Simon Giuliano

Évaluation multicritère des performances de systèmes de culture maïsicoles alternatifs à la monoculture de maïs conventionnelle et à faible impact sur l'environnement: Les systèmes de culture maïsicoles (SdCM) conventionnels irrigués se définissent, dans les principaux bassins de production en climat tempéré, par des rotations très courtes associées à un usage d’intrants important : fertilisation azotée minérale >150 kg ha-1, irrigation >150 mm ha-1, travail du sol intense et utilisation d’herbicides quasi-systématique. Cette conduite provoque des impacts environnementaux importants sur l’eau, le sol, l’air et la biodiversité, amplifiés par les effets du changement climatique. Ces impacts sont prégnants dans le Sud-Ouest de la France, en particulier concernant les problématiques de gestion quantitative et qualitative de l’eau (nitrate, herbicides). Afin de répondre à ces problématiques, une expérimentation a été menée depuis 2010 à Lamothe (Seysses, 31, Occitanie) afin de co-concevoir et évaluer différents SdCM alternatifs à la monoculture de maïs irriguée conventionnelle. Les SdCM mis en oeuvre se répartissent selon un gradient de réduction de travail du sol et de diversification des rotations, dans l’objectif d’atteindre différents niveaux de réduction d’utilisation d’intrants fixés lors de la phase de co-conception. Afin de mener une évaluation multicritère, des suivis scientifiques sont menés afin de mesurer les performances économiques et sociales des SdCM, de quantifier les niveaux d’intrants utilisés mais aussi de mesurer l’impact des systèmes de culture sur la qualité de l’eau de drainage, les rendements et l’évolution de la flore adventice. Les résultats montrent que les performances environnementales (émissions de gaz à effet de serre, irrigation, fertilisation azotée…) de la monoculture de maïs mobilisant les leviers de la production intégrée (cultures intermédiaires, choix variétal, désherbage mécanique, pilotage fin des intrants…) sont améliorées de 15 à 50% par rapport à la monoculture de maïs conventionnelle, alors même que leurs performances économiques et sociales sont similaires. En revanche, les SdCM réduisant le travail du sol ont obtenu des performances globalement plus faibles. Ces différences peuvent s’expliquer par une évolution de la communauté adventice et un plus faible niveau de maîtrise de celle-ci sur les SdCM avec peu de travail du sol, bien que d’autres facteurs – probablement édaphiques – viennent également impacter les rendements. Par ailleurs, le SdCM conventionnel et les SdCM avec peu de travail du sol ont utilisé plus d’herbicide et ont enregistré davantage de pertes d’herbicide par drainage que les SdCM en production intégrée. Pourtant, il apparaît que la relation entre quantité d’herbicide appliquée et pertes par lixiviation est limitée, le volume de drainage étant le premier facteur explicatif des pertes en herbicide par lixiviation dans le contexte de l’expérimentation menée. Du point de vue méthodologique, ces résultats encouragent à poursuivre l’évaluation multicritère de la durabilité des systèmes de culture en y incluant l’évaluation des impacts environnementaux ainsi que des services écosystémiques. La revue bibliographique menée pointe l’intérêt de mesurer la dynamique des performances des systèmes de culture au cours du temps afin d’en évaluer leur résilience, face aux aléas climatiques notamment. Enfin, l’amélioration des performances permise par la combinaison de leviers agronomiques doivent favoriser les démarches de territorialisation et de transition agroécologique des SdCM.

Conventional Maize Monoculture (MM), a dominant Cropping System in South-Western France, is now questioned for environmental reasons (nitrate leaching, pesticide use and excessive irrigation). Three low-input Cropping Systems (CS) using diverse weeding strategies (MM_LI, a Low-Input MM implementing ploughing, a combination of on-row spraying and in-between row cultivation and cover crops; MM_CT, Conservation Tillage MM implementing chemical control and cover crops; Maize-MSW, maize managed similar to MM_LI but rotated with soybean & wheat) were compared to a reference system (MM_Conv, a conventional MM with tillage and a high quantity of inputs). Potential of Infestation of weeds (PI), weed biomass and crop production of these CS were compared during the first five years after their establishment. Yields were also assessed in weed-free zones hand-weeded weekly in 2014 and 2015. Weed communities did not drastically differ among CS. PI and weed biomass were higher in MM_CT, especially for Echinochloa crus-galli (L.) P.Beauv. and were comparable between MM_Conv, MM_LI and Maize-MSW. Analysis of covariance between CS and weed biomass did not reveal a significant interaction, suggesting that weed biomass affected yield similarly among the CS. Comparison between weedy and weed-free zones suggested that weeds present at maize maturity negatively affected yields to the same extent for all four CS, despite having different weed biomasses. Grain yields in MM_Conv (11.3 ± 1.1 t ha⁻¹) and MM_LI (10.6 ± 2.3 t ha⁻¹) were similar and higher than in MM_CT (8.2 ± 1.9 t ha⁻¹. Similar yields, weed biomasses and PI suggest that MM_LI and Maize-MSW are interesting alternatives to conventional MM in terms of weed control and maize productivity and should be transferred to farmers to test their feasibility under wider, farm-scale conditions.

The need for medicinal and aromatic plants for industrial uses creates an opportunity for farmers to produce alternative crops. Stevia rebaudiana Bertoni, a perennial shrub originating from Paraguay, is of increasing interest as a source of zero-calorie natural sweeteners: the steviol glycosides (SVglys). The aim of this study was to investigate the relevance of nitrogen (N) supply for leaf yield and for SVgly concentrations in leaves, which are the two major components of S. rebaudiana productivity. In this regard, the relationship between leaf N concentration, CO₂ assimilation, leaf production and SVgly accumulation was investigated. The experiments were conducted consecutively in growth-chamber (CC: controlled conditions), in greenhouse (SCC: semi-controlled conditions) and in field conditions (FC) on two genotypes. In CC and SCC, three levels of N fertilization were applied. Plants were grown on four locations in the FC experiment. Both N supply (CC and SCC) and location (FC) had a significant effect on N content in leaves. When light was not limiting (SCC and FC) N content in leaves was positively correlated with CO₂ assimilation rate and biomass accumulation. Irrespective of the growth conditions, N content in leaves was negatively correlated with SVgly content. However, increased SVgly content was correlated with a decreased ratio of rebaudioside A over stevioside. The evidence that the increased SVgly accumulation compensates for the negative effect on biomass production suggests that adequate SVgly productivity per plant may be achieved with relatively low fertilization.

Because organic systems present complex environmental stress, plant breeders may either target very focused regions for different varieties, or create heterogeneous populations which can then evolve specific adaptation through on-farm cultivation and selection. This often leads to participatory plant breeding (PPB) strategies which take advantage of the specific knowledge of farmers. Participatory selection requires increased commitment and engagement on the part of the farmers and researchers. Projects may begin as researcher initiatives with farmer participation or farmer initiatives with researcher participation and over time evolve into true collaborations. These projects are difficult to plan in advance because by nature they change to respond to the priorities and interests of the collaborators. Projects need to provide relevant information and analysis in a time-frame that is meaningful for farmers, while remaining scientifically rigorous and innovative. This paper presents two specific studies: the first was a researcher-designed experiment that assessed the potential adaptation of landraces to organic systems through on-farm cultivation and farmer selection. The second is a farmer-led plant breeding project to select bread wheat for organic systems in France. Over the course of these two projects, many discussions among farmers, researchers and farmers associations led to the development of methods that fit the objectives of those involved. This type of project is no longer researcher-led or farmer-led but instead an equal collaboration. Results from the two research projects and the strategy developed for an ongoing collaborative plant breeding project are discussed.