Wheat performance with subclover living mulch in different agro-environmental conditions depends on crop management

Intercropping has been proposed as a useful strategy for reducing external inputs in cereal-based cropping systems, while maintaining adequate crop yield. Intercropping of wheat and subclover, implemented as living mulch, is recommended, but there is limited experimental proof for its suitability in different environments. The main objective of this study was to provide an overview and evaluation of wheat-subclover intercropping under different agro-environmental conditions. Coordinated field experiments were conducted over a two-year period in six sites located in four agro-environmental zones [Atlantic North (Neu-Eichenberg, Germany), Continental (Freising, Germany – Tänikon, Switzerland), Mediterranean North (Viterbo, Italy), Mediterranean South (Sidi Alla Tazi and Sidi El Aidi, Morocco)]. Wheat–subclover intercropping was compared with a pure wheat. Additionally, other treatments adopted in specific sites were: soil tillage (conventional and minimum tillage); fertilization input (high and low level); cropping system (conventional and organic). The measurements recorded were: soil coverage, wheat and subclover phenological stages, wheat grain yield and yield components, subclover and weed biomass. The data of each site were analyzed separately and were also used for a meta-analysis to obtain an overview of how pedo-climatic conditions affect the interactions of subclover living mulch with wheat and weeds. Subclover biomass was the highest at Viterbo (228 g m−2 of DM) proving its adaptability to the climatic conditions of Mediterranean North characterized by mild temperature and abundant rainfall. Wheat-subclover intercropping reduced weed infestation (from 22 to 75% in Mediterranean South and North, respectively). Intercropping also resulted in grain yield losses compared to pure wheat in Mediterranean North and Continental (on average −16 and −14%, respectively), probably because of the competition between the intercropped species. In the agro-environmental zones where subclover growth was limited by cold temperatures (Atlantic North) or dry conditions (Mediterranean South), hardly any grain yield reduction of intercropped wheat was observed. Subclover biomass and wheat grain yield were also negatively correlated and yield reductions were generally due to a reduced number of fertile spikes. The yield gap between intercropped and pure wheat was reduced when: (i) there was a proper spatial arrangement of subclover and wheat; (ii) the amount of added mineral nitrogen fertilizer was reduced, while compost application did not influence the cropping systems. The use of subclover living mulch in wheat appears to be most suitable for low input systems. Future research should focus on the development of appropriate crop management practices for intercropping in order to avoid wheat yield loss.