Intercropping, particularly cereal-legume combinations, offers benefits such as reduced input needs and improved environmental quality. However, the low degree of mechanization in many developing countries presents challenges due to labor scarcity. Strip intercropping, with appropriately chosen strip widths, offers a potential solution by allowing mechanization. This research addresses the optimal strip width in a maize/soybean relay intercropping system (MSR) to maximize grain yields of both crops while accommodating machinery. Previous studies have shown conflicting results regarding the optimal strip width, with some suggesting wider strips lead to higher yields and others advocating for narrower strips. Therefore, this two-year field experiment aimed to determine the optimal strip width that maximizes yield and interspecific interactions in MSR, while considering mechanization needs. The study hypothesized that narrow strips would reduce soybean yield, wider strips would increase yields without negatively impacting maize, and that wider strips would overall be more beneficial than narrower strips.

The literature on optimal strip width in intercropping systems is mixed. Some studies report that wider strips (3m, 3.1m, 3.3m, 3.4m, and 6m) result in higher land equivalent ratios (LER) compared to narrower strips. This suggests that wider strips may facilitate mechanization and high profits. However, other research indicates that an optimal strip width of 1m exists, with wider strips leading to decreased intercropping benefits and reduced LER. For instance, studies on maize/common bean intercropping systems found a decrease in LER when row ratios (and thus strip width) increased from 2:2 to 5:5. Cereal-legume intercropping systems, particularly maize/soybean relay intercropping (MSR), are of increasing interest due to their potential to reduce inputs and boost nitrogen fixation. While MSR offers yield stability and resource utilization advantages, trade-offs between intercrop species exist, with soybean often suffering from maize shading. This study aimed to find the optimal strip width to mitigate this shading while improving yields and accommodating machinery.

A two-year field experiment was conducted at the Sichuan Agricultural University Research Farm in Ya'an, China. The experiment used soybean (Gongxuan-1) and maize (Chuandan-418). Five treatments were established with three replications: sole maize (SM), sole soybean (SS), and three MSR treatments with varying strip widths: T1 (narrow-strips, 180 cm total width), T2 (medium-strips, 200 cm total width), and T3 (wide-strips, 220 cm total width). In the MSR treatments, two rows of soybean were intercropped with two rows of maize. Standard fertilization practices were followed. Measurements taken included photosynthetically active radiation (PAR) transmittance using LI-191SA quantum sensors, leaf area index (LAI), dry matter, photosynthetic characteristics (Pn, Tr, Gs, Ci) using a Li-6400 photosynthesis system, grain yield components (grain number, grain weight, grain yield), land equivalent ratio (LER), and competition ratio (CR). Economic analysis, including total expenditure, gross income, and net income, was also performed. Statistical analysis using Statistix 8.1 was conducted to determine significant differences among treatments.

The study found that medium-strips (T2, 200 cm) produced the highest grain yields for both maize and soybean. Maize and soybean in T2 achieved 98% and 77% of the yields obtained in sole cropping systems, respectively. This treatment also resulted in the highest total grain yield (8429.4 kg ha⁻¹ on average) and total LER (1.76 on average), significantly exceeding the other treatments. Narrow-strips (T1, 180 cm) resulted in significantly lower soybean yields compared to medium and wide strips. Wide-strips (T3, 220 cm) showed a decline in both maize and soybean yields compared to medium strips. PAR transmittance, LAI, dry matter accumulation, and photosynthetic rates were significantly higher for soybean in T2 compared to T1, indicating reduced shading stress. Economic analysis revealed that medium-strips had the highest net income (1233 US$ ha⁻¹ on average), significantly higher than other treatments. Competition ratios indicated improved complementarity between maize and soybean in medium-strips.

The results strongly support the hypothesis that narrow strips reduce soybean yield, partially support the hypothesis that wider strips increase soybean yield without affecting maize yield, and partially support the hypothesis that wider strips are more beneficial than narrow strips. The optimal strip width of 200cm balances the competitive interactions between maize and soybean, maximizing the benefits of intercropping. Increased inter-row spacing in medium-strips improved PAR transmittance for soybean, mitigating shading stress and improving growth parameters. The higher net income in medium strips demonstrates the economic viability of this approach. The reduced yields and LER in wide-strips suggest that exceeding the optimal spacing leads to increased intraspecific competition, diminishing the advantages of intercropping. These findings highlight the importance of considering both inter- and intraspecific competition when designing intercropping systems for optimal resource utilization and yield.

This study demonstrates that a medium strip width of 200 cm optimizes the maize/soybean relay intercropping system, maximizing yields, improving resource use efficiency, and enhancing net income. The findings emphasize the importance of selecting appropriate strip widths to balance the competitive interactions between species and to achieve the full benefits of intercropping. Future research could explore the impact of different planting densities within the optimal strip width and investigate the feasibility of adapting farm machinery to effectively utilize this optimal strip width in large-scale farming operations.

The study was conducted in a specific location with particular soil conditions and climate. The results might not be directly generalizable to other regions with different environmental conditions. The economic analysis was based on local market prices and costs, which may vary across different regions and years. The study focused solely on maize and soybean, and findings may not be applicable to other intercropping combinations.