Guénaëlle Corre-Hellou

Increasing crop trait diversity in oilseed rape (OR, Brassica napus L.) cropping systems by introducing frost-sensitive legume species could improve weed suppression and crop productivity. Intercrops and sole crops were compared over two years in the field in Western France. Winter OR was intercropped simultaneously with either spring faba bean (Vicia faba L.) or common vetch (Vicia sativa L.) in a row replacement design without herbicides. Each species was sown at 50% of the recommended sole crop density in alternate rows. Due to the high values of faba bean aboveground traits (height, leaf area, and biomass) and the strong competitive ability for soil N of OR, both species appeared complementary in resource utilization, and thus less soil N and light were available for weeds. The OR–faba bean intercrop was able to reduce weed biomass by 41% compared to the OR–common vetch intercrop. Furthermore, growth and competitive ability of OR for soil N were increased when intercropped with faba bean. Both grain yield and number per plant were three times higher in OR–faba bean intercrops compared to OR sole crops. Under high weed infestation, the presence of faba bean with OR reduced weed aboveground biomass by 35% and weed N accumulation by 11% compared to the OR sole crop. No change was observed in the weed community composition. We observed that a level of aboveground biomass greater than 2 t ha⁻¹ and a soil N uptake at approximately 80 kg ha⁻¹ was needed to reduce biomass and N content of weeds.

Calls for the application of ecological principles in agriculture have gained momentum. Intercropping systems are designed by growing two, or more, annual crop species in the same field, aiming for a better resource use efficiency. However, assembly rules for their design are lacking. Notably, it is unknown whether species performances are maximized during both the vegetative and reproductive phases given the sensitivity of reproductive allocation rules to resource limitation. Interestingly, ecological theory provides expectations regarding putative invariance of plant reproductive allometry (PRA) under non‐limiting conditions for plant growth. Here we examined whether and how PRA changes in response to plant–plant interactions in intercropping systems, which can inform both ecological theory and the understanding of the functioning of intercropping systems. We analysed a dataset of 28 field cereal–legume intercropping trials from various climatic and management conditions across Western Europe. PRA was quantified in both mixing and single‐species situations. Plant reproductive allometry was positively impacted in specific management conditions, leading to a greater increase in yield for a given increase in plant size. Variations in PRA were more beneficial to legumes grown in unfertilized mixtures, which explains their use as a key component in actual intercropping systems. The response for cereals was similar but less pronounced in magnitude, and was greater under resource‐limiting conditions. Focusing on intercropping conditions, hierarchical competition (indicated by biomass difference between intercropped species) appears as a strong driver of the reproductive output of a given species. Synthesis and applications. Plant reproductive allometry (PRA) behaves in crop species in the same way as it does in wild species. However, contrary to theoretical expectations about an overall invariance of PRA, we highlighted taxon‐specific and context‐dependent effects of plant–plant interactions on PRA. This systematic deviation to PRA expectations could be leveraged to cultivate each species up to its reproductive optimum while accounting for the performance of the other, whether farmer's objective is to favour one species or to reach an equilibrium in seed production. Sowing density and cultivar choice could regulate the biomass of each component, with specific targets derived from allometric relationships, aiming for an optimal reproductive allocation in mixtures.

Lupin (Lupinus sp.) produces protein-rich grains, but its adoption in cropping systems suffers from both its low competitive ability against weeds and its high yield variability. Compared with legume sole cropping, grain legume–cereal intercropping benefits include better weed suppression and higher yield and yield stability. However, the potential of enhancing crop competitive ability against weeds in additive winter grain legume–cereal intercrops is not well-known, and this potential in long crop cycles is even less studied. We studied how intercropping with a triticale (×Triticosecale) alters weed biomass and productivity of winter white lupin (Lupinus albus L.). The experimental setup consisted of eleven sites during a two-year period in western France. In each site-year, winter white lupin sole cropping was compared to winter white lupin-triticale intercropping in an additive sowing design. We found that intercropping reduced weed biomass at lupin flowering by an average of 63%. The rapid growth and high soil N acquisition of triticale compensated for the low competitive ability of lupin against weeds until lupin flowering. Competition from triticale in the intercrop reduced lupin grain yield (−34%), but intercropping produced a higher total grain yield (+37%) than did lupin sole cropping while maintaining the total protein grain yield.

Early strategies of crop growth and N acquisition can be critical for determining competitive interactions between weeds and crops. Grain legumes and especially lupins are known to be poor competitors against weeds. Grain legumes are known to have low mineral soil N uptake abilities. However, inter- and intraspecific differences in N uptake ability in relation to below-ground traits have received little attention. Our objectives were (i) to measure differences among lupins for a set of traits associated with early growth and N acquisition; (ii) to examine how this variation compares to differences between lupin and a cereal, triticale, and (iii) to assess if mixing lupin with triticale provides a higher potential than does pure lupin regarding plant biomass and mineral soil N acquisition early in the crop cycle. Lupin (12 genotypes) and triticale plants were grown separately and in mixed species pairs in a replacement design for 1 and 2 months in three rhizotron experiments. Shoot and root biomass, root length, root expansion dynamics, N₂ fixation and mineral soil N uptake were measured. Differences among lupin species and genotypes regarding traits related to early growth and to mineral soil N uptake were observed, but all lupins demonstrated slow early growth and low ability to absorb mineral soil N compared to triticale. In lupin–triticale mixture, a contrast in early growth strategies between species induced a higher total soil mineral N uptake compared with pure lupin. Complementarity between lupin and triticale persisted during the second month, when interactions began. This complementarity may allow for reduced competition between species, favouring higher triticale biomass production than in pure triticale, without compromising lupin growth.