Research for Development – Scaling  Rhizobial Inoculation for Leguminous Forage Crops

The International Center for Agricultural Research in the Dry Areas (ICARDA), in collaboration with its national research and development partners in Tunisia, has engaged in concerted efforts over several years to enhance feed and forage crops and associated technologies. This initiative recognizes that the scarcity of sufficient and nutritious feed poses a significant challenge to Tunisian animal husbandry. To address this, co-design workshops were conducted with farmers and other stakeholders in the Agroecological living landscape of the Kef-Siliana transect. The participatory findings suggest that the suboptimal production of leguminous forage crops might result from inadequate rhizobial inoculation.

Sulla (Hedysarum coronarium L.) is a leguminous plant native to Tunisia, notable for its drought tolerance. It holds significant potential to alleviate poverty among rural families, whose livelihoods depend heavily on livestock. Sulla’s robust agronomic performance in marginal, drought-prone environments, along with its diverse uses, makes it a promising crop. As a biannual crop, Sulla produces considerable biomass, suitable for grazing, cut-and-carry systems, or hay production. Moreover, Sulla is also a high-protein forage crop with low water demands. In cereal-based systems prevalent in semi-arid regions, the use of Sulla has been demonstrated to enhance the productivity and sustainability of crop-livestock integrated farming systems. This efficacy is attributed to Sulla’s capacity for biological nitrogen fixation and soil organic matter maintenance, crucial for agricultural resilience. Furthermore, Sulla’s deep and complex root structure positions it as an effective solution for mitigating soil erosion. The distinct features of its secondary roots enhance biological nitrogen fixation, particularly on sloping lands, thereby reducing erosion rates and improving overall soil health at farm and landscape scales.

In this regard, experimental trials were co-designed in collaboration with the National Agronomic Research Institute (INRAT) and the office of livestock and pasture (OEP) to examine the impact of rhizobial inoculation on the production of the forage crop Sulla. As part of this initiative, INRAT scientists successfully developed a rhizobial biofertilizer specifically adapted to Sulla, utilizing carefully selected Rhizobium sullae strains known for their efficiency in atmospheric nitrogen fixation and plant growth promotion.

Experimental results showed that the inoculated Sulla plants developed abundant, large, pink nodules widely distributed across their roots, whereas the non-inoculated plants exhibited no nodulation. Rhizobium inoculation increased average biomass by 66.3% compared to non-inoculated plants, positively influencing early crop establishment. Non-inoculated controls had the lowest emergence density, highlighting the critical role of inoculation. Additionally, inoculated plants showed a 37.05% increase in dry matter yield compared to the non-inoculated controls, further validating the effectiveness of Rhizobium sullae inoculation in enhancing forage yields and supporting sustainable agricultural practices.

The nitrogen content of Sulla plants significantly increased due to inoculation with specific Rhizobium sullae strain. The analysis revealed a significant increase in nitrogen content, with inoculated plants exhibiting an over 80 % improvement compared to non-inoculated controls. This improvement is due to the symbiotic relationship between the legume and rhizobia, where the bacteria fix atmospheric nitrogen into forms usable by the plant, thereby boosting nitrogen assimilation. Such increases in nitrogen content lead to improved plant growth, higher productivity, and reduced reliance on chemical fertilizers. The phosphorus content of Sulla plants was also significantly influenced by inoculation with specific Rhizobium sullae strains. The analysis revealed a 21.75% increase in phosphorus content in inoculated plants compared to non-inoculated controls. These results underscore the unique contribution of Rhizobium inoculation in enhancing phosphorus availability and uptake in Sulla plants.

Leguminous forage crops, including Sulla, faba beans, and vetch, establish symbiotic relationships with specific Rhizobia bacteria in their root nodules, enabling biological nitrogen fixation. This innovation involves inoculating forage legume seeds with meticulously selected rhizobia strains, which are specifically adapted to the crop species and prevailing environmental conditions of the local ecosystem. The specific Rhizobia bacteria have been demonstrated to enhance nitrogen fixation, thereby leading to improved biomass yield, superior forage quality (higher protein content), and increased crop resilience. Post-harvest, residual fixed nitrogen enriches the soil, benefiting subsequent crops, such as cereals, by reducing the need for synthetic nitrogen fertilizers. This lowers production costs for farmers, improves soil fertility, and minimizes the ecological impact of agriculture. By integrating rhizobial inoculation into forage crop systems, this innovation promotes sustainable agricultural practices, improves soil health, fosters crop productivity, and ensures long-term agroecological balance.

Research for Development

The importance of rhizobial inoculation for leguminous forage seeds, along with the evidence generated, was disseminated by INRAT

within the framework of the agroecology initiative and the PROSOL-SWC@Scale project. Both projects were coordinated in Tunisia by ICARDA and contributed to significant investments by GIZ – ProSol project and our common partner in Tunisia OEP, in establishing an inoculum

multiplication unit. The unit has been inaugurated on February 26^th, 2025 at the forage seed center of OEP in El Grine, Kairouan. With a capacity of 40 l it can produce in 48 hours enough rhizobial bacteria to inoculate forage seeds needed to plant 60 ha. The inoculum is sold either as a liquid or solid substrate at 60 TD (25$) per liter/kg. The inoculum multiplication unit is the first of its kind in Tunisia and will significantly contribute to scaling up the legume seed inoculum technology.

This is a notable example of successful collaboration among research and development actors, donors, and others in promoting agroecological technologies. Agricultural research institutes have provided scientific evidence on positive impact of seed inoculation on forage legume production in Tunisia, whereas development partners like GIZ and OEP get engaged in multiplying and scaling the inoculum. This complementarity is key for the sustainable introduction and scaling of innovations.

Authors:Udo Rudiger (ICARDA) ; Imen Hemissi (INRAT); Aymen Frija (ICARDA); Zied Idoudi (ICARDA)