Studies around the world indicate a range of advantages of intercrops compared to monocultures, such as increased land use efficiency (Paul et al. 2023), higher yield stability (Martin-Guay et al. 2018), weed suppression (Nelson et al. 2021), and improved grain quality (Peoples et al. 2009). However, in Europe, food crops are not widely grown as intercrops by farmers, though mixtures are widely used in sown grassland and cover crops. A range of challenges of intercropping for food production have been identified, especially the complexity of the management and harvesting of mixtures (Timaeus et al. 2022). Another challenge is a lack of knowledge on the advantages that might accrue to farmers when adopting intercrops under European conditions. Farmers need guidance on which species combinations and management might be most advantageous under their growing conditions, but this insight is simply not available because the available information is too fragmented and has not been synthesized. Intercropping is complex. There is a vast amount of possible crop species combinations and for each there are multiple design options (cultivar combinations, sowing time and density, spatial pattern) and management practices (fertilization, irrigation, disease and weed management). For monocultural arable cropping systems there are well-studied substantial interactions among genotypes (G), environment (E) and management (M, such as sowing density or fertilization) on yields, yield components and quality. Understanding these GxExM interactions enables evidence-based optimization of cropping systems. In intercrops, we have to deal with more complex interactions (G1xG2xExM), hugely complicating the issue of finding the best combination of species, varieties, design and management.

A large number of experimental studies investigating the yield of intercropping systems has been conducted over the last decades, globally. A simple search in web of science core collection in for intercrop* OR "crop mixture" OR "species mixture" OR "relay crop" OR "strip crop" yielded 10,276 articles (search conducted 12.06.2023 in title/abstract/keywords). Several global meta-analyses of these data have been conducted, e.g Yu et al. (2015), Yu et al. (2016), Martin-Guay et al. (2018), Li et al. (2020) and Li et al. (2023). Furthermore, some meta-analyses of productivity of intercrops have been conducted for specific crop species combinations (e.g. maize/soybean; Xu et al. (2018); or maize/peanut; Feng et al. (2021)) or for specific regions such as China (Li et al. 2020, Mudare et al. 2022) or Africa (Mudare et al. 2022). However, apart from a synthesis of studies in France, there is no meta-analysis on the productivity of intercropping under European conditions. It is important to conduct a study specifically for Europe because growing conditions and growing objectives and constraints in Europe differ from those in China or Africa. While production systems in China are characterized by very high and sometimes excessive inputs to reach high yields, those in Africa are often characterized by lack of fertilizer input. Systems in Europe vary from low input systems on poor soils with limiting water, often with organic management, to higher input systems under more favourable growing conditions, either with organic or conventional management. Due to a lack of synthesis, there is currently no good overview of the comparative advantageousness of these European systems and the variation in performance according to species choice, intercrop design, growing conditions and management.

We are working on the first meta-analysis of the relative performance of intercropping systems under European growing conditions. We focus on grain producing intercropping systems such as cereal/legume combinations because these have received the most attention in research because of their potential to provide grain production at low environmental costs. We ask the following questions:

1. What is the average land equivalent ratio (LER) of European intercropping systems?
2. How does the LER vary over European regions, and between conventional and organic farming?
3. How is the relative performance of intercrop vs sole crop systems affected by the climatic conditions and the soils?
4. How is the LER affected by temporal complementarity between species, nutrient inputs, and the comparative plant density in intercrops?

A literature search was conducted on the Web of Science Core Collection (WoSCC) on 24^th of November 2023 without any restriction on time or document type. The final search was conducted and resulted in 2070 articles after removal of duplicates.

Data extraction and analysis is ongoing, and the intermediate results will be presented.

Madhuri Paul

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