model we calibrated can help make such an assessment in silico.
The loss of food production due to the production of the green manure could be partially compensated by growing a grain legume that provides food to the farmer (eg groundnut) with the green manure. Experiments in Southern Africa are currently devoted to the evaluation of these 'dual' legume rotations.
But it would be necessary to make sure of an existing market for these productions and of a sufficiently remunerative price. Finally, the cultivation of dual-use legumes (producing both grain and fodder) could be one of the necessary elements in the compromise to be found to improve soil fertility and provide fodder.
Time is running out, but we are convinced that change can take place quickly to transform the current cereal farming system towards greater productivity and sustainability. The solutions, like green manures, exist, but extensionists, researchers, value chain actors and policy makers need to help farmers invest in those techniques that will maximize production in the short term, but be more sustainable in the long run. term.
The last decades have seen the emergence of many talented African researchers. It is now up to the leaders to make the decisions.
Lalaina Ranaivoson, Researcher in agronomy, CIRAD; Aude Ripoche, Researcher in cropping system agronomy and modeller, CIRAD; Francois Affholder, Principal Investigator in Agro-ecology and sustainable intensification research unit, CIRAD; Gatien Falconnier, Agronomist Ecologist, CIRADand Louise Leroux, PhD, Remote sensing scientist, CIRAD