Updated January 23, 2026:
Across the multi-state network (AR, IA, KS, IL, and IN), the near-complete dataset and preliminary analyses indicate clear systems-level effects of wheat–soybean relay intercropping on early-season weed dynamics. Broadly, relative to full-season soybean, relay intercropping consistently reduced early-season weed emergence and establishment, consistent with mechanistic expectations that the standing wheat phase sustains high canopy cover and soil surface shading, thereby restricting the effective weed recruitment window. This response was observed across multiple site-years and was most pronounced for major management-challenging taxa, including pigweed species (e.g., Palmer amaranth and waterhemp) and key annual grasses. These findings demonstrate that relay intercropping can function as a meaningful cultural control tactic within integrated weed management programs.
Early-season weed suppression in relay system was often comparable under no additional herbicide input and under a one-pass herbicide program, reinforcing that the cropping-system configuration itself (wheat with interseeded soybean) was the dominant driver of early weed suppression. In environments where early-season weed pressure was inherently low and a one-pass program was employed, full-season soybean achieved weed control similar to that of relay intercropping. This pattern indicates that the relative advantage of relay systems is most consequential under conditions conducive to early weed recruitment and establishment. Collectively, these results position relay intercropping as a management option capable of reducing early-season weed density and buffering weed management risk in systems where early recruitment pressure is a recurring constraint.
The residual wheat application of pyroxasulfone (Zidua®) contributed little additional weed control and did not produce consistent improvements in system yield or net economic outcomes across sites. Where statistically detectable differences occurred, effect sizes were small relative to those associated with the relay system itself and with the structure and timing of the subsequent soybean herbicide program. These results indicate that, under the environmental and management conditions represented in this project, biological suppression associated with the wheat phase was the primary driver of early-season weed outcomes, thereby diminishing the marginal value of adding a soil-residual herbicide at wheat application.
In contrast to weed suppression, yield responses were more context dependent and reflected tradeoffs associated with interspecific crop competition during the wheat–soybean overlap period. Soybean yield in relay systems ranged from near equivalence with full-season soybean in some environments to sites in which greater wheat competitive dominance resulted in soybean yield reductions. Full-season soybean yields were most vulnerable under reduced-input scenarios when early weed pressure was insufficiently constrained, emphasizing the interaction between weed management intensity and yield stability.
Survey responses collected through October 7 (n = 129) indicate that the most frequently cited perceived limitations to adoption are operational, particularly equipment access and management complexity, followed by concerns related to economic risk and cost. Despite these constraints, respondent interest in intercropped soybean systems was substantial, suggesting openness to adoption where agronomic risk can be effectively managed.
Taken together, this integrated, multi-site dataset provides a strong justification for continued research and system refinement. Beyond short-term weed suppression, wheat–soybean relay intercropping has the potential to reallocate a portion of weed regulation away from sole reliance on herbicides toward crop competition and canopy-mediated ecological interference. Such diversification of selection pressures is expected to reduce the frequency and intensity of herbicide exposure events that contribute to rapid evolution of herbicide-resistant weed populations, while improving the resilience of weed management across variable environments. Although these results provide a robust multi-environment foundation and clear mechanistic inference, they also indicate that favorable outcomes occur within a constrained suite of environmental and management conditions. Continued multi-year evaluation across environments and management scenarios is therefore warranted to define best-fit conditions, characterize the stability of economic returns, and support translation of these findings into adoption-ready recommendations.
Key survey results were presented at the 2025 American Society of Agronomy–Crop Science Society of America–Soil Science Society of America (ASA–CSSA–SSSA) Annual Meeting (Salt Lake City, UT). A broader synthesis of opportunities and challenges identified from early relay intercropping research, initiated in Arkansas during 2022–23 and expanded during 2024–25 to Kansas, Illinois, Indiana, and Iowa under current funding, will be presented at the 2025 Weed Science Society of America Annual Meeting (Raleigh, NC).
This project examined whether growing wheat and soybean together through relay intercropping can improve weed control while maintaining productive and profitable farming systems, and whether adding a residual herbicide to wheat (Zidua®) provides extra benefit in five soybean growing central US states ((AR, IA, KS, IL, and IN). Broadly, wheat–soybean relay intercropping consistently reduced early-season weed pressure compared with full-season soybean. In simple terms, the standing wheat provides early ground and canopy cover, which reduces the time and space available for weeds to emerge and become established. This early advantage was most noticeable for hard-to-control weeds such as pigweed species (including Palmer amaranth and waterhemp) and important grass weeds.
Specifically, when herbicide strategies were compared, the relay system often provided similar early weed control under reduced-input approaches, ranging from no additional herbicide to a single postemergence application. When overall weed pressure was low, full-season soybean with a one-pass program performed similarly to relay intercropping, indicating that the relay advantage is most important in fields where early-season weed pressure is typically higher. In contrast, applying Zidua to wheat offered little added weed control in most cases and did not consistently improve overall system yield or profitability. These results indicate that the cropping system itself, not the added residual herbicide, was primarily responsible for the early weed-suppression benefit.
Yield responses varied more than weed control and depended on how well the system was balanced during the period when wheat and soybean were growing at the same time. In some environments, soybean yields in relay systems were similar to full-season soybean, while in others, stronger wheat competition during the overlap period contributed to reduced soybean yield. These results highlight both the opportunity and the management challenge of relay intercropping: while early weed suppression was consistent, achieving reliable soybean yields requires careful attention to system setup and timing.
To better understand adoption potential, the project also included a multi-state survey of soybean producers and agronomists, with 129 responses compiled through October 7. The most commonly reported barriers to adoption were equipment requirements and management complexity, followed by concerns about risk and cost. Despite these challenges, overall interest in relay-intercropped soybean was strong.
Overall, the project shows that wheat–soybean relay intercropping can support more resilient and sustainable weed management by increasing suppression from crop competition rather than relying solely on herbicides. This approach could help reduce repeated herbicide use over time, which is increasingly important as herbicide resistance and control costs continue to rise. These results provide strong encouragement for continued refinement and evaluation across environments to identify where the system fits best, improve yield and economic consistency, and develop practical, adoption-ready recommendations.
Key survey results were presented at a national agronomy meeting in 2025, and a broader summary of the multi-state project will be presented at a national weed science meeting in 2025.