Shade avoidance conditions soybean productivity in relay intercropping with winter canola

Relay intercropping (RI) involves cultivating two crops in overlapping growth cycles to enhance land use efficiency and optimize resource utilization. While RI systems can increase overall productivity, their success depends on effectively managing interspecific interactions that influence crop establishment, development, and yield potential. This study aimed to evaluate how nitrogen (N) fertilization and winter canola plant population density influence crop establishment, development and yield in a winter canola–soybean RI system compared to sole cropping (SC). Field experiments were conducted across multiple environments in Ontario, Canada during the 2022 and 2023 growing seasons. Treatments included two N levels and varying winter canola densities. Crop growth, yield components, and physiological traits such as internode elongation and harvest index (HI) were measured. When spring N was applied, total seed yield in the RI system was comparable to SC soybean, with a 40% increase in winter canola yield offsetting an 18% decline in soybean yield. However, soybean yield alone declined by 58–61% in RI compared to SC. Despite these reductions, LER values consistently exceeded 1.0 (0.93–1.67), confirming that RI improved land-use efficiency across environments and N levels. In contrast, TOI values were generally below 1.0 (0.68–1.30), indicating that combined intercrop yield rarely surpassed the most productive sole crop, except in environments where SC soybean yields were very low. RI soybeans exhibited significantly greater epicotyl elongation, particularly under N fertilization, indicating a strong shade avoidance response. This was accompanied by reduced biomass, lower yield, and increased variability in reproductive partitioning. While mean HI was similar between systems (0.58), RI soybeans showed a 45% wider range and 76% higher coefficient of variation in HI. The results demonstrate that increased winter canola productivity in RI systems can intensify interspecific interference, primarily through canopy shading and N-mediated effects, limiting soybean yield potential. Despite reduced soybean yield, the overall productivity and land equivalence ratio (LER) of the RI system suggest that winter canola is a viable intercrop partner. These findings underscore the need for refined agronomic strategies, such as optimized row spacing and fertility recommendations, to mitigate interspecific interference and enhance system-level performance in RI systems. • Relay intercropping matched sole crop yield when spring nitrogen was applied. • Yield losses in soybean coincided with early-season shade avoidance responses. • Epicotyl elongation explained 25% of yield variation in intercropped soybean. • Intercropped soybeans showed greater variability in reproductive partitioning.