Is sunflower-soybean intercropping an efficient solution for increasing natural resources use efficiency and yield production?

 Intercropping (IC) is the simultaneous growing of two or more species in the same field for a significant period. Intercropping could be considered as a way of ecological intensification allowing the increase of natural resources use efficiency by positive interspecific interactions. Previous studies on grain legume-cereal intercrops have shown advantages in low input systems such as the increase of the overall yield and the grain protein concentration improvement of the cereal. Few papers dealing with sunflower-soybean intercropping are available in the literature; they mainly come from African and Asian conditions. The land equivalent ratio (LER) for yield or gross margin reported for sunflower-soybean intercrops was generally high (1.2 to 1.6), indicating the agronomical and economic advantages of intercrops in comparison to sunflower and soybean sole crops. One of the interests of sunflower-soybean intercrops is based on the ability of the soybean to get the atmospheric nitrogen thanks to its symbiotic fixation, leaving more soil mineral nitrogen available per sunflower plant. Moreover, the two rooting systems (deep for sunflower and shallower for soybean) can explore different soil layers which could lead to niche resource complementarity for nutrients and water. The aim of this study was to analyze the dynamical functioning of sunflower-soybean intercrops and their performances in order to determine their efficiency and the conditions to maximize resources use.  Three field experiments were carried out in South-West of France at CETIOM and INRA experimental stations in 2010 and 2011. CETIOM experiment was carried out in non-limiting conditions for water and nitrogen. The two other experiments carried out at INRA were cropped in low inputs conditions without irrigation and with only one herbicide spreading at sowing. Two spatial row design corresponding to two structures (2/2 and 2/4 rows, i.e. 2 adjacent rows of each specie and 2 sunflower row alternated with 4 soybean rows) and two types of cultivar (early and late) of each specie were studied. Each crop was also grown in sole crop in order to evaluate the resource use efficiency based on the land equivalent ratio. In intercrop and sole crop, each crop was sown at the same row density. The biomass production, light absorption, N uptake and N2 fixation, and crop yield formation components of both species were measured during the crop cycle but only results obtained at harvest are presented here.  Our results clearly indicated that the best performances of sunflower-soybean intercrops were obtained in low inputs conditions. When crops were conducted in high inputs conditions, the sole crops were found more efficient than intercrops (LER lower or equal to 1) due to the strong interspecific competitions of sunflower on soybean in particular for light. Intercropped sunflower always presented a competitive advantage (partial LER higher than its relative density partial LER) on the intercropped soybean leading to low soybean yields. As hypothesized, the N2 fixation rate was higher in the intercropped soybean than the sole crop. The four soybean rows alternated with two sunflower rows was the most efficient spatial organization in order to re-equilibrate interspecific competitions and optimize natural resources use. Intercropping the late soybean cultivar with the earliest sunflower allowed obtaining the best results. In that case, the overall yield was higher in intercrops than for the two sole crops average; the highest LER was 1.24, indicating a true potential of sunflower-soybean intercrop for yield production in rainfed conditions.  Our work confirms that intercrops are more efficient in low inputs conditions. This clearly emphasized that there are various possibilities for improving the use of abiotic resources, such as exploring precocity, aerial architecture of genotypes (sunflower and soybean), but also adapting intercrop row structure and management. Further research based on genotype-environment interactions analysis is needed for optimizing interspecific complementarities between both species in intercrop.  Intraspecific and interspecific interactions were changing within the growth cycle and depended on intercrop structure. This highlights the needs for further researches based on functional intercrop analysis under various conditions and with different sunflower and soybean genotypes.